# Flywheel squat

## Flywheel Squats Versus Free Weight High Load Squats for Explosive Actions in Football

The investigators will aim at recruiting 45 recreationally active football players, that will be randomly allocated to three groups: 1) flywheel (FW) group (n=15), 2) barbell free weight (BFW) group (n=15) and 3) control group (n=15).The FW and the BFW group will participate in an intervention where they perform a squat exercise either with a FW device or with BFWs twice a week over six weeks (in total 12 sessions) as a part of their preseason preparations. The control group will be instructed to not perform lower body resistance exercise and only to perform their teams' preseason preparations and acted as controls. During the intervention period, all players in all groups will be instructed to avoid other resistance exercises for their lower body, while no restrictions will be given in regard to resistance exercises for their upper body in their spare time.

This study will be carried out in accordance with international ethical standards for sport and exercise science and to the Declaration of Helsinki; prior to pre-tests, all the players will be informed of the purpose of the study and its associated risks and benefits, before providing oral and written informed consent. The Norwegian Data Protection Service has approved the study and the storage of personal data (Approval reference number: 374030). No further Regional Ethical approval per applicable institutional and national guidelines for sport and exercise science.

Prior to the interventions, the players will undergo pre-tests in the following order on the same test day: 1) 10 meter sprint time, 2) CMJ and 3) 1RM in a free barbell partial squat, carried out as 90° ROM in the knee joint (standing position = 180°).

10 meter sprint: The 10 meter sprint test will be performed on artificial grass indoors. Photocells mounted to the floor and walls recorded the sprint times, where the photocells at the starting and the finishing line are placed 20 cm and 100 cm above the ground, respectively. A marker is placed 30 cm behind of the starting timing gate, where the players chose their starting position behind the marker. The players starts their test on their own initiative, and without verbal encouragement, by breaking the laser beam at the starting timing gate and sprinted to the finishing line as fast as they can. Each player is given three attempts with three minutes recovery between each sprint.

Countermovement jump:

the players will test their jump height with a countermovement jump on a force platform that measures the vertical jump height in centimetres (cm) by calculating the centre of mass displacement from force development (take-off velocity) and body mass. Starting from an upright standing position with their feet shoulder-width apart and with both hands placed on their hips, the players will make a preliminary downward movement (eccentric phase) by flexing their hips and knees to approximately 90° (knee-flexion) before performing the concentric phase of the vertical jump off the ground by extending the knees and the hips, respectively. Each player is given three attempts with three minutes recovery between each jump.

One repetition maximum in squat:

Following the jump test, the players will perform a one repetition maximum (1RM) partial range of motion (90° knee joint angle) back squat test using an Olympic barbell. The players will warm up by lifting the Olympic barbell (20 kg) without additional weights for 8-10 repetitions, and thereafter perform two sets of progressively decreasing repetitions (6 and 3 repetitions, respectively) and increasing the weights based on their perceived effort in the previous warmup set. Thereafter, the players attempts their 1RM trials with increasing weights (2.5-10 kg) until failure. Failure is defined as inability to lift the barbell to standing (starting) position (180° knee joint angle). A goniometer will be held to the lateral part of their knee joint by an instructor to ensure that the players reach 90° of knee flexion before they are given a verbal "go" and they can start the concentric phase of the lift. The kilograms (kg) lifted in the last approved set is considered their 1RM and recorded in kg.

Exercise interventions Over the course of the interventions, all players in all three groups will be instructed to adhere to their two-three weekly football practices and preseason matches of their team (~one weekly match). The players in the FW and BFW groups start their exercise interventions the week following pre-tests. All intervention sessions were performed in the same laboratory and supervised by the same instructor. The players in both intervention groups are expected to experience a large increase in 1RM partial squat strength and thus a predominant increase in quadriceps muscle force is expectable with respects to hamstrings force. Therefore, the players will also perform the Nordic hamstring exercise to avoid a large quadriceps-to-hamstring strength ratio and thereby potentially reduce the risk for hamstring strains.The Nordic hamstring exercise will be performed at the end of each exercise session (for both interventions) and involve three sets of four repetitions (week 1) where the number of repetitions is progressively increasing to five in week 2, six in week 3-4, eight in week 5, and 10 in the final week of the interventions.

Flywheel group:

The players allocated to the FW group will be equipped with a west around their upper body connected with a band to the FW device (#215 YoYo Squat Unlimited Pro, nHance, YOYO Technology, Stockholm, Sweden). The players start in a deep squat position (~120° knee angle), and perform at first a standardized warmup set with six repetitions using the #1 inertia FW (0.025 kg·m-2). Thereafter, the players perform their maximal intended mobilization of force contraction sets by starting with three slow repetitions to allow the players to get into the flow of the squat exercise movement, where they will be given a verbal "go" when starting to push with maximal intended mobilization of force from deep squat starting position to standing position. The band connecting the west and the FW device will be strapped tightly making the players stop at 175° knee joint angle in the standing position when the FW band is unwound. When the FW continues to rewind again and produces kinetic force in the pivoting shaft, this immediately will force the players to bend their knees and begin the eccentric contraction phase. The players are instructed to over-win the kinetic energy with the highest possible mobilization of muscular force, and immediately start a new concentric maximal intended mobilization of force contraction. During the sets and sessions, the load (Watt) is monitored using the manufacturer´s application (Bluebrain, Kuopio, Finland) on a portable tablet (Samsung Galaxy S4, Samsung Electronics, Daegu, South Korea) connected to the FW device through Bluetooth. If the players produces on average >4 watts·kg-1 from each repetition of one set, the FW size is increased, to #2 (0.05 kg·m-2) and later to #3 (#1 + #2= 0.075 kg·m-2) and finally #4 (0.1 kg·m-2). Throughout the sessions, the players are given verbal encouragement. In week 1 and 2 of the intervention, the players will perform six sets with six repetitions, thereafter, in week 3, 4 and 5-6, the players will perform 3x5, 4x5 repetitions and 4x4 repetitions, respectively. Recovery time between sets is set to ≥3 minutes.

Barbell free weight sqaut group The BFW group performes a specialized warmup with three sets of progressively increasing intensity in the squat exercise; eight repetitions at 30%-, six repetitions at 50%- and six repetitions at 70% of 1RM, respectively. In all sessions, the players are instructed to perform the concentric phased lift with maximal intended mobilization of force and are given verbal encouragement throughout the sessions. The first two sessions (week 1) consists of familiarization to the squat exercise movement with three sets of eight repetitions at ~70% of 1RM. Thereafter, from the third session (week 2), the players are instructed to perform four sets of four repetitions with high loads (preferably >85% of 1RM) throughout the remaining sessions with progressively increasing the load with 5kg if they can perform five repetitions within one set.

Sours: https://clinicaltrials.gov/ct2/show/NCT04113031

## Exxentric kBox4 Flywheel Training Review

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See our disclosures page for more information.

The kBox4 from Exxentricis a revolutionary piece of training equipment that is unlike anything we've tested before. Not only is the flywheel system employed by the kBox4 effective, but the build quality and precision of the machine is unparalleled.

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Sours: https://www.garagegymreviews.com/exxentric-kbox4-flywheel-training-review

## Flywheel squats versus free weight high load squats for improving high velocity movements in football. A randomized controlled trial

• Research article
• Open Access
• Published:

BMC Sports Science, Medicine and Rehabilitationvolume 12, Article number: 61 (2020) Cite this article

• 2272 Accesses

• 3 Citations

• 4 Altmetric

• Metrics details

### Background

High load (HL: > 85% of one repetition maximum (1RM)) squats with maximal intended velocity contractions (MIVC) combined with football sessions can be considered a relevant and time-efficient practice for maintaining and improving high velocity movements in football. Flywheel (FW) resistance exercise (RE) have recently emerged with promising results on physical parameters associated with football performance.

### Methods

In this randomized controlled trial over 6 weeks, 38 recreationally active male football players randomly performed RE with MIVCs two times per week as either 1) FW squats (n = 13) or 2) barbell free weight (BFW) HL squats (n = 13), where a third group served as controls (n = 12). All three groups conducted 2–3 football sessions and one friendly match a week during the intervention period. Pre- to post changes in 10-m sprint, countermovement jump (CMJ) and 1RM partial squat were assessed with univariate analyses of variance.

### Results

The FW and BFW group equally improved their 10-m sprint time (2 and 2%, respectively, within group: both p < 0.001) and jump height (9 and 8%, respectively, within group: both p < 0.001), which was superior to the control group’s change (between groups: both p < 0.001). The BFW group experienced a larger increase (46%) in maximal squat strength than the FW group (17%, between groups: p < 0.001), which both were higher than the control group’s change (both p < 0.001).

### Conclusion

Squats carried out with FWs or BFWs where both are performed with MIVCs and combined with football sessions, were equally effective in improving sprint time and jump height in football players. The BFW group experienced a more than two-fold larger increase in maximal partial squat strength than the FW group in maximal partial squat strength. This presents FW RE as an alternative to BFW HL RE for improving high velocity movements in football.

### Trial registration

ClinicalTrials.gov Identifier: NCT04113031 (retrospectively registered, date: 02.10.2019).

Peer Review reports

### Background

Maximal and high velocity forces are considered decisive for human movement performance. In modern football, the importance of performing rapid and high velocity movements, such as sprints and jumps, has gradually increased [1,2,3,4,5]. Maximal lower limb muscle strength is associated with lower limb muscle power [6, 7], where an increase in lower limb muscle strength is likely to result in an increased sprint performance [8].

High power resistance exercise (RE) with high velocities and low external loads is effective for improving rapid and high velocity movements [8,9,10,11]. However, independent of external loads, the intention of maximal velocity while performing RE is likely the most prominent factor for increasing the neural drive to the muscles, resulting in an increased velocity in the mechanical response [8, 12,13,14]. This is likely explained by short time to peak tensions, high rates of torque development, high motor unit discharge rates and an early and fast motor unit recruitment [15,16,17,18]. Consequently, RE with high external loads (HL: ≥85% of 1RM) and subsequently low velocity movement is also likely effective, as long as the intended velocity during the contractions is maximal [13]. In fact, HL RE is reported to be effective in football players when it is combined with performing high velocity movements (e.g. sprints and jumps) in football practice [8, 19]. Additionally, although the intensity in HL RE is high, the low number of repetitions and sets allows the total RE volume to be low. Due to the challenges of incorporating all important physical aspects while also ensuring sufficient recovery time in football players’ weekly exercise and competitive schedules [20], HL RE can be considered a relevant and time-efficient exercise modality for maintaining [21] and improving [22, 23] sprint and jump performance in football.

As eccentric muscle contractions allows for higher force production compared to the concentric contractions [24, 25], exercises with eccentric overload, such as inertia spinning YoYo™ flywheel (FW) devices [26], have been suggested as an alternative or supplement to the established exercise modalities [24, 25]. In FW devices, a band is connected to a pivoting shaft, where pulling the band unwinds the band and kinetic energy is subsequently produced in the shaft due to the inertia of the spinning FW. When the band reaches its maximal length, the FW keeps spinning and rewinds the band again and high muscle force is produced during the eccentric phase if the individual is trying to slow the spinning of the FW, where peak muscles forces are produced if the individual is instructed to break the eccentric movement towards the end of the rewound band [26, 27].

Over the past two decades, a substantial number of studies have assessed the utility of FW RE for improving sports performance, with positive effects on maximal strength, muscle power, jump height, sprint performance and changes of direction movements [26, 27]. Although the evidence for improved performance is compelling, there are fewer studies comparing FW RE to other RE modalities, which is necessary to determine whether FW exercise could have similar effects compared with the established RE modalities.

To our knowledge, no study has compared the effect of FW RE versus free weight RE using the same motion path, which consequently stimulates the same muscles. Additionally, no study has compared the effect of FW exercise and free weight using HL with maximal intended velocity contraction (MIVC)s combined with football sessions, which can be considered a relevant and time-efficient exercise modality for improving high velocity movements in football while also improving maximal strength [22, 23]. Such information can be highly applicable for coaches in football clubs, who should use the best practice in relation to total exercise load to optimize performance of the players, at least in elite clubs. Thus, the objective of this study was to compare the effect of FW RE versus free weight HL RE on 10-m sprint time, countermovement jump (CMJ), and 1RM partial 90° range of motion (ROM) squat strength in football players. In this randomized controlled trial, both the FW RE and the free weight RE were carried out in a squat exercise with MIVCs and combined with football sessions. We hypothesized 1) that RE using FW and barbell free weight (BFW) combined with football practices will equally improve sprint time and jump height, and 2) that squats carried out in a BFW exercise will result in superior improvements in 1RM partial squat compared with FW squats.

### Design

In this randomized controlled trial, we randomly allocated 49 players into three different groups using Research Randomizer [28] (three sets, 17 numbers per set, ID-number range 1–49, “every number unique”, “no sorted order” and “no place marker”); 1) flywheel (FW) group (n = 16), 2) barbell free weight (BFW) group (n = 16) and 3) control group (n = 17). Due to drop out (22.5%), the final number in the three groups were 13, 13 and 12 players in the FW, BFW and control group, respectively. The FW and the BFW group participated in an intervention where they performed a squat exercise either with a FW device or with BFWs twice a week over 6 weeks (in total 12 sessions) as part of their preseason preparations. The control group was instructed not to perform lower body RE and only to perform their teams’ preseason preparations and acted as controls. During the intervention period, all enrolled players were instructed to avoid complementary REs for their lower body, while no restrictions were given regarding REs for their upper body. Our outcome measures were 10-m sprint time, CMJ and 1RM partial squat, which we measured pre- and post the 6 week long intervention.

This study was carried out in accordance to the Declaration of Helsinki; prior to pre-tests, all the players were informed of the purpose of the study and its associated risks and benefits, before providing oral and written informed consent. The Norwegian Data Protection Service approved the study and the storage of personal data (Approval reference number: 374030), without further Regional Ethical approval per applicable institutional and national guidelines for sport and exercise science [29, 30].

### Subjects

In the pre-season period in Norway, from January to March 2019, 49 recreationally active football players volunteered to participate. Recruitment period was January 5th to January 31st, data collection was from February 1st to March 31st. The players played at the two highest regional levels in the Norwegian national league system, which is the 5th and 6th levels in Norway. After contacting multiple 5th and 6th level teams’ coaches, the included players were recruited from teams with similar overall exercise load with the following inclusion criteria; 1) two or three 60 min football sessions and 2) one friendly football match a week. Exclusion criteria was no injury or disease preventing from participation in RE and football practice. The flow and random allocation of participants are illustrated in Fig. 1. Four of the 49 recruited players reported to be unfamiliar with RE, while the remaining players reported to perform 1–6 weekly RE sessions beside their teams’ football sessions. Four players withdrew from the study prior to study completion due to illness and injuries not related to the study interventions, and seven players did not show up for post-tests. As a result, 38 players completed the study. The descriptive baseline test characteristics are shown in Table 1; there were no differences in baseline characteristics between the intervention groups (all p ≥ 0.20).

### Test procedures

Prior to the interventions, the players underwent pre-tests in the following order on the same test day: 1) 10-m sprint time, 2) CMJ and 3) 1RM in a barbell free weight partial squat, carried out as 90° ROM in the knee joint (standing position = 180°). The players’ height was assessed on a portable scale (Seca 217, Seca GmbH & Co., KG, Germany) and body mass on a portable force platform (Hurlab FP4, HUR Labs Oy, Kokkola, Finland), which was connected to a portable laptop (ThinkPad, Lenovo Group Ltd., Beijing, China) through a USB cable and monitored with the manufacturer’s software (Force platform software suite, HUR Labs Oy, Kokkola, Finland). Body mass index (BMI) was calculated. Prior to testing, the players jogged for 15 min at progressively increasing intensity (easy to moderately paced jogging) with various exercises (e.g. knee raises, heel kicks, lunges, and frontal vertical kicks to their hands) on artificial grass, supervised by an instructor. The subjects wore jogging shoes and light clothing. Following the 15 min jog, the players performed two progressive 15-m sprints instructed to be at 95% of self-determined maximum acceleration.

### 10-m sprint test

The 10-m sprint test was performed on artificial grass indoors. Single-beam photocells (ATU-X, IC Control AB, Stockholm, Sweden), mounted to the floor and walls recorded the sprint times, where the photocells at the starting and the finishing line were placed 20 cm and 100 cm above the ground, respectively. Within-subject coefficient of variation of single-beam photocells is reported to be 2% [31]. A marker was placed 30 cm behind the starting timing gate, where the players chose their starting position behind the marker. The players started the test on their own initiative, and without verbal encouragement, by breaking the laser beam at the starting timing gate and sprinted to the finishing line as fast as they could. Each player was given three attempts with 3 min recovery between each sprint. The fastest sprint time was recorded.

### Countermovement jump

Following ≥3 min rest from the sprint test, the players performed the CMJ test on the portable force platform (Hurlab FP4, HUR Labs Oy, Kokkola, Finland) following the body mass measurement. Portable force platforms is found to measure CMJ jump height within a 2% accuracy compared to a laboratory floor mounted force platform (Type 9281B Kistler, Instrumente AG, Winterthur, Switzerland) [32] and with a 2.8% within-subject coefficient of variation [33]. Starting from an upright standing position with their feet shoulder-width apart and with both hands placed on their hips, the players were instructed to make a preliminary downward movement (eccentric phase) by flexing their knees to approximately 90° (knee-flexion) before performing the concentric phase of the vertical jump off the ground by extending the knees and the hips, respectively. Each player was given three attempts with 3 min recovery between each jump. If an incorrect jump was performed (e.g. typical mistake was lifting the heel prior to extending the knees), the player was given a new attempt. The force platform measures the vertical jump height in centimetres (cm) by calculating the centre of mass displacement from force development (take-off velocity) and body mass. The sampling rate was set to 1200 Hz. The highest vertical jump was recorded.

### One repetition maximum in partial squat

Following the CMJ test, the players performed a partial ROM (approximately 90° knee joint angle) back squat test using an Olympic barbell (Eleiko, Halmstad, Sweden) for the assessment of 1RM. We used a slightly modified 1RM protocol used by Helgerud et al. [34]. The players first warmed up by lifting the Olympic barbell (20 kg) without additional weights for 8–10 repetitions, and thereafter performing two sets of progressively decreasing repetitions (6 and 3 repetitions, respectively) and increasing the weights based on their perceived effort in the previous warm-up set (Helgerud et al. [34] specified no 1RM warm up). Thereafter, the players attempted their 1RM trials with increasing weights (10 kg) until failure (Helgerud et al. [34] used 5 kg increments). Failure was defined as inability to lift the barbell to standing (starting) position (180° knee joint angle). A mechanical goniometer was held to the lateral part of their knee joint by an instructor to ensure that the players reached 90° of knee flexion before they were given a verbal “go” and they could start the concentric phase of the lift. The kilograms (kg) lifted in the last approved lift with one repetition was considered their 1RM and recorded in kg. One repetition maximum was normally reached between 3 and 6 sets, ≥3 min recovery was given between each attempt. The coefficient of variation for 1RM squat is reported to be 2.9% [35]. As 1RM strength divided by body mass may be imprecise where a heavier individual may be overestimated and a lighter individual underestimated [34, 36], the kg lifted was also allometrically scaled as kg lifted in the squat exercise multiplied by body mass raised to the power of 0.67 (kg lifted·kg body mass-0.67) [34, 36].

### Exercise interventions

An overview of the exercise programs is presented in Table 2. Over the course of the interventions, all players in all three groups were instructed to adhere to their two-three weekly football sessions and friendly matches of their team. The players in the FW and BFW groups started their RE interventions the week following pre-tests, which did not coincide with their football sessions (i.e. RE and football sessions was separate). Prior to both intervention groups’ sessions, the players performed a 10 min self-selected low intensity aerobic warm-up on a motorized treadmill (ELG 70, Woodway Inc., Waukesha,Wisconsin, United States) or an ergometer bike (Pro/Trainer, Wattbike Ltd., Nottingham, United Kingdom). For both groups, the players were instructed to perform their exercise with MIVCs and were given verbal encouragement throughout the sessions. All intervention sessions were performed in the same laboratory and supervised by the same instructor. The players in both intervention groups were expected to experience a large increase in knee extensor strength. Therefore, the players performed the Nordic hamstring exercise to avoid a large quadriceps-to-hamstring strength ratio and thereby potentially reduce the risk for hamstring strains [37].

The Nordic hamstring exercise was performed at the end of each exercise session (for both interventions) and involved three sets of four repetitions (week 1) where the number of repetitions were progressively increased to five in week 2, six in week 3–4, eight in week 5, and 10 in the final week of the interventions. At the end of the 6-week exercise interventions, the participants performed post-tests in the same order as the pre-tests.

### Flywheel squat group

The players allocated to the FW group was equipped with a vest on their upper body connected with a band to the FW device (#215 YoYo Squat Unlimited Pro, nHance, YOYO Technology, Stockholm, Sweden). In the FW device, different sized spinning inertia FWs can be connected to the pivoting shaft (size #0.5: 0.0125 kg·m− 2, #1: 0.025 kg·m− 2, 2#: 0.05 kg·m− 2, 4#: 0.1 kg·m− 2). The first two sessions (week 1) were familiarization sessions, which consisted of three sets with six repetitions. Thereafter, from week 2, the players performed three sets with six repetitions with MIVCs followed by 3 × 5, 4 × 5 and 4 × 4 repetitions in week 3, 4 and 5–6, respectively. Recovery time between sets was set to ≥3 min. The players started in a partial squat position (~ 90° knee angle) and performed first a standardized warm-up set with six repetitions using the #1 inertia FW (0.025 kg·m− 2). In all exercise sets, the players started with three slow repetitions to get into the flow of the squat exercise movement before beginning their scheduled MIVC sets (6 × 3, 5 × 3, 4 × 5, 4 × 4 depending on exercise week), where they were given a verbal “go” when starting to push with MIVCs from starting position (~ 90° knee joint angle) to standing position. The band connecting the vest and the FW device was strapped tightly making the players stop at approximately 175° knee joint angle in the standing position when the FW band was unwound. When the FW continued to rewind again, this immediately forced the player to bend their knees and begin the eccentric contraction phase of the next repetition. The players were instructed to over-win the kinetic energy with the highest possible mobilization of muscular force at the end of the eccentric movement (~ 80° knee joint angle), and immediately start a new concentric MIVC. During the sets and sessions, the load (Watt) was monitored using the manufacturer’s application (Bluebrain, Kuopio, Finland) on a portable tablet (Samsung Galaxy S4, Samsung Electronics, Daegu, South Korea) connected to the FW device through Bluetooth. The starting inertia at week 2 was set to #1 (0.025 kg·m− 2). If the players produced on average > 4 watts·kg− 1 from each repetition of one set, the FW size was increased, to #2 (0.05 kg·m− 2) and later to “#3” (#1 + #2 = 0.075 kg·m− 2) and finally #4 (0.1 kg·m− 2).

### Barbell free weight squat group

The players in the BFW group performed a specialized warm-up with three sets of progressively increasing intensity in the squat exercise; eight repetitions at 30%-, six repetitions at 50%- and six repetitions at 70% of 1RM, respectively. The first two sessions (week 1) consisted of three sets with eight repetitions at ~ 70% of 1RM. Thereafter, from the third session (week 2), the players were instructed to perform four sets of four repetitions at > 85% of 1RM throughout the remaining sessions with progressively increasing the load with 5 kg if they could perform five repetitions within one set (e.g. if performing 5 repetitions, the load in the next set was increased, which could be set 1, 2, 3 or 4 in the exercise session). Recovery time between sets was set to ≥3 min. Figure 2 illustrates the logged progression of the BFW group.

### Statistical analyses

The Shapiro Wilk test confirmed all data to not deviate from normal distribution, both prior (all p ≥ 0.11) and following randomization (all p ≥ 0.052), which were confirmed by inspection of the Q-Q plots. We performed paired sample t-tests to assess pre- to post-test changes within groups. One-way univariate analyses of variance (ANOVAs) with Bonferroni corrected post-hoc tests were used to examine differences in baseline characteristics, and in the change score (post-pre) from pre-to post-test between the groups. Effect sizes were calculated as Cohen’s d where determination of magnitude was considered according to Rhea’s recommendation for RE interventions of moderately fit individuals; trivial: < 0.35, small: 0.35–0.79, medium: 0.80–1.49, large: ≥1.50 [38]. For pre- to post effect size within groups, we divided the mean change score by the standard deviation (SD) of the change score. We calculated between groups effect size by the pooled SD of the two groups of interest (e.g. FW vs BFW, FW vs control, BFW vs control) divided by the difference in mean change score of the two groups of interest using the following formula:

$$\sqrt{\frac{\left({n}_1=1\right)\times {SD}_{1^2}+\left({n}_2-1\right)\times {SD}_{2^2}}{n_1+{n}_2-2}/}{m}_1-{m}_2$$

Where n1 and n2 represents the groups’ n, SD12 and SD22 represents the groups’ SD squared, m1 and m2 represents the two groups’ mean change score, respectively. We used Pearson’s correlations to assess the association between the change in sprint time and jump height, respectively, and the change in maximal partial squat strength. We adopted linear regressions to assess whether inclusion of changes in body mass could explain more of the variation in the association than maximal partial squat change alone. We performed a pilot study where we observed a mean decrease of 0.0243 ± (SD) 0.0215 s in the 10-m sprint test following 6 weeks of partial squat exercise at > 85% of 1RM characterized by 4 × 4 repetitions. Sprinting 0.02 m·s− 1 faster over 10 m would result in a ~ 10 cm difference, which can be considered a shoulder length ahead of an opponent and thus a game changing and relevant difference [14]. With 80% power and an alpha level of 0.05, we calculated to need 12 participants in each group. We assumed a 25% dropout and thus aimed to recruit at least 45 participants (15 in each group); following dropouts (22.5%), we ended up with 13 (FW), 13 (BFW) and 12 (control) in our three groups for the final analyses. Data are shown as mean ± SD unless otherwise is stated. All statistical analyses were performed using the Statistical Package for Social Sciences (SPSS, Version 26, IBM, Armonk, NY, United States).

### Results

The pre- and post-test results are presented in Table 3. There were differences in changes in the 10-m sprint test between the groups (between subjects effect: p < 0.001), where the FW and the BFW group equally decreased their 10-m sprinting time from pre- to post-test by 2% (between groups: p = 1.00, Cohen’s d: 0.00, pre- to post-test: FW group: p < 0.001, Cohen’s d: − 0.97; BFW group: p = 0.005, Cohen’s d: − 0.96), while the control group did not decrease their sprinting time (p = 0.39, Cohen’s d: 0.26; difference between FW and BFW vs control: both p < 0.001, both Cohen’s d: − 1.39) (Table 3). The individual change in 10-m sprint time from pre- to post-test and the association with 1RM partial squat change is illustrated in Fig. 3. Two out of the 13 in the FW group did not experience a game changing relevant change in 10-m sprint performance (≥0.02 s decrease in 10-m sprint time; range FW group: 0.02 to − 0.08 s, mean increase: − 0.03 ± 0.01 s). Four out of the 13 in the BFW group (range: − 0.01 to − 0.04 s, mean increase: − 0.03 ± 0.03 s) and 11 out of the 12 players in the control group (0.02 to − 0.02 s, mean increase: 0.003 ± 0.01 s) did not experience a game changing relevant change in 10-m sprint time (Fig. 3).

There were differences in changes in the CMJ test between the groups (between subjects effect: p < 0.001), where the FW and the BFW group equally increased their jump height in the CMJ test from pre- to post-test by 9 and 8%, respectively (between groups: p = 1.00 Cohen’s d: − 0.16; pre-to post-test: FW: p < 0.001, Cohen’s d: 1.70; BFW: p < 0.001, Cohen’s d: 1.54), while the control group did not increase their jump height (p = 0.75, Cohen’s d: 0.09; difference between FW and BFW vs control: both p < 0.001, Cohen’s d: FW vs control: 2.15, BFW vs control: 1.94) (Table 3). The individual CMJ change from pre-to post-test and the association with 1RM partial squat change is illustrated in Fig. 4. All players in FW group experienced an increase in jump height (range 0.37–7.01 cm, mean increase: 3.07 ± 1.80 cm). In the BFW group, 11 out of the 13 increased their jump height (range: − 0.43 to 6.10 cm, mean increase: 2.78 ± 1.80), while seven out of the 12 players in the control group experienced an increased jump height from pre- to post-test (range: − 1.64-1.02 cm, mean increase: 0.07 ± 0.72 cm) (Fig. 4).

There were differences in changes in the 1RM partial squat test between the groups (between subject effect: p < 0.001), where the BFW group increased their 1RM squat by 46%, which is more than the FW group’s increase of 17% (difference between groups: p < 0.001, Cohen’s d: 3.43, pre- to post-test: FW: p = 0.001, Cohen’s d: 3.13, BFW: p < 0.001, Cohen’s d: 3.17), and the BFW and the FW group increased their 1RM squat more than the control group (difference between FW and BFW vs control: both p < 0.001, Cohen´s d: FW vs control: 2.71, BFW vs control: 4.93, pre-to post-test control group: p = 0.10, Cohen’s d: 0.51). When scaling 1RM partial squat strength to the power of 0.67, the results remained unchanged (Table 3). For individual pre- to post 1RM partial squat changes, all players in FW and BFW group increased their 1RM (FW: range: 10–30 kg, mean increase: 21.5 ± 6.9 kg; BFW: 40–90 kg, mean increase: 62.3 ± 15.4), while three out of the 12 players in the control group increased their 1RM (range: 0–20 kg, mean increase: 0.07 ± 0.72 cm) (Figs. 3 and 4).

We observed a negative linear association between the change in maximal partial squat strength and the change in sprint time (1RM: r = 0.39, r2 = 0.15, p = 0.02) (Fig. 3). We observed a positive linear association between maximal partial squat strength and jump height (r = 0.52, r2 = 0.27, p = 0.001) (Fig. 4). These associations were unchanged when including change in body mass as independent variable, and when changing 1RM to scaled 1RM (data not shown).

### Discussion

In this randomized controlled trial of recreationally active football players, FW and BFW HL squats equally improved 10-m sprinting time and CMJ height while BFW HL squats was superior to FW squats in improving maximal partial squat strength. Finally, we observed linear associations between changes in maximal partial squat strength and changes in 10-m sprinting time and CMJ, respectively.

The equal improvement for both intervention groups in 10-m sprint time and jump height is in line with the latest systematic review assessing the effect of RE in football players [8], and also with previous studies assessing the effect of BFW HL partial squats combined with football sessions [22, 34]. Although not always consistent [39], sprint improvements following FW squats is reported previously [40, 41], while improvements in jump height following FW RE seem to be a consistent observation [39,40,41].

Although we observed linear associations between improvements in maximal squat strength and improvements in sprint time and jump height, which is in line with the latest review on the effect of RE in football players [8], the explained variances are low (10-m sprint change: 15% (r2 = 0.15), Fig. 3; CMJ change: 27% (r2 = 0.27), Fig. 4), indicating that other factors than increased maximal squat strength may explain the improved 10-m sprint and jump performance. These similar improvements between the BFW and FW groups are likely explained by neuromuscular adaptations induced by MIVCs [12]. For example, using novel high density surface electromyography recordings, a recent study showed an increased motor unit discharge rate accompanied by a decreased motor unit recruitment threshold following 4 weeks of isometric MIVCs [42]. Moreover, it seems that peak rate of force development is associated with peak motor unit discharge rate, which also seem to be generated prior to maximal force development [16], which thus seem to explain the underlying neural mechanisms for improvements of high velocity movements following RE [12].

However, it is reported that neural adaptations preliminary occurs within the first 1–2 weeks of RE [25, 43]. Thus, although the strength of the associations between change in sprint time or jump height and change in maximal squat strength were unchanged when including body mass change as independent variable, we cannot rule out whether our 6 week long intervention induced morphological changes (e.g. increase in pennation angle, fascicle length and cross-sectional area), which normally occur as a result of longer exercise programs. For example, a previous study assessing the effect of FW RE revealed changes in muscle fascicle length and pennation angle, which was paralleled with hypertrophy gains [44].

The BFW group experienced a more than two-fold larger increase in 1RM squat (46%) than the FW group (17%). The 17% increase in the FW group is in line with previous reported increases following squat RE in football players [8], while the 46% increase in the BFW group is towards the highest reported increases in 1RM partial squat in the literature for football players (52%) [8, 34]. A meta-analysis reported that FW RE is not superior to traditional RE for strength improvements [45], which corroborate our findings. Nevertheless, the difference in 1RM squat strength between the BFW and the FW group in our study is likely an effect of test specificity where the exercise performed by the BFW group was isotonic to the test; this is shown previously for the squat exercise [46]. Consequently, we urge for cautious interpretation when comparing 1RM gains between the BFW and FW group.

A previous meta-analysis comparing concentric and eccentric RE reported superior gains in maximal strength following eccentric RE [24]. However, their stratified analysis of exercise intensity revealed no differences between the two exercise modalities [24]. In fact, in studies comparing solely concentric low intensity (75% of 1RM) contractions with concentric and subsequent eccentric overload contractions (> 100% of 1RM), superior 1RM gains are reported from subsequent eccentric overload [47, 48]. While studies comparing solely concentric higher intensity (maximal 6- and 10RM and > 85% of 1RM) with subsequent eccentric overload reported similar gains in 1RM [49, 50]. This may suggest that as long as the concentric phase is performed with heavy loads (~ ≥ 85% of 1RM), no extra maximal strength gains can be derived from additional eccentric overload [24]. This indicate that high external loads (> 85% of 1RM) should be applied to easily recruit the higher threshold motor units [14], which is responsible for the highest force productions [13].

### Strengths

To our knowledge, this is the first randomized controlled trial comparing FW RE to HL RE practices for maintaining [21] and improving [22, 23] sprint and jump height performance in football. Due to the comparison in our study, one can assess the applicability of FW RE in football. Such information is highly applicable for coaches in football clubs, which should use the best practice in relation to total exercise load to optimize performance of the players, at least in elite clubs.

### Limitations

Some limitations need to be addressed. First, football involves multiple changes of direction at high velocities [51]. As changes of direction involves decelerations and subsequently accelerations in a different direction, the ability to utilize the elastic energy stored in tissues from deceleration during eccentric contractions into a subsequent concentric acceleration phase can be decisive in football [51]. Flywheel RE comprise of such decelerations with high force production, and FW RE is found to improve changes of directions [52]. We did not assess the ability to perform changes of direction our study. Future research investigating whether FW or HL BFW squat exercise results in superior performance in changes of direction tests is warranted.

Further, we did not match exercise intensity between the two intervention groups, which introduce the possibility of the external loads employed in the interventions influencing our results (i.e. the exercise intensity per se, not exercise modality). One study demonstrated that increasing FW inertia increases coupling time (transition from eccentric to concentric contraction during the movement) and reduces power output [53]. Thus, increasing FW inertia might have hindered maximal improvements in high velocity movements (sprint and jump height) for the players in the FW group. However, force production increased by increasing inertia [53] and the intended velocity per se (not actual movement velocity) is responsible for improving high velocity movements following RE [12]. As increasing force production with increasing inertia can be considered higher load RE than not increasing inertia, we increased inertia following mean > 4 watts∙kg− 1 in one set to label both intervention groups’ exercise intensity “HL RE” and make exercise intensity between groups more comparable. Thus, we tried to keep similar progression in exercise load in both intervention groups, where reaching a certain limit (FW: > 4 watts∙kg− 1, BFW: ≥ 5 repetitions) resulted in an increased load in the next set. This also ensured individualized progression, as highlighted as an important factor for optimizing improvements in sprint performance from FW RE [53].

Furthermore, by performing 4 × 4 repetitions and increasing load when reaching five repetitions in the BFW, without any mid-test 1RM to adjust relative load, there could have been a possibility of some players in the BFW group exercising at < 85% of their actual 1RM as their actual 1RM increases during the intervention. However, this protocol is proved highly effective in improving maximal strength [21, 22, 34] and moreover, the increase from week to week was high in this group (Fig. 2), ultimately leading to a 46% increase in 1RM, which is towards the highest reported increases in 1RM in football players [8].

Hamstring muscle strength is associated with sprint performance [54,55,56], and antagonist co-contraction may have contributed to the increase in force production by an exercise-induced increase in reciprocal inhibition [57]. As both intervention groups performed the Nordic Hamstring exercise, the control group should also have performed this exercise allowing us to solely compare the effects of FW and BFW squats. However, the potential effects of Nordic Hamstring on sprint and jump height performance in our two intervention groups should influence our results in similar proportional order. Nevertheless, it seems that antagonist co-contraction plays a greater role in joint protection in RE, suggesting that they may play a minor role in the actual movement velocity [57]. Moreover, the effect on sprint performance following Nordic hamstring exercise is usually small [58, 59] or non-existing [60].

Finally, this study included recreationally active football players. Elite football players are reported to sprint faster than lower level players [1] and have a larger total exercise load resulting in limited recovery time between exercise sessions [20]. Whether differences in sprints, jump height and maximal strength gains from FW and BFW squats would be present in elite football players are currently unknown. However, as the players in our study experienced similar gains from BFW squats on sprint, jump height and 1RM partial squat as previously reported in elite football players [8, 22, 34], one may consider our study’s findings generalizable to elite football players, at least until proven otherwise by future research.

### Conclusion

Squats carried out with FWs or HL BFWs where both are performed with MIVCs and combined with football sessions, were equally effective in improving sprint time and jump height in football players. The BFW group experienced a more than two-fold larger increase in maximal partial squat strength than the FW group. This presents FW RE as an alternative to HL free weight RE for improving high velocity movements in football players.

### Availability of data and materials

All data generated or analysed during this study are included in this published article and its supplementary information files.

### Abbreviations

Resistance exercise

Flywheel

High load

Barbell free weight

Standard deviation

One repetition maximum

Maximal intended velocity contraction

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Getting started with flywheel training

Open Access

Peer-reviewed

• Darjan Spudić ,
• Darjan Smajla ,
• Nejc Šarabon

Contributed equally to this work with: Darjan Spudić, Darjan Smajla, Nejc Šarabon

Roles Funding acquisition, Investigation, Methodology, Project administration, Software, Supervision, Visualization, Writing – review & editing

* E-mail:[email protected]

Affiliations Faculty of Health Sciences, University of Primorska, Izola, Slovenia, InnoRenew CoE, Izola, Slovenia, S2P, Science to Practice, Ltd., Laboratory for Motor Control and Motor Behaviour, Ljubljana, Slovenia, University of Primorska, Andrej Marušič Institute, Koper, Slovenia

• Darjan Spudić,
• Darjan Smajla,
• Nejc Šarabon

x

### Abstract

Although the popularity of flywheel (FW) devices in sports research is increasing, to date, no study has been designed to test the reliability of electromyographic (EMG) variables during FW squats as a basic lower-body FW resistance exercise. At the primary level, our study was conducted to determine the minimum number of the consecutive flywheel (FW) squat repetitions that need to be averaged in a single set to obtain excellent reliability of peak, mean and three position-specific EMG variables. At the secondary level, comprehensive analysis for peak and mean EMG variables was done. Intra-set reliability was investigated using the minimum number of repetitions determined from the primary level of the study. Twenty-six participants performed five sets of seven squats with three FW loads (0.05, 0.125, 0.225 kg∙m2). EMG signals were collected from eight leg muscles. By averaging twelve consecutive repetitions, we obtained ICC2.k > 0.95 for mean and peak EMGRMS regardless of the muscle, load or phase of the squat (concentric vs. eccentric). Due to the heterogeneity of the results at the primary level, position-specific variables were excluded from the inter-set reliability analysis at the secondary level. Trustworthy mean and peak EMG variables from the primary level showed good to excellent inter-set reliability. We suggest averaging twelve consecutive squat repetitions to achieve good to excellent intra-session reliability of EMG variables. By following the proposed protocol, activation of leg muscles can be confidently studied in intra-session repeated-measures study designs.

Citation: Spudić D, Smajla D, Šarabon N (2020) Intra-session reliability of electromyographic measurements in flywheel squats. PLoS ONE 15(12): e0243090. https://doi.org/10.1371/journal.pone.0243090

Editor: Dragan Mirkov, University of Belgrade, SERBIA

Received: September 10, 2020; Accepted: November 15, 2020; Published: December 3, 2020

Copyright: © 2020 Spudić et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript.

Funding: This study was supported by the Slovenian Research Agency in the form of salaries for NŠ and DSM, within the framework of the project “Body asymmetries as a risk factor in musculoskeletal injury development: studying etiological mechanisms and designing corrective interventions for primary and tertiary preventive care” (L5-1845) and a Research Program Fund within the framework of the project “Kinesiology of monostructural, polystructural and conventional sports” (P5-0147). S2P, Science to Practice, Ltd. also provided support in the form of a salary for NŠ. The specific roles of this author are articulated in the ‘author contributions’ section. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have read the journal’s policy and have the following competing interests: NŠ was employed by S2P, Science to Practice, Ltd. This does not alter our adherence to PLOS ONE policies on sharing data and materials. There are no patents, products in development or marketed products associated with this research to declare.

### Introduction

Despite the increasing popularity of flywheel (FW) devices, especially in the fields of research, sports and health care, only a several studies have assessed electromyographic (EMG) muscle activation during FW loading conditions [1–10]. Lower EMG activity in the eccentric—compared to concentric—phase of the contraction is obvious for the exercises with equal gravity-based load (i.e. weight-stack or barbell) [11,12]. In contrast, studies using FW load have indicated greater muscle activation during the eccentric phase compared to gravity-based exercises in both open [1] and closed [8] kinetic chain exercises. Most recently Alkner & Bring (2019) [9] measured higher mean EMG activation during the eccentric phase of the contraction when comparing the FW leg press to a following gravity-based resistance (GB) exercises: barbell front squat, weight stack leg press and weight stack knee extension. One of the shortcomings of the recent studies comparing EMG muscle activity between FW and GB resistance exercises was the relativization of load selection (FW vs. weights) and the tempo of the exercise being executed (FW all-out vs. fluent concentric). In this manner, it can also be speculated that performing such GB exercises required a more controlled approach compared to the all-out effort from the first repetition on, applicable in the FW devices. Most of the FW resistance protocols were, therefore, power-oriented and were targeting improvements in neuromuscular activation. In contrast, for the GB resistance exercises, load was determined by the maximum number of repetitions performed with fluent concentric repetitions, meaning that it was submaximal during most of the set repetitions [9]. The variable tempo of the exercise execution using FW resistance, which is oriented towards high power outputs, significantly influences the rate of force development, resulting in burst-like muscle activation patterns that potentially decrease the reliability of measurements [13]. Therefore, the reliability of the EMG variables using FW resistance should be questioned.

Due to stochastic nature of an EMG signal [14], in order to obtain representative insight into EMG activation, the average of consecutive repetitions should be considered. To date, there has been a lack of consensus across studies about the representative number of repetitions and muscles analysed during FW leg press movement patterns. To our knowledge, previous studies used signals from three [9] to ten [4] consecutive repetitions, which were post-hoc averaged. In contrast, an average of five sets of 10 repetitions during the FW squat [8] were used in comparing quadriceps muscle activity between FW and GB resistance. Signals were averaged from the following muscles: m. vastus medialis (vm) [4,8–10], m. vastus lateralis (vl) [4,8–10], m. rectus femoris (rf) [8,9], m. gastrochnemius medialis (m.gas) and lateralis (l.gas) [4]. To date, only one study [4] reported between-participant (n = 17) reliability of mean vl, vm, m.gas and l.gas muscle activation for the concentric and eccentric phase of the squat using intraclass correlation coefficient (ICC) and within-participant coefficient of variation (CV). Reliability was highest for vm (ICC = 0.95, CV = 9.9%) and lowest for l.gas (ICC = 0.22, CV = 17.4%) muscles. Additionally, only Alkner & Bring (2019) [9] analysed position-specific EMG variables during FW leg press movement pattern. EMG activity during the concentric and eccentric actions were averaged over position-based—10° knee angle width—intervals from 85° to 155° knee extension joint angles.

Altogether, questions concerning the reproducibility of EMG variables during FW squats, remain open. To reliably follow training adaptations and related underlying mechanisms in future research, intra-session reliability concerning leg muscles at different FW loading conditions should be assessed. Although the popularity of FW devices in sports research is increasing, no study to date has been specifically designed to test the reliability of EMG variables during FW squats. Consequently, the primary level of our study was conducted to determine the minimum number of consecutive repetitions that need to be averaged to obtain reliable intra-session measures of EMG outcome variables. At the secondary level, the inter-set reliability was investigated using trustworthy EMG variables determined in the primary level. Using three different FW load conditions and signals from eight leg muscles, we hypothesized that averaging a higher number of consecutive repetitions improves the reliability of the selected EMG variables. Three chosen loading conditions (0.05, 0.125, and 0.225 kg∙m2) represent very fast, medium, and slow velocity squat movements, therefore EMG acquisition was covered during equidistantly different training conditions, which are representative of strength, power, or speed regimens. Furthermore, the trustworthy variables from the primary level were expected to provide us with good to excellent inter-set reliability at the secondary level. The results are proposed to contribute to the standardization of the methodology for assessing leg muscle EMG measurements using FW squats.

### Participants

Twenty-six physically active volunteers participated in the study—for details see Table 1. The inclusion criterion was strength-training experience (strength exercises at least two times per week in the last five years). The exclusion criteria were: knee injuries, chronic diseases, history of lower back pain or acute injuries in the past 6 months. The study was approved by the National Medical Ethics Committee (no. 0120-690/2017/8) and adhered to the tenets of the Oviedo Convention and Declaration of Helsinki. The individual in this manuscript has given written informed consent (as outlined in PLOS consent form) to publish these case details. Participants were informed about the testing procedures prior to signing an informed consent. They were instructed to avoid any strenuous exercise at least two days prior to the testing session.

### Experimental design

A repeated-measures design was used to assess (a) the reliability of the EMG outcome variables depending on the number of averaged repetitions and (b) inter-set reliability for each FW load.

### Testing procedures

The participants performed squats on a custom-made FW device (Fig 1). Three FW loading conditions were used, i.e. 0.05, 0.125, 0.225 kg∙m2. Before each testing session, participants performed a 10-min warm-up as described in detail elswhere [15]. A draw-wire sensor (d = 1250 mm; linearity = ± 0.02%; Way-Con SX-50, Taufkirchen, Deutschland) was fixed perpendicularly to the FW device below the standing surface and a draw-wire was attached to the lifting harness (between legs). The sensor setup provided us with vertical position-time data for the concentric and eccentric phases of the squat. A bilateral force plate system (Type 9260AA, Kistler Instrumente AG, Winterthur, Switzerland) with Kistler MARS software (S2P Ltd., Ljubljana, Slovenia) was used to acquire ground reaction force (F) data during maximal voluntary isometric (MVC) contractions. For EMG activity assessment, we used a Trigno Delsys Wireless System (Delsys Inc., Massachusetts, USA), with pre-amplified self-adhesive wireless electrodes (dimensions: 27 x 37 x 15 mm; mass: 14.7 g; electrode material: silver; contact dimension: 5 x 1 mm). After skin preparation (shaving, light abrasion, and cleaning with alcohol; < 5 kΩ), the electrodes were unilaterally placed over soleus (sol), l.gas, semomembranosus (semi), biceps femoris (bf), vm, vl, rf and glutes maximus (glut) muscles according to recommendations for the surface EMG of non-invasive assessment of muscle [16] and secured using flexible adhesive tape (Fig 1). Electrodes were placed on the dominant leg—which was determined as the opposite one to the dominant leg when kicking a ball—in vertical jumping. Ground reaction F and vertical position data were simultaneously acquired using a USB Data Acquisition System (synchronized with Delsys Trigger Module and triggered by Kistler MARS software).

Download:

Fig 1. Testing setup.

The flywheel (FW) exercise device utilized the inertia of a spinning FW (A) to produce resistance. The FW standing platform (F) with plates (B) size was 1.1 x 0.6 m, rotary shaft diameter was 0.03 m and pulling rope diameter was 0.006 m. A harness (C) was used to aid in performing FW and isometric squats. A draw-wire sensor was installed under the device. The wire originated directly above the center of the axis—to avoid diagonal vertical displacement (D). The distal part of the wire was attached to the harness rope attachment (between legs) (G).

https://doi.org/10.1371/journal.pone.0243090.g001

Following warm-up, MVC repetitions were performed for the purpose of EMG normalization. Three repetitions (5 s) of maximal isometric exertion against external resistance were performed for each movement: (i) harness squat on FW device in a 90° knee and hip position [17,18] for vm, vl, rf, (ii) good morning deadlift for semi, bf and glut, and (iii) 90° ankle plantar flexion in an upright standing position with fixed pelvis and shoulders for sol and l.gas. Rest periods between repetitions were 60 s and 5 min between the MVC tasks. The participant’s knee and hip angle during normalization was determined with a long arm steel analog goniometer (Saehan Co., Masan, Korea), centered at the lateral epicondyle of the knee or greater trochanter. Loud verbal encouragement by the examiner was provided during all MVC trials.

Thereafter, a total of 15 sets of FW squats were performed. FW loads were applied in counter-balanced random order among the subjects to avoid any systematic inter-load effect. Participants performed 5 sets of 7 repetitions with each of the three loads. The testing protocol was intentionally divided onto sets to reduce the bias of the EMG variables due to fatigue response. The first two repetitions (excluded from data analysis) were intended for FW acceleration and squat amplitude stabilization. The following 5 repetitions were executed with maximal effort and analyzed post-hoc. While the intra-set concentric power output is influenced by the flywheel load used, [19] only 5 repetitions were selected to maintain a high power output—regardless of the load. Participants performed the squat movement from the lower (90° knee angle) position to the full extension of the knees (0° knee angle). Arms were crossed with hands on the opposite shoulders and ankle plantar flexion was not allowed. The participants were instructed to perform the concentric phase as fast as possible while delaying the braking action in the first third of the eccentric phase. Loud verbal encouragement was given to the participants during all testing sessions. To standardize the range of motion, squat amplitude was monitored (real-time feedback from draw-wire sensor on a computer monitor in front of the subject). Moreover, squatting technique (hip and knee flexion angles) was carefully controlled by an experienced researcher. There was 60 s break between sets (same load) and 5 min break between different loads. A numerical rating scale (1–10) [20] in the middle of the rest period was used to record fatigue responses (higher scores indicate more severe fatigue perception).

### Data analysis

Vertical position and EMG activity data were simultaneously collected during FW squats, while ground reaction F was collected only during MVC measurements. Data was sampled at a frequency of 1,000 Hz. Position and F data were filtered using a moving average filter with 50-ms window, while the EMG data was, firstly, bandpass filtered using Butterworth second-order filter (20–500 Hz) and, secondly, rectified using root mean square (RMS) function (100 ms window length). Raw and processed EMG signals for each representative subject are presented in the Fig 2.

Download:

Fig 2. Representation of typical raw and processed vertical position and EMG signal.

Data are presented for 12 consecutive squat repetitions at the 0.225 kg∙m2 load. The first row represents raw (left) and processed (right) position data. In rows 2–8 raw (left) and processed (right) EMG signals for eight muscles are presented. Repetitions were determined from position data cycles, starting at the highest (approximately 0° knee angle) going through the lowest (approximately 90° knee angle) position and stopping at the highest vertical position. Position data for 12 consecutive repetitions was later time-domain normalized and superimposed (first row, right column). EMG data were firstly filtered and then rectified using root mean square (RMS) function (100 ms) and expressed as a percentage of peak EMG activity during MVC trials (%MVC). Average values (solid line) and standard deviations (grey area) for 12 consecutive time-normalized and superimposed traces are presented in the right column. The concentric area represents the propulsive (concentric) movement and the eccentric area represents braking (eccentric) movement while executing the squat.

https://doi.org/10.1371/journal.pone.0243090.g002

The main outcome variables for the concentric and eccentric phase of each repetition were: (a) peak EMG activity (maximal EMGRMS on the 10% moving window average from position-time data), (b) mean EMG activity (mean EMGRMS from position-time data), and (c) three position-specific variables; mean EMG activity in the first (1./3mean), second (2./3mean) and third (3./3mean) part of the vertical displacement length during the squat derived from the position-time data. The 1./3 corresponds to approximately 9–27°, the 2./3 to 36–54° and the 3./3 to the 63–81° knee flexion angle. Variables were expressed as percentage of peak EMG activity during the MVC trials (%MVC) (calculated as peak value of MVCRMS on a 1 s time window for the peak isometric ground reaction F produced).

### Statistical analysis

The obtained averaged outcome variables are reported as means ± standard deviations. Typical error (TE = SDdiff/√2), coefficient of variation (CV = 100 ∙ (eRMSE/100–1) ≈ 100 ∙ RMSE; RMSE, Square root of the mean square error in the repeated measures ANOVA output) and intraclass correlation coefficient (ICC) were calculated according to [21] and Koo and Li (2016) [22]. ICC values were interpreted according to recent guidelines (< 0.5: poor reliability, 0.5–0.75: moderate reliability, 0.75–0.9: good reliability, and > 0.90): excellent reliability. At the primary level of the analysis, 5 sets of 5 “all out” repetitions were merged and intra-session reliability was calculated between the 25 consecutive repetitions, progressively until all the repetitions were averaged. Values of ICC2.k > 0.95 were considered trustworthy and were included in further analyses. Inter-set reliability was calculated at the secondary level. Twenty-five consecutive repetitions were split into halves and the reliability components (TE, CV, ICC2.1 with 95% confidence interval and bias) between the means of the first twelve repetitions in each half were then calculated. The systematic bias between sets was analysed using paired samples t-test. Differences in fatigue scores between loading conditions were tested for statistical significance using one-way repeated measures ANOVA. The assumptions for normality were confirmed using Shapiro-Wilk test and sphericity using Mauchly’s test. Level of significance was set at p < 0.05.

### Results

On average, the fatigue statistics scores significantly increased from 4.48 ± 1.96 after the first loading condition, to 5.04 ± 1.77 after the second and 5.52 ± 1.73 after the third loading condition, F(2, 48) = 6.804, p < 0.05.

At the primary level, the results showed increasing reliability (ICC2.k) with the higher number of averaged repetitions for all EMGRMS variables (Fig 3). Table 2 represents the minimum number of consecutive repetitions to meet the trustworthy criteria. An overall average of 12 consecutive repetitions showed to be the cut-off value for trustworthy (ICC2.k > 0.95) reliability of peak and mean EMGRMS for all muscles in the concentric and eccentric phase of the squat with the exception of the glut muscle. Moreover, 89% of position-specific variables (1./3mean, 2./3mean, 3./3mean) meet the trustworthy criteria (ICC2.k > 0.95) when averaging 12 consecutive repetitions. Due to the heterogeneity of the results and total quantity of data, position-specific variables were excluded from further analyses.

Download:

Fig 3. The number of averaged repetitions to assure ICC2.k > 0.95 (dashed horizontal line) for four representative muscles in the concentric and eccentric parts of the squat.

The dashed vertical line represents the post-hoc determined cut-off value for the number of consecutively averaged repetitions to meet the reliability criteria for peak and mean EMGRMS outcome variables.

https://doi.org/10.1371/journal.pone.0243090.g003

Inter-set reliability components from the secondary level of the analysis are presented in Table 3. On average, we found comparable inter-set reliability for peak and mean EMGRMS variables, regardless of the FW load. The muscle activation variables of the eccentric phase of the squat provided us with lower ICC2.1 reliability compared to the concentric phase. ICC values ranged from 0.57 (rf mean EMGRMS at load 0.05 kg∙m2) to 0.99 (glut peak EMGRMS at load 0.05 kg∙m2) for the concentric phase and from 0.49 (glut peak EMGRMS at load 0.225 kg∙m2) to 0.96 (glut peak EMGRMS at load 0.05 kg∙m2) for the eccentric phase related variables. Systematic inter-set bias was found in 23% of the concentric and eccentric phase variables.

### Discussion

The main aim of the study was to define the minimum number of consecutive repetitions that need to be averaged to obtain reliable intra-session EMG variables and, consequently, to asses inter-set reliability of the defined variables. At the primary level of the analysis, we confirmed our first hypothesis with the finding that a minimum of 12 consecutive repetitions should be averaged to obtain trustworthy intra-session EMG outcome variables (ICC > 0.95), excluding position-specific variables due to heterogeneity of the results. Trustworthy intra-session variables provided us with good to excellent inter-set reliability, regardless of muscle, FW load or type of contraction (concentric vs. eccentric). Therefore, we confirmed our secondary level hypothesis. According to the findings, it can be suggested that the minimum number of repetitions that should be averaged in one set is 12 to ensure trustworthy intra-session reliability of the peak and mean EMG variables. To ensure that influence of fatigue is excluded from the testing results, we suggest performing two sets of six repetitions at a certain load to achieve the suggested number of intra-set repetitions.

In the FW resistance exercise, P and F vary depending on the tempo of execution, which may highlight the imprecision of prescribing FW loading and reflect the lack of reliability in performance testing. We observed that 12 consecutively averaged repetitions represented the cut-off value that ensures trustworthy reliability of the EMG variables among all three FW loads used, when excluding position-specific variables and glut muscle from the first phase of the analysis. A conclusion of trustworthiness (ICC2.k > 0.95) was made due to the possible influence of inter-individual variability on the magnitude of ICCs [23]. Due to the high heterogeneity of subjects (high CV), a large ICC can be obtained even when consistency is poor [24]. Moreover, when analysing specific muscles (e.g. only vl), less than 12 repetitions are adequate to meet the trustworthy intra-session criteria—with the help of the Table 3. Position-specific variables showed lower reliability when averaging several consecutive repetitions and higher result variations. When processing position-specific EMG signals—in respect of different muscles—from 2 to 25 repetitions should be averaged and, consequently, the results should be interpreted with caution.

The main advantage of our study is the quantity of valuable data collected using valid modern technology, i.e. force plates, draw-wire linear positional sensor and 8-channel wireless EMG system. Moreover, direct transfer rope-FW offers basic FW resistance exercise conditions, enabling easily controllable exercise intensities. Although we used a custom-made FW device with three FW loading conditions, we do not see a functional divergence to the commercially available devices that are frequently used for this sort of training. The results of our study are reproducible for simultaneous measurements of vertical displacement and muscles EMG activity. Some commercially available devices enable calculation of mechanical variables (i.e. vertical displacement) from axis rotation data alone. In such cases, researchers should be cautious about the following characteristics of the FW devices, as they can affect the fundamental metric characteristics: strap/rope winding around the axis, direct/pulley mechanism rope to axis transfer and cylinder/cone shaped axis. In terms of fatigue rating, although the scores increased from the first to the last FW load, fatigue influence should be equally distributed between different loads as these were executed in a different random order for each participant.

There were several limitations with the testing procedure that should be noted. At the transition from the eccentric to the concentric phase of the squat, we observed a certain decrease in the participant’s balance and therefore inter-participant variability. Unsteadiness can potentially affect squatting performance, especially using high FW loads, although we have done our best to ensure maximum squat execution among all FW loads. On some occasions, FW harness discomfort could also have influenced squatting performance. Sabido et al (2018) [19] emphasised the importance of the familiarization process, showing that the participants’ experience plays an important role in some variables, such as peak P output and eccentric overload. As yet, we lack information about EMG variables concerning the familiarization process. Familiarization in our study was shorter than suggested [19]. Nevertheless, we found good to excellent inter-set reliability using each of the three FW loads. We believe that the consistency of the muscle activation results reflects the highly-strength-trained participants and of the equipment. The direct transfer rope-FW shaft used offers better, more fluent movement feeling, and consequently better squat depth control. Based on these findings, stabilization, comfort requirements, familiarization procedures and consequently inter-visit reliability should be taken into account and explored further.

In the present study, we only concentrated on the inter-set reliability of the peak and mean EMGRMS variables due to the large dataset involved. It should be noted that the main findings of the study are also applicable when analysing position-specific variables, especially when exploring the neuromechanical principles responsible for adaptations in FW resistance training. It has been found that training adaptations relating to the depth of a squat differently influences adaptations in strength, sprinting and jumping abilities [25].

Similar to pedalling motion [26], we found that consecutive FW squat repetitions result in onsets and offsets of the main burst of EMG activity. We believe such bursts are consequences of mechanical restraints of FW loading conditions and are therefore vertical displacement dependent. In future research, the range of the active phase should be defined (duration between the onset and the offset of the muscle activity), which should also positively influence result reliability, especially with respect to position-specific results. In addition, we suggest analysing the EMG amplitude to F ratio while following specific training adaptations [27]. With additional research, it is possible that the linear slope coefficient of the EMG amplitude to the squat vertical ground reaction F spectrum may be useful for examining neural vs. hypertrophic adaptations to strength training [28] in a specific—i.e. FW—conditions.

By using reliability data as the decision-making criteria in this process, the testing protocol has likely been optimised. The results should contribute to the optimization of EMG measurements using FW squat devices and therefore help research practitioners to obtain confident results. According to the findings, it can be suggested that the minimum number of repetitions that should be averaged to ensure trustworthy intra-session reliability of EMG variables is 12. Moreover, our data demonstrates that 12 consecutive averaged squat repetitions in a single set achieves good to excellent inter-set reliability of the EMG variables. The results are expected to lead the standardization of a methodology for quick and less prone to fatigue assessing EMG activity of leg muscles using FW squats. Taking these results into account, activation of leg muscles can be confidently studied in intra-session repeated-measures study designs. In addition, researchers should be aware of their FW device’s characteristics to obtain the most relevant EMG results.

### Acknowledgments

The authors would like to thank all the participants for their effort during the study.

### References

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26. 26. Fonda B, Panjan A, Markovic G, Sarabon N. Adjusted saddle position counteracts the modified muscle activation patterns during uphill cycling. J Electromyogr Kinesiol. 2011;21: 854–860. pmid:21684759
27. 27. Moritani T, DeVries H. Neural Factors Versus Hypertrophy. Am J Phys Med. 1979;58: 115–130. pmid:453338
28. 28. Michael L, Stock M, Chappell A. Electromyographic Amplitude vs. Concentric and Eccentric Squat Force Relationships for Monoarticular and Biarticular Thigh Muscles. J Strength Cond Res. 2014;28: 328–338. pmid:23897014

### Subject Areas ?For more information about PLOS Subject Areas, click here.We want your feedback.Do these Subject Areas make sense for this article? Click the target next to the incorrect Subject Area and let us know. Thanks for your help!

• Electromyography
• Legs
• Knees
• Material fatigue
• Muscle analysis
• Bandpass filters
• Butterworth filters
• Musculoskeletal mechanics
Sours: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0243090

## The Flywheel Box - The Best Piece of Training Equipment You've Never Heard Of

### Takeaway Points:

• Recently, I discovered the flywheel box, a lesser-known piece of equipment that relies on the rotation of a flywheel to generate resistance.

• This piece of equipment is uniquely useful for some exercises due to its small footprint, making it incredibly useful at home.

• I cover some of the adaptations, tips, and tricks needed to get the most use out of this unique device.

Given that I live in London these days, it’s been very hard to assemble a home gym. While I’ve written before about how assembling a home gym can be a lot easier, cheaper, and better value than you might think, I’ve still found that it’s hard to assemble the kind of home gym I’d need to train effectively, where I live.

Since space is at a premium in a big city like London, we’ve had to make do with living in a lot less space than we were previously used to. Even when we moved to a bigger house (with a proper back garden) early this year, we don’t really have a ton of free space.

When lockdown started, I had to make the hard decision not to go into my local gym anymore. While I had plenty of home workout equipment to facilitate bodyweight and lightly-weighted workouts (including a pair of 50lb quick change dumbbells), my big problem was my lack of ability to get a good squat rack. Without a good rack, bar, and plates, I was going to have a hard time getting in the heavy workouts I was used to. As a result, I had to pivot mostly to relying on a very different kind of training. I leaned out in order to learn new calisthenics and gymnastics movements I had never had the training time to focus on before.

However, I still missed the barbell quite a lot, especially when it came to certain movements. While bodyweight training methods are generally great at training the upper body and core, and they can be exciting when training the lower body for a while, the naturally greater strength of your lower body means that soon you end up either wanting to add more weight, or having to focus on doing very endurance-heavy lower body workouts that get long and boring. I got as creative as I could to keep workouts interesting, but even then it started to get boring after a while.

Unfortunately, my options for acquiring a rack are limited. Without a car, picking up used equipment is more difficult. Of the options which deliver, most suppliers have been entirely sold out of many common items since the beginning of the pandemic, and the options which remain are all either prohibitively expensive or of inferior quality for my needs.

This is all on top of the fact that here at home the only place a rack would fit is our back garden, where it would quickly rust in the winter rain. It would also take up a significant amount of our back garden, leaving little space for everything else. We’re renting, so it seemed silly to buy an expensive piece of equipment at artificially inflated prices, only to have to sell it or tear it down in the case that we need to move. So ultimately, we decided that it’s probably not worth it, as much as it pained me to admit.

### Recently, I discovered an entirely different option - flywheel training - thanks to my buddies at the Stronger By Science Podcast.

Flywheel training works on a principle similar to the way that a rowing machine works. The device resembles a typical step box, but with an attached wheel somewhere on the device, and a cable attached to the wheel. By pulling on the cable, the wheel spins, and this spinning generates significant resistance - and similar to the way that a rowing machine works, it generates more resistance the harder and faster the pull. This means that even a relatively light amount of weight on the flywheel itself can generate a lot of resistance if the exerciser pulls on it with enough force.

Flywheel boxes are typically most useful for lower body exercises, which can be performed by means of harnesses or handles attached to the cable. Exercises which can be performed on the flywheel box are similar to those which can be performed on a cable machine with the cable set very low to the ground - the only difference is that the flywheel box is held to the floor only by your own bodyweight, so at least some amount of your weight has to be centered on the box to prevent it from moving around during the lift. I’ve also found that using my 50lb dumbbells to hold the box down works decently well in some situations and enables me to do a few other exercises I wouldn’t be able to do normally.

Flywheel boxes aren’t cheap - most of them are about as costly as a full squat rack setup, though I managed to find a budget brand which makes them a bit more affordable, and I already had the money saved for a squat rack anyway. Due to the wide range of exercises the box enables, I find that it was well worth the cost.

Another huge benefit of flywheel training is that it can pack a pretty hard workout into a rather budget-sized amount of space. My flywheel box has the footprint of a standard step aerobics step, meaning that it fits very nicely into my office alongside my other home gym workout equipment. Aside from the resistance plates, it doesn’t weigh very much and is easily portable in the case of a move.

I find that there are a lot of exercises that it does very well - belt squats, deadlifts, and romanian deadlifts are the most exciting, but I can also do strongly weighted split squats, lunges, bicep curls, rows, and upright rows - all movements which I’m excited to do more of, and have been missing during quarantine.

I also find that the box makes it very easy and tempting to get in very challenging workouts around my daily schedule - if I’ve got a bit of downtime during work while waiting for a program on my computer to load, or I need to step back and muddle over something without staring at the computer for a bit, or I need to move a bit to keep myself from getting too stiff, it’s super easy to step onto the box and get in a set or two.

As a very strong lifter, I’m used to a lot of resistance. One of the biggest worries with a piece of equipment like this would naturally be whether or not it’s capable of putting up enough resistance to satisfy even the stronger lifters out there. Luckily, I find that my unit is capable of putting up more than enough to really knock me out quickly. My box comes with the option to add up to three weight plates of varying sizes, and just 2 of the largest plates is enough to really destroy me on squats and deadlifts, where I used to regularly squat 300+lbs and deadlift 400+.

### It’s also challenging in a very different way than typical weight.

All exercises are sorted into three types of movement phases - concentric (lifting the weight), eccentric (lowering the weight) and isometric (any time the weight is not moving, whether this is because it’s reached the top or bottom of the motion, or because the lifter is pushing as hard as they can but this isn’t sufficient to move the bar). Most of the time what we think of as “lifting”, is primarily the concentric (raising) phase of the movement, though in most cases, lifters know that it’s a good idea to slow down the eccentric (lowering) phase of the movement in order to be able to properly control the bar. Most lifts are naturally comprised of all three types of movement to varying degrees, so we don’t normally think about these different phases of the lift too much.

Eccentric training is known to generate a disproportionate amount of soreness and muscle damage compared to concentric training. Some lifts contain little to no eccentric phase, and thus generate a smaller than normal amount of soreness as a result. Many lifts can be purposefully made to emphasize the eccentric phase, causing it to generate additional soreness and muscle damage.

In the past, it was believed that soreness is a direct measure of the quality of a workout, and thus that eccentric-focused training might be superior in terms of producing additional gains. However, this has largely been proven false - eccentric training is inferior when it comes to improving concentric strength (which most people care about a lot more), and is not necessarily any better than traditional concentric training when it comes to building muscle mass. However, eccentric training does potentially have some other unique benefits, including protection against certain kinds of injuries.

With my flywheel device, the wheel spins in one direction as a result of the concentric phase of the lift, and then you must forcibly resist the momentum (and reverse it) on the way down. As a result, the training is much more eccentric-focused than traditional training, and is another reason why it packs such a punch.

I find that flywheel training pairs very well with a more autoregulated kind of training. Because the resistance of the wheel depends on the speed of the lift, you find that you spend a couple reps getting into the groove of the lift, then there’s a few fast, hard reps which are wonderfully tailored to your current energy levels, and then you start to get tired out and pull slower, and as a result the wheel produces less resistance, until your reps get pitifully slow as you’re just exhausted. And this all happens relatively quickly - a set of 10 is often enough to knock you out with more strength-focused lower body lifts, and 15 is often enough with slightly more endurance-focused upper body lifts.

This means that training can be really simple and quick. 3x10-15 per lift, with the intent to move the wheel as quickly as possible, is more than enough. Over time, the lift will automatically get harder as you get faster and stronger, so you don’t even have to worry about changing the weight very often. Eventually if you get strong enough, you may find that you’re hitting the limits of speed and you can’t meaningfully move any faster, so you would have to add additional resistance on the flywheel, but this would occur a lot less frequently than with typical barbell/dumbbell loading.

### I do have some issues with the system I’m using, and it’s not perfect.

Because I bought a more budget option, my flywheel box uses woven belts for the pull system, and these belts rub on the metal sides of the device, causing them to fray and need to be replaced over time. There’s a rubber guard on the metal sides, but it’s not actually anchored to anything, so I found that in practice it was constantly popping off the instant the belt rubbed on it. I purchased some super glue to properly anchor the rubber guard to the device, and that’s helped minimize excess wear on the belt. Of course, the belts are replaceable as well, but it still feels mildly irritating - I’d guess I’ll have to replace the belt periodically with regular use.

The device is also initially very low to the ground - using it on the carpet in my office caused the wheel to rub fiercely on the carpet while rotating, damaging the carpet and causing a ton of noise. The company sells additional feet which raise the device by a couple more inches, which solved the problem but probably should have come standard rather than being sold separately for an extra \$125. While I ordered the feet and waited for them to arrive, I placed a sheet of cardboard between the box and the ground so that the cardboard got torn up instead of my carpet. I could also imagine that it wouldn’t be too hard to craft some wooden blocks or other homemade feet and save the cash, though my options on that front were limited.

I also find that the device takes a bit of time to get used to. Because the resistance starts rather abruptly during the eccentric phase, you can sort of “stutter” a bit awkwardly during the top of the lift as you adjust to the sudden pull of the resistance. You also need to constantly adjust the length of the pull belt so that it’s just long enough for the lift you’re currently doing - too short, and it will abruptly jerk you down before you hit full range of motion - too long, and it will awkwardly go slack at the top of the lift before abruptly jerking you down right when you aren’t expecting it. However, if you get a good feel for the right belt lengths for each lift, this becomes more natural and less of an issue. I initially found some of the movement very awkward, but have since gotten much more used to them and am able to push hard without losing control over the movements.

Flywheel training is something that I’d never heard of until just recently, so I’m really pleasantly surprised by how useful it’s been and how well it’s complimented my existing equipment set. Since I’ve been following a gymnastics program for the last couple months, the flywheel box has perfectly complemented that training with more difficult lower body work. It fits perfectly in with my training needs, and has made training crushing again in a fun way that I’ve missed out on since going to the gym.

### This was the one thing my training routine was missing - now, I don’t mind at all that I’m working out from home.

If you’re interested in picking up your own, I can recommend the strexbox brand that I have - they’re on the cheaper end for flywheel boxes, they work well for home use, and they’ve got a sale on their basic model at the time of writing.

### About Adam Fisher

Adam is an experienced fitness coach and blogger who's been blogging and coaching since 2012, and lifting since 2006. He's written for numerous major health publications, including Personal Trainer Development Center, T-Nation, Bodybuilding.com, Fitocracy, and Juggernaut Training Systems.

During that time he has coached hundreds of individuals of all levels of fitness, including competitive powerlifters and older exercisers regaining the strength to walk up a flight of stairs. His own training revolves around bodybuilding and powerlifting, in which he’s competed.

Adam writes about fitness, health, science, philosophy, personal finance, self-improvement, productivity, the good life, and everything else that interests him. When he's not writing or lifting, he's usually hanging out with his cats or feeding his video game addiction.

Follow Adam on Facebook or Twitter, or subscribe to our mailing list, if you liked this post and want to say hello!

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Sours: https://www.gains.af/blog/flywheel-training
Kabuki Strength Kratos Flywheel Review: Dual-Axis Home Gym Training?!

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Sours: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7277616/

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