Does Flywheel Paradigm Training Improve Muscle Volume and Force? A Meta-Analysis : The Journal of Strength & Conditioning Research

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Brief Review

Does Flywheel Paradigm Training Improve Muscle Volume and Force? A Meta-Analysis

Nuñez Sanchez, Francisco J.; Sáez de Villarreal, Eduardo

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Journal of Strength and Conditioning Research: November 2017 - Volume 31 - Issue 11 - p 3177-3186
doi: 10.1519/JSC.0000000000002095
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Abstract

Introduction

Resistance training produces muscular adaptations at multiple structural and functional levels (1,28,41,44). Numerous studies have demonstrated that chronic resistance exercise can produce neural changes in the first several weeks and increase hypertrophy (after 4–8 weeks) and strength within the first 4 weeks of training (54,57). Different studies conclude that a training protocol that utilizes on the concentric (CON) or only the eccentric (ECC) phase of the movement increases both muscle mass and strength (13,28,31,55), and the increases are magnified when combining CON and ECC actions (26,28,30). Considerable controversy is noted regarding the muscle load during the CON and ECC movement given that muscle fibers generate greater force during an ECC action compared with a CON action (14,19,24,32,33,58,62,63). Some authors argue that training protocols where exercise is overloaded during the ECC phase of movement achieve greater strength gains than those movements in which the load is constant during the CON-ECC cycle (8,17,29), giving rise to the concept of “eccentric overload” (EO).

Different training systems have been designed to use the inertia of rotating flywheel(s) to provide maximal resistance load during CON-ECC movements favouring EO (Flywheel Paradigm). Berg and Tesch (6,7) introduced YoYo Technology (Stockholm, Sweden). YoYo Technology generates resistance by opposing the trainee's effort with the inertial force generated by a lightweight rotating flywheel such that the same inertia must be overcome during each repetition by means of accommodated loading (45). Training loads on the YoYo Technology can be regulated by increasing the speed of movement or by adding flywheels weights. There are several types of YoYo devices that use the inertia of flywheel(s) to provide resistance load, including YoYo Squat and YoYo MultiGym (7), YoYo Leg Curl (6), YoYo Knee Extension (59), and other devices (2). The Inertial Training and Measurement System (ITMS) is another system designed and constructed by an inter-university group from the Faculty of Physical Culture in Gorzow Wielkopolski (department of the University School of Physical Education in Poznan) and the Faculty of Mechanics University of Zielona Gora (42). This device comprises a steel frame attached to the ground, encompassing an inertial wheel (flywheel) with a 506-mm radius. A rope is mounted on the flywheel circumference. Training loads on the ITMS are regulated by increasing the speed of movement or by adding weights (43). With ITMS, exercises can be performed in multiple planes of motion. Versapulley (VP, Costa Mesa, CA, USA) provides a source of linear resistance from a tether wrapped around a vertical cone-shaped shaft (12). The tether winds around the cone. The concentric action unwinds the tether, and the eccentric action occurs during rewinding. Training loads on the VP can be regulated by increasing the speed of movement or by adding weights. With VP, exercises can also be performed in multiple planes of motion. In all systems, the kinetic energy from the concentric portion of the exercise is transferred to the eccentric portion, and an equal eccentric impulse is necessary to halt the rotation of the disc. Given that impulse is a function of both force and time, a greater amount of eccentric average force can be induced by performing the eccentric portion in less time than the concentric portion (45,46). When this kinetic energy is decelerated in a restricted portion of the ECC action, the force exceeding that generated during the corresponding CON phase must produce ECC overload (45).

Several studies have confirmed the efficacy of flywheel training for improving or benefiting muscle volume (2,21,43,45,56,59), force (15,42,43,45,46,48,56,59), and EMG activity (42,43,46,56). The flywheel paradigm also has been used as an aid in the treatment and prevention of tendon injuries (24,52) and muscle injuries (4,15). Most studies compared this methodology with traditional training (16,29,46,48). The results mixed acute (10,21,22,40,47,61) and chronic (16,21,24,42,43,45,46,48,56,59) training effects. Training protocols are applied to subjects between 17 and 71 years of age, and an equivalent level of physical activity was developed: sedentary, athletically active, trained subjects, injured subjects or subjects who suffered a muscular atrophy produced by weightlessness. Although many studies are available, the beneficial effect of flywheel training is not clear. Therefore, the purpose of this systematic review and meta-analysis was to determine if chronic training using a flywheel paradigm with healthy people increase muscle volumen and forcé.

Methods

Experimental Approach to the Problem

In this investigation a meta-analysis of 13 studies with a total of 18 effect sizes was performed to analyse the role of various factors on the effectiveness of flywheel paradigm training to increases muscle volume and force. The following inclusion criteria were employed for the analysis: (a) randomized studies; (b) high validity and reliability instruments; (c) published in a high quality peer-reviewed journal; (d) healthy participants; (e) studies where the eccentric programme were described; and (f) studies where increases in muscle volume and force were measured before and after training. To determine the effects of the categorical independent variables on muscle volume and force effect sizes (ES), an analysis of variance (ANOVA) was used. In the case of quantitative independent variables a Pearson's (r) correlation test was used to examine the relationships between muscle volume and force ESs and variable values.

Procedures

To evaluate the effectiveness of flywheel eccentric training for increases in muscle volume and force (N), a meta-analysis was conducted. Literature searches were conducted electronically to identify investigations that examined these topics. The research assessed ADONIS, ERIC, SPORTSDiscus, EBSCOhost, Google Scholar, MedLine and PubMed electronic databases between October and November 2015 and was updated in March 2016. Moreover, manual searches were performed in sport science-relevant journals. The references of identified articles were examined to identify additional studies that were eligible for the review. The search included all studies published in English and studies in any language for which the abstract was available in English. Key words used included eccentric, eccentric overload, flywheel, resistance training, and inertia. No age or gender restrictions were imposed during the search stage.

For the selection of studies to further review, we performed 3 steps: (a) the article titles were read, (b) the abstracts were read, and (c) the entire articles were read. In this review, only human studies and full primary research papers (i.e., not a conference abstract, letter to editor, thesis or review) were eligible for inclusion.

Studies were included if they met the following criteria based on recommendations by Campbell and Stanley (9): (a) randomized studies, (b) high validity and reliability instruments, (c) published in a high quality peer-review journal, (d) healthy participants, (e) studies where the eccentric programme was described, and (f) studies where muscle volume and force (Table 1), were measured before and after training. Following this search process, 13 articles were included in the analysis (Figure 1).

T1
Table 1.:
Summary of characteristics of all studies meeting the inclusion criteria.*†
F1
Figure 1.:
Flow of study selection.

Each article was read and coded by 2 investigators for the following variables: (a) descriptive information (age, body mass, and height), previous experience with eccentric exercises (familiarized and not familiarized), physical activity (trained, physically active and sedentary), and gender (male, female and both); (b) programme exercises: type of exercise (knee extension, squat, leg press, leg curl, shoulder adduction, front step, and combined), eccentric overload, and type of work (unilateral or bilateral); (c) programme variables: frequency of weekly sessions, programme duration, total number of sessions, number of sets per day, number of repetitions per day and rest intervals; and (d) outcome measurements: the type of test used to identify performance gains (e.g., load cells, magnetic resonance imaging MRI, dual X-ray absorptiometry DEXA, and Bioimpedance). The mean agreement was calculated using an intraclass correlation coefficient (ICC). The coding agreement between investigators was determined by dividing the variables coded by the total number of variables. A mean agreement of 0.90 is accepted as an appropriate level of reliability for such coding procedures (27). Any coding differences between investigators were scrutinized and resolved a priori to the analysis.

Gain effect size (ES) was calculated using Hedges and Olkin's g using the formula (1): g = (MpostMpre)/SDpooled, where Mpost is the mean for the post test, Mpre is the mean for the pretest, and SDpooled is the pooled SD of the measurements (2):

The ES is a standardized value that permits the determination of the magnitude of the differences between the groups or experimental conditions. It has been suggested (50) that ES should be corrected for the magnitude of the sample size of each study. Therefore, correction was performed using the formula (3): 1−3/(4m−9), where m = n−1, as proposed by Hedges and Olkin (27).

Statistical Analyses

To determine the effects of the categorical independent variables (previous experience, fitness activity, gender, and programme exercises [type of exercise, eccentric overload, type of work and training sessions]) on muscle volume and force effect sizes (ES), an analysis of variance (ANOVA) was used. In the case of quantitative independent variables (e.g., age, duration of the treatment in weeks, number of total sets, number of sets per day, number of repetition per day and rest intervals) a Pearson's (r) correlation test was used to examine the relationships between muscle volume and force ESs and variable values. The following criteria were adopted to interpret the magnitude of the correlation (r) between the different measures: ≤0.1, trivial; >0.1–0.3, small; >0.3–0.5, moderate; >0.5–0.7, large; >0.7–0.9, very large; and >0.9–1.0, almost perfect (Hopkins et al. 2009). The α level was set at p ≤ 0.05 for statistical significance. In addition, data were also assessed for clinical significance using an approach based on the magnitudes of change. Threshold values for assessing magnitudes of ES were <0.35, 0.35–0.80, 0.80–1.50 and >2.0 for trivial, small, moderate and large, respectively (50).

Results

The analysis showed that the average ES of the experimental group (0.68; n = 18) was significantly increased (p ≤ 0.05) compared with the ES of controls (0.32; n = 2).

Muscle Volume

The analysis of muscle volume showed that the average ES the experimental group (0.75; n = 7) was significantly higher (p ≤ 0.05) compared with the ES of controls (0.39; n = 1). Regarding the subject's characteristics, the results indicate a significant correlation for age (r = −0.771, p = 0.042) with the magnitude of the ES, and no significant correlation for body mass (r = −0.229) or height (r = 0.155) with the magnitude of the ES (Table 2). ANOVA results revealed significant effects for some of the variables measured (i.e., previous experience, p = 0.014). In addition, significant relationships (p ≤ 0.05) were noted between the frequency of weekly sessions (FWS) (r = 0.752) and total sessions (TS) (r = 0.797) with muscle volume ES (Table 3).

T2
Table 2.:
Muscle volume.*†§
T3
Table 3.:
Muscle volume.*†

Force (N)

The analysis of force revealed that the average ES in the experimental group (0.6; n = 11) was significantly increased (p ≤ 0.05) compared with the ES of controls (0.28; n = 1). Regarding subject characteristics, no significant correlation for age (r = 0.282), body mass (r = −0.269) or height (r = −0.020) with the magnitude of the ES was noted (Table 4). ANOVA results revealed significant effects for some of the variables measured (i.e., gender, p = 0.038; eccentric overload, p = 0.001). In addition, a significant relationship (p ≤ 0.05) was noted between the number of repetitions per set (Nrs) (r = 0.604) with force ES (Table 5).

T4
Table 4.:
Force (N).*†
T5
Table 5.:
Force (N).*†

Discussion

The objective of this review was to establish the chronic effects of different training protocols for the flywheel paradigm in healthy people, i.e., muscle hypertrophy and strength capacity. The main findings from this review were as follows: (a) Increases in muscle mass and force were noted through the use of flywheel systems during short periods of training. (b) A previous familiarization period with the flywheel system facilitates the improvements in muscle volume. (c) The increase in muscle mass does not appear to be influenced by the existence of EO during the exercise. (d) The increase in force was significantly higher with the existence of EO during the exercise.

Several studies argue that eccentric contractions cause greater muscle damage (3,22,23,37,49), increased production of muscle fiber proteins (5,39), and therefore, greater muscle hypertrophy compared with concentric contractions (5,26,28,30,39,51). The muscle exhibits an increased capacity to generate force in the eccentric phase compared with concentric phase (14,19,32,33,34,58,62,63). However, studies that overload the eccentric phase with respect to the concentric phase reported small increments in hypertrophy (18,28) or no effects (13), suggesting that the use of EO per se may not sufficiently stimulate the generation of muscle mass (45). Related to our systematic review, some authors claim that flywheel paradigm training, which allows for brief episodes of enhanced force in eccentric over concentric actions, prompted increases in muscle size (20,45,56,59). However, in our analysis, we identified no differences between the existence of eccentric overload and an increase in muscle mass in the studies included in this review (Table 2). For participants performing knee extension exercises in an ITMS system without EO (3 times per week and 3 × 28−31 repetitions each day), Naczk et al. (43) reported increases of 9.8 (ES 0.85) to 15.1% (ES 0.83) in quadriceps muscle mass. These values were slightly higher than the values (6.8–6.6%, ES 1.5) reported by other studies using a knee extension exercise in the same age group (56), and in a group of older participants (45,59), performing an eccentric training with EO. These values are also greater than the thigh muscle mass increase reported by Fernandez-Gonzalo et al. (21) for both men (4.6%, ES 0.71) and women (5.4%, ES 0.67) training with an EO on a leg press YoYo system. Using a similar protocol in a YoYo system knee extension, Lundberg et al. (36) proposed that a leg undergoes exhaustive aerobic work 6 hours before training, whereas the other leg exclusively trains with the flywheel system. The group that performed aerobic work exhibited an increase in muscle mass very similar to that obtained without EO (14%, ES 0.62). This study demonstrates a large increase in hypertrophy of type I fibers, an increase in skeletal muscle aerobic capacity, and a loss of muscle strength. These findings suggest that this hypertrophy was influenced by aerobic work. However, Carmona et al. (10) showed that squat training performed in a Versapulley system obtained significant serum increases in fast myosin isoforms after 48 hours, the improvements were maintained after 144 hours, and the slow myosin isoform concentration in blood did not change. All these results could indicate that the higher increase in muscle mass without eccentric overload systems could be influenced by an increased number of repetitions per set. It is possible that the technique used to assess the increase in muscle mass might influence the results of this review; however, the analysis shows no significant difference. Despite these increases through the evaluation with bioelectrical impedance, the results obtained were two-fold enhanced compared with that obtained by MRI or DXA (Table 1).

Our analysis reveals a negative correlation between subject age and the size of the effect produced by increased muscle mass after training processes (Table 2). Although almost no study used a long familiarization period, the ANOVA results revealed significant effects for previous experience (Table 2). Seynnes et al. (56) proposed monitoring the evolution of increased muscle mass over 35 days of training with the flywheel system. During the first 10 days, an apparent increase in muscle mass was not generated, and an increase of 6.8% with a large effect sizes (ES 1.5) was attained 35 days after starting training. The increased muscle mass through flywheel paradigm systems during the 5-week training periods was not significantly different from that obtained with traditional systems with the same workload (45). Norrbrand et al. (45) reported two-fold greater quadriceps hypertrophy with training on YoYo knee extension system (6%, ES 0.38) compared with the weight stack system (3%, ES 0.11) without significant differences. Similar to our analysis, Tesch et al. (59) revealed a significant increase in quadriceps muscle mass with the flywheel system, whereas hypertrophy increased significantly in the rectus femoris exclusively with the weight stack system. Seynnes et al. (56) used the same protocol and reported an increase in muscle mass similar to Norrbrand et al. (45). A significant increase in the fascicular length of the vastus lateralis of the quadriceps was observed. These authors argued that training with EO in the flywheel system compared with traditional systems led to a faster increases of sarcomeres in series than in parallel. This finding could explain why no differences in the cross section of muscle are noted after short periods of training with both systems. It is interesting to evaluate the increase in muscle mass after long training periods. In fact, our analysis reveals a positive correlation with the number of sessions per week and the total number of sessions (Table 3). However, consulted studies measuring the increase in muscle mass do not exceed 6 weeks or 15 training sessions.

The training based on the flywheel paradigm produced a significant increase in the MVC over short periods of training (5 weeks), 2–3 days per week, with an average of 3–4 sets per day and a rest between sets of 2 minutes (Table 1). Dynamic training with flywheel paradigm improved the specific capacity to generate force during both the concentric and eccentric phases. Through our analysis, we can see how the protocols that did not generate eccentric overload exhibited significantly greater effect sizes than the protocols that generated EO. Thus, performing 15 training sessions of knee extension without EO in a ITMS system with and without an additional 10-kg load, Naczk et al. (43) achieved 25.2% (ES 0.93) and 23.3% (ES 0.77) increase in maximum force. Similarily, Naczk et al. (42) applies the same ITEMS system for shoulder abduction-adduction training in adult women during 12 sessions with and without 5-kg overload and achieved increases of 11.8% (ES 0.55) and 13.7% (ES 0.81) in maximum force, respectively. Most of the protocols using an eccentric overload achieved a small effect size (45,46,56,59). However, Seynnes et al. (56) achieved an increase of 38% MVC with experienced subjects, but did not provide the effect size. Possible explanations include the following: protocols potentially used an eccentric overload and inexperienced subjects, and the ability to produce an eccentric overload appears to require some experience or a process of familiarization with the flywheel paradigm system. Tous-Fajardo et al. (61) analysed the acute effect of a series of 6 repetitions with the YoYo Leg Curl system and observed that subjects, with experience in flywheel training, generated a higher peak force during the ECC phase than the CON phase (EO). Importantly, this effect did not occur in inexperienced subjects. The experience and inexperienced CON average force were higher than ECC average force. Moras and Vázquez-Guerrero (40) and Norrbrand et al. (47) assessed a squat exercise in the Versapulley system and a leg press in the YoYo multi gym, respectively, using subjects that were not familiarized with flywheel systems, where EO was not generated, and the average CON force was higher than ECC. Nevertheless, Askling et al. (4) trained experienced subjects for 10 weeks for 1–2 days per week with 4 × 8 repetitions on a YoYo Leg Curl system and noted increased peak eccentric (16%, ES 0.95) and concentric (13%, ES 0.95) torque. These authors argued that this increase produced a 67% reduction in the incidence of hamstring injuries. These results are consistent with those obtained by De Hoyo et al. (15) who reported a 65% (ES 0.48) reduction in injury severity and a significantly reduced incidence of match play per 1,000 hours when performing YoYo squat and YoYo leg curls for 10 weeks, 1–2 days per week, and 3–6 × 6 repetitions.

Training for 5 weeks of training with flywheel paradigm systems produces a greater increase in force than in muscle volume (42,43,45,56,59). This finding can be attributed to increased motor unit recruitment during exercise (25), increased synchronization of motor units (38,53) and an increase in EMG activity (41,46). We must emphasize that we could not include in this review an analysis of the variable “EMG activity” due to the small number of studies using the same “gold standard” for measuring this variable. However, conflicting results regarding the correlation of increased strength and root mean square EMG activity are noted that demonstrate a nonexclusive influence of neural processes in the increase of the maximum voluntary force. Thus, in a study of sedentary subjects that trained for 5 weeks on a YoYo knee extension, Tesch et al. (59) reported a 11–12% (ES 0.14) increase in MVC in both CON and ECC phases without modifying the EMG activity of the vastus lateralis, vastus medialis and rectus femoris of the quadriceps. Performing a similar training, Seynnes et al. (56) reported proportional increases in both the maximal voluntary contraction and the EMG activity of the vastus lateralis of the quadriceps during the entire training process, reaching increases of 38.9 and 34.8%, respectively, at the end of the training. Similarly, performing the same training in untrained subjects, Norrbrand et al. (46) achieved an 8.1% (ES 0.52) increase in maximal voluntary contraction and an increase in EMG activity of the vastus lateralis of the quadriceps, whereas the EMG activity of the vastus medialis during CON phase and all muscles during ECC phase did not change. One of the possible reasons for the non-linearity between MVC and EMG involves the differences in the type of fibre that composes the structure of the muscle evaluated (35,64), which was not analysed in this review. Another possible explanation involves previous experience in training with these systems. Studies on chronic and acute effects using flywheel systems showed that during the execution of the exercise, the EMG activities were similar than obtained during MVC in subjects without experience (2,47) and higher in subjects with experience (61) compared with those obtained during MVC. Onambele et al. (48) showed that EMG activity was 32% higher during the CON phase and 42% lower during the ECC phase in older subjects without prior experience using a YoYo knee extension compared with a traditional machine. Nevertheless, Norrbrand et al. (47) denote a trend (p ≤ 0.05) for increased EMG activity in the ECC phase of the rectus femoris in experienced subjects using a YoYo multi gym compared with using a barbell in half squat exercises.

The literature concludes that the use of a training system based on flywheel paradigm generates higher improvements in maximum voluntary force compared with traditional methodologies (45,48). Norrbrand et al. (45) performing a training with a YoYo knee extension system, reported an increase of 11.6% (ES 0.47) in the MVC at 90° knee-flexion, and they found no differences at 120° knee-flexion or in both angles using a weight stack system. Performing a similar training, Onambele et al. (48) reported an increase of 17% (ES 2.4) in the MVC, which was significantly higher than the 8% (ES 0.66) obtained with a traditional machine. Nevertheless, Norrbrand et al. (46) did not obtain significant differences between the flywheel and weight stack despite the two-fold increase in MVC with the YoYo knee extension system. Similarly, De Hoyo et al. (16) compared a front step exercise VP vs. half traditional squat for 6 weeks and reported that training under flywheel paradigm resulted in a 10% improvement in the MVC and a higher improvement in the effect size compared with the traditional system (ES 1.02 vs. 0.45); however, significant differences between both training methods were not achieved.

There are no studies that measure the MVC exclusively in women, but studies using a mixed sample (men and women) obtained a small effect size compared with studies in men that obtained a moderate effect size. Fernandez-Gonzalo, et al. (21) reported a 25% (ES 7.6) increase in 1 RM in men and a 20% (ES 3.6) increase in women without significant differences between sexes. Similarly, Caruso et al. (11) assessed the acute effects of the execution of twin exercises and soleus 3 × 10 reps. Men generated more EMG activity than women for soleus and lateral twin exercises, but medial calf activity only occurred in the first repetition. Regarding force, men have a greater ability to increase absolute and relative levels; however, more studies are needed to corroborate this assessment.

Practical Applications

The ability to produce an eccentric overload in the flywheel system appears to require some experience. A significant increases in muscle mass was noted through the use of flywheel systems during short periods of training. A previous familiarization period with the flywheel system facilitates the improvements obtained. The increase in muscle mass appears was not influenced by the existence of EO during the exercise. In addition, chronic training under flywheel paradigm systems is related to improve the force. The increase in force was significantly higher with the existence of EO during the exercise. The responses identified in this analysis are essential and should be considered by strength and conditioning professionals regarding the most appropriate dose response trends for flywheel paradigm systems to optimize the increase in muscle volume and force.

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Keywords:

strength; eccentric overload; hypertrophy

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