Eccentric overload differences between loads and training variables on flywheel training

ECCENTRIC OVERLOAD DIFFERENCES BETWEEN LOADS AND TRAINING VARIABLES ON FLYWHEEL TRAINING Abstract Objective: there is considerable debate about the existence of a real eccentric overload in flywheel exercises. This study aimed to analyse the differences in concentric:eccentric mechanical output ratios between different loads and variables in the flywheel squat exercise. Methods: twenty physically active men (22.9 ± 2.2 years, height: 1.8 ± 0.1 m, weight: 79.6 ± 8.2 kg) performed a loading test using five moments of inertia. Angular speed was measured using a rotary encoder, while the vertical force was measured using force plates. For each variable (angular speed, angular acceleration, power, vertical force, and torque), mean and peak values were calculated for concentric and eccentric phases to allow comparisons across the loads. We tested the possible differences in Load x Phase (concentric and eccentric) and Load x Variable. The level of significance was established as p < 0.05. Results: a significant Load x Phase interaction was found in mean angular speed, peak vertical force, peak angular acceleration, peak power and peak torque. Higher eccentric overload values were observed with speed-derived variables (angular speed, angular acceleration and power). Conclusions: in conclusion, speed-derived peak variables and lower loads are more likely to show an eccentric overload and can be used to monitor responses to flywheel training. Keywords: eccentric training, injury prevention, resistance training, strength, squat INTRODUCTION Athletes must move in all directions and with frequent changes of direction to meet the performance needs of their sport (1). These movements are carried out at different speeds and may require high-intensity accelerations and decelerations, which can impact physical loading and fitness (2). There are different training equipment to improve acceleration or deceleration abilities; for instance, traditional resistance training can improve acceleration ability (i.e., the initial part of a sprinting action) (3). In contrast, flywheel resistance training devices have mostly been used in training programs to improve performance during the eccentric phase of the movement (4). With these devices, the athlete first pulls a rope or a strap at the desired (i.e., maximal) intensity, thus spinning a flywheel disk during the concentric phase. Then the rope or strap recoils around the rotary shaft during the eccentric phase, where the athlete is instructed to decelerate the kinetic energy produced during the concentric phase (5). During flywheel resistance training, practitioners try to achieve a so-called eccentric overload with their athletes, which can be explained as the production of a greater mechanical output during the eccentric phase compared to the concentric one (6,7). In a recent meta-analysis, Muñoz-López et al. (8) showed that most of the research that used flywheel resistance training with the purpose to achieve an EO neither measured it nor achieved that. EO can be easily measured using the eccentric:concentric ratio (E:C) (9). E:C is a parameter used to quantify the application of kinetic variables (e.g., force) during the eccentric phase of a movement in comparison to the concentric phase (6). Therefore, if the kinetic energy measured is higher during the eccentric phase than the concentric phase, an E:C would be greater than 1 and EO would be achieved (6). However, a known problem related to EO during flywheel resistance training is its accurate quantification and its reliability (7,10). The absence of an EO during flywheel resistance training can be related to the following reasons: 1) the participant is not able to decelerate the kinetic energy towards the end of the eccentric phase, 2) low experience or poor familiarization of the participants with flywheel resistance training (5), 3) the proper selection of the mechanical output used to monitor it (8), or 4) the external load used (11,12). An effective selection of the inertial load may be critical to induce EO (13). Some authors showed significant differences in speed (14–16), power (11,15,17), angular acceleration (14,17), or vertical force (16,17) between different moments of inertia in flywheel squat exercises. Some authors reported that the lower the moment of inertia, the higher the speed (11,14,16). For the leg extension exercise, Martinez-Aranda and Fernandez-Gonzalo(12) showed that EO is more likely to be produced with a moment of inertia of 0.0375 kg·m2. However, they calculated the EO only using peak force, while Muñoz-López et al. (8) showed an influence of the variable selected when the occurrence of EO is assessed. Sabido et al. (11) showed that a lower moment of inertia (i.e., 0.025 kg·m2) resulted in higher concentric peak power outputs, whereas higher inertial loads (i.e., 0.075 kg·m2) were more likely to produce higher eccentric peak power outputs. However, other researchers reported that a generalization about the use of moments of inertia for all exercises is not possible because mechanical outputs are very dependent on the exercise selected, the characteristics of the athletes and their previous familiarization with the devices (7). Therefore, it is impossible to state that lower moments of inertia are more suitable for EO production because several factors can affect kinetic and kinematic parameters in both concentric and eccentric phases (7). A common approach to understanding the neuromuscular profile for a given exercise is to study the force-velocity profile (18). Typically, the force-velocity profile is calculated using the force and speed of the concentric phase using several external loads in a progressive loading test. Although it has been widely studied with stack machines and free weights (here categorized as traditional resistance equipment), to our knowledge, only a few studies have analyzed it using flywheel resistance training devices (18,19), reporting both concentric and eccentric outputs. Flywheel devices are equipped with a rotary encoder instead of a linear encoder (used with traditional resistance equipment), which can help to understand better the mechanical rotation demands of both concentric and eccentric phases. To the best of our knowledge, no study has yet investigated the force-velocity relationships using angular variables (i.e., angular speed or torque), nor have investigated those relationships during the eccentric phase of the exercise. Considering the increasing interest in implementing flywheel resistance training in strength and conditioning programs, especially due to the opportunities to increase muscle activation (20), acute performance, and chronic adaptations (21), it is important to know how to monitor EO over a range of moments of inertia effectively. Therefore, this study aimed to determine the differences in EO between the moments of inertia and the mechanical outputs used in a flywheel squat, which is one of the preferential exercises implemented in those programs. A secondary objective was to compare the force-velocity profile of the concentric and eccentric phases of this exercise. We hypothesized that, first, the kinetic and kinematic outputs will vary across the loads used, and second, the EO level is variable- and load-dependent. MATERIALS & METHODS Experimental Approach to the Problem This study used a randomized observational design to measure the mechanical output during the concentric and the eccentric phase of the flywheel squat exercise using different loads. Subjects performed a randomized loading test using five different moments of inertia (e.g., loads). Before testing, subjects conducted two familiarization sessions across all the loads used during the test. Subjects A total of 25 physically active males participated in this study (age: 22.9 ± 2.2 years, height: 1.8 ± 0.1 m, weight: 79.6 ± 8.2 kg). The main inclusion criteria were having a minimum of two years of experience in the squat exercise and having participated for a minimum of 6 months in weekly resistance training programs involving the lower body muscles. At the beginning of the familiarization sessions, none of the subjects had experience executing flywheel squat exercise. We also instructed the subjects not to change their daily nutritional habits or consume any ergogenic aid at least 48 h before testing execution. The minimum sample size required for the statistical test used (two-way repeated-measures ANOVA), considering 80% statistical power, an alpha error of 0.05, and a moderate f effect size of 0.3, was 16 subjects (Power = 0.84). The study protocol followed the guidelines of the Declaration of Helsinki and was approved by the local ethics committee. Intervention Prior to the execution of the tests, the subjects warmed up for 10 minutes. The warm-up consisted of 5 minutes of cycling on an electronic ergometer at a self-selected submaximal pace, followed by mobilization exercises for the ankle, knee, hip, and trunk joints for 5 minutes. Furthermore, the subjects ended the warm-up with five countermovement jumps and six submaximal repetitions of the harness squat exercise with the lowest load used in the tests. The squat exercise was performed on a cylindrical shaft flywheel resistance device (kBox 3, Exxentric, AB Bromma, Sweden). The loading test consisted of 5 sets (one set for each load in random order), with 5 minutes of rest interspersing the sets. The loads used for each set were 0.025 kg·m2, 0.050 kg·m2, 0.075 kg·m2, 0.100 kg·m2, 0.125 kg·m2. The radius width of the cylinder was 0.025 m. Two submaximal repetitions were used to accelerate the flywheel disk initially. From the third repetition (the first for further analyses), subjects had to accelerate the flywheel disk at maximum voluntary effort, pulling until the end of the concentric phase. We instructed the participants to decelerate as hard as possible the spinning flywheel disk at the end during the eccentric phase. Only subjects who committed to completing the five loads were considered (n= 20); therefore, 5 subjects were excluded. Data were recorded using a rotary encoder (EMS22Q, Bourns, Riverside, CA, USA) connected to the flywheel shaft and a prototype multichannel acquisition system (SmartCoach MultiChannel, SmartCoach Technologies Inc., Pleasanton, CA, USA), with specific computer software (SmartPlot V4.7.0, SmartCoach Technologies Inc., Pleasanton, CA, USA). We calculated the mean and peak values for the speed, acceleration, power and torque variables from the raw speed captured for each concentric and eccentric phase movement. In addition, we recorded the vertical ground reaction force using two force platforms (SmartCoach Europe AB, Stockholm, Sweden), synchronized and connected to the SmartCoach Multichannel. From the force platforms, we calculated the mean and peak force. The sampling rate was set at 100 Hz for both capture systems. Finally, we calculated the E:C value as the eccentric phase value divided by the concentric phase value. The average from the three fastest repetitions of each set (i.e., with the highest concentric speed) was considered for further analysis. Further information on data acquisition and computation is published elsewhere (17). Statistics Data are shown as mean ± standard deviation (SD). Data normality was checked using the Shapiro-Wilk test. The sphericity assumption was tested using Mauchly's sphericity test and corrected using the Greenhouse-Geisser correction in the case of being positive. We used a twoway ANOVA repeated measures test with two within-subjects factors (Load (i.e., moments of inertia) x Phase (i.e., concentric or eccentric)). In addition, we tested the differences between loads in the E:C using a two-way ANOVA repeated measures test with two within-subjects factors (Load x Variable). When a significant interaction was found in both cases, we performed a post hoc analysis using the Bonferroni correction. Alpha level to determine significant differences was 5%, therefore p = 0.05. We used the partial eta squared (η2p) to measure the effect size of the differences for the interactions such as 0.01-0.059 (small), 0.06-0.139 (medium), and >0.14 (large). We used the Cohen’s d to measure the effect size of the post-hoc differences as follows: < 0.20 (trivial), 0.2–0.59 (small), 0.6–1.19 (moderate), 1.2–2.0 (large), and > 2.0 (very large)(22). Finally, we calculated and plotted the mean ± SD of force, torque, power, and velocity to represent different mechanical relationships for each movement phase. We used the JASP software for Windows (JASP Team, Version 0.16) for all statistical tests. RESULTS Concentric and eccentric outputs The mean and peak values for each variable during the concentric and eccentric phases are shown in Table 1 and Table 2, respectively. From the mean variables, only the Mean Angular Speed (p < 0.001, η2p = 0.45 – large) showed a significant Load x Phase interaction. In addition, the eccentric Mean Angular Speed was significantly higher than concentric Mean Angular Speed for 0.050 kg·m2 (p < 0.001, d= 0.54 - moderate), 0.075 kg·m2 (p < 0.05, d= 0.48 - small), 0.100 kg·m2 (p < 0.001, d= 0.72 - moderate), and 0.125 kg·m2 (p < 0.001, d= 0.60 - moderate). The Peak Vertical Force (p < 0.001, η2p = 0.44 - large), the Peak Angular Acceleration (p < 0.001, η2p = 0.35 - large), the Peak Power (p < 0.001, η2p = 0.23 - large), and the Peak Torque (p = 0.009, η2p = 0.16 - large) showed a significant Load x Phase interaction. In addition, the Peak Vertical Force concentric outputs were higher than the eccentric outputs at 0.075 kg·m2 (p < 0.001, d= 0.70 – moderate), 0.100 kg·m2 (p < 0.001, d=1.08 – large), and 0.125 kg·m2 (p < 0.001, d= 1.23 – large). For the Peak Angular Acceleration, eccentric outputs were higher than concentric outputs for 0.025 kg·m2 (p < 0.001, d= 1.44 – large), 0.050 kg·m2 (p < 0.001, d= 1.68 – large), 0.075 kg·m2 (p < 0.001, d= 0.99 – large), and 0.100 kg·m2 (p < 0.05, d= 1.02 – large). For the Peak Power, the eccentric outputs were higher than concentric outputs for 0.025 kg·m2 (p < 0.001, d= 0.76 – moderate). Eccentric:concentric ratio between variables differences Figures 1 and 2 show the E:C for the mean and peak variables, respectively. The mean and peak variables did not show a significant Load effect, but a Variable significant effect (p < 0.001) was detected. For mean variables, E:C was higher than 1 (i.e., eccentric overload) for Mean Angular Acceleration, Mean Torque, Mean Power, and Mean Angular Speed, but lower than 1 (i.e., no eccentric overload) for Mean Vertical Force. For peak variables, E:C was higher than 16. Núñez FJ, Galiano C, Muñoz-López A, Floria P. 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Available from: http://www.johk.pl 29. Beato M, Maroto-Izquierdo S, Hernández-Davó JL, Raya-González J. Flywheel Training Periodization in Team Sports. Frontiers in Physiology. 2021;12(November):1–6. 30. Muñoz-López A, Galiano C, Nuñez FJ, Floría P. The flywheel device shaft shape determines force and velocity profiles in the half squat exercise. Journal of Human Kinetics. 2022;81(January):15–25. FIGURE CAPTIONS Figure 1. Eccentric:concentric ratio for each moment of inertia and mechanical variable mean values. The dotted horizontal line represents a ratio equal to 1. Values above this line represents an eccentric overload. Figure 2. Eccentric:concentric ratio for each moment of inertia and mechanical variable peak values. The dotted horizontal line represents a ratio equal to 1. Values above this line represents an eccentric overload. Figure 3. Force-, Torque-, and Power-Velocity profiles for the concentric (black squares) and eccentric (grey circles) phases. The vertical and the horizontal lines represents the individuals standard deviations for each y-variable (vertical lines) and x-variable (horizontal lines).