Background: To determine the role of muscle mass in sex-dependent differences in power output during flywheel resistance training (FRT). Methods: Twenty recreationally active (≥ 2 resistance exercise bouts per week), subjects (10 M, 10 F) completed 2 bouts of resistance exercise using a flywheel resistance training (FRT) device (Exxentric kbox 4 Pro) separated by at least one week. Each session consisted of 3 sets of 4 exercises (squat, bent-over row, Romanian deadlift, and biceps curl) with varying moments of inertia (0.050, 0.075, and 0.100 kg/m2, respectively) in random order. Each set consisted of 5 maximal effort repetitions with 3-minute recovery between sets. Average power, peak concentric and peak eccentric power were recorded and normalized to whole-body skeletal muscle mass (as calculated from bioelectrical impedence analysis). Additionally, linear regression analysis was used to determine the association between muscle mass and highest power output observed among all three inertial loads. Results: Absolute average, peak concentric and peak eccentric power for all lifts was significantly higher for males compared to females except for peak eccentric power for biceps curl which showed no significant difference. After normalizing to skeletal muscle mass, power output remained significantly higher for men in Row average power and peak concentric power as well as average power for biceps curl. A significant main effect of inertial load was noted for both absolute and relative power output for all exercises except for squat average power and peak concentric power. Regression analysis revealed that power output increases linearly with skeletal muscle mass (R2 = 0.37-0.77). Conclusions: Differences in power output between sexes during resistance exercise can largely be explained by differences in muscle mass. Indeed, muscle mass accounts for approximately 37-77% of the variance in power output during FRT depending on the exercise. Increasing inertial load tends to decrease power output during FRT.

Background: Training to increase muscular power is essential for improving athletic performance in most sports. Weight training (WT) is a common means for training muscular power. Another modality, flywheel resistance training (FRT), may be superior for improving muscular power. However, few studies have examined if FRT is kinematically similar to WT, or if FRT kinematics change with increasing inertial load. The purposes of this study were to determine how sagittal plane joint kinematics are affected by increasing inertial load during FRT squats, and to determine how FRT squat joint kinematics compare to WT squat joint kinematics. Methods: Subjects (n=9) completed three visits for this study. On the first visit subjects completed squat 1 repetition maximum (1RM) testing. The second visit served as a full FRT familiarization session in which subjects performed one set of 5 maximal effort FRT squats at each inertial load (0.050, 0.075, and 0.100 kgm2). On the third visit, subjects were videoed in the sagittal plane while performing the FRT squat protocol. Subjects then completed 5 maximal velocity repetitions of WT squats with the barbell loaded according to the Kansas Squat Test (KST) protocol. Kinematic differences between inertial loads were determined via 1-way repeated measures ANOVAS while differences between FRT and WT were determined with paired T-tests. Results: There were no differences in peak sagittal plane knee, trunk-hip, trunk (absolute) or ankle angles between inertial loads. Peak and mean joint angular velocities decreased with increasing inertial loads at the knee and trunk-hip. Mean joint angular velocities decreased at the ankle with increasing inertial loads, while peak and mean trunk (absolute) angular velocities were unaffected. No statistical analyses were conducted for FRT and WT comparison as not enough subjects met the criteria (n=3). Conclusions: Sagittal plane joint kinematics are largely maintained despite increasing inertial load during FRT squats. Lower extremity joint angular velocities decreased with increasing inertial load. If training for muscular power and knee extensor velocity is the goal, then the inertia of 0.050 kgm2 is most suitable.