The aim of the randomized controlled study, which included physical education students, was to investigate the mechanical and electromiographic (EMG) outcomes of motor tests and vastus lateralis (VL) architecture after eight weeks of lower extremity resistance training with different flywheel loads (FW). Loads were individually determined based on Force-velocity-Power (F-v-P) characteristics during FW squats. The Popt group (n = 16) performed squats with the FW load that maximazed power (P) output and P70 group (n = 18) with the load that corresponded to 70% of maximal theoretical P (Pmax). The control group (C; n = 13) received no intervention. Mixed model analysis of variance (time [pre, post], factor group [C, Popt, P70]) revealed different changes between groups in a) maximal voluntary isometric contraction during knee extension, b) squat jump push-off rate of force development, force (F), and P, c) countermovement jump (CMJ) height, push-off F, P, and braking P, (d) all F-v-P outcomes in the concentric phase of FW squats and the maximal theoretical force (F0) in the eccentric phase of the FW squats, (d) the maximal theoretical velocity (v0) and Pmax derived from loaded CMJ F-v-P profile, and finally e) the VL muscle thickness and the pennation angle (all p < 0.05). There was a trend toward higher improvements in the mechanical variables in the P70 group; nevertheless, we found no differences in the adaptations between the two training groups (p > 0.05). Training with higher inertial load (P70) resulted in improvements of F0 in FW squats and conversely in improvements of v0 in loaded CMJ. The improvements in mechanical variables can be explained to a greater extent by the adjustment in VL muscle thickness than by the neural activation enhancement.
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