Impact of resistance training-induced changes in muscle quality on muscle power: a post-hoc analysis.

Background: This study aimed to determine the effects of changes in muscle mass and quality on muscle function and physical performance following an 8-week resistance training (RT) intervention.

Methods: Thirty-three healthy young women (23.1±2.2 years) underwent high- or low-velocity concentric knee extension RT at 60% one-repetition maximum strength (1RM) load for 10 repetitions × 4 sets, 3 times a week for 8 weeks. Before and after the RT intervention, quadriceps femoris muscle thickness (MT) (muscle mass index) and echo intensity (EI) (muscle quality index) were measured using B-mode ultrasound. Muscle function, including maximum isokinetic muscle strength, 1RM, rate of velocity development (RVD), and peak power, was assessed. Physical performance was evaluated using the countermovement jump (CMJ). Percentage changes (%change) in all parameters before and after the intervention were calculated for subsequent analyses. Multiple regression analysis was used to examined the contributions of changes in EI and MT to various muscle functions and physical performance, with the intervention group included as the covariate.

Results: MT %change was associated with 1RM %change (std.β=0.51, P=0.003), whereas EI %change was correlated with RVD %change (std.β=-0.39, P=0.025), peak power %change (std.β=--0.39, P=0.025), and CMJ %change (std.β=-0.44, P=0.011).

Conclusions: RT -induced increases in muscle mass contributed to improvements in 1RM, whereas enhancements in muscle quality were associated with gains in peak power, RVD, and CMJ. These results suggest training programs aimed at improving dynamic explosive performance in young adults should not only target muscle hypertrophy but also incorporate strategies to enhance muscle quality.