Muscle growth mediates the increases in muscle strength in untrained older women

To investigate the potential role of muscle size changes on changes in strength following regimented resistance training, 175 older women completed 12 weeks of no intervention time-matched control (Ctrl, n = 84) or resistance exercise training (RET, n = 91). One-repetition maximum (1RM) strength of knee extension exercise and muscle thickness of the anterior and lateral thigh (B-mode ultrasound) were assessed pre- and post-12 weeks. The effect of RET on muscle thickness relative to the Ctrl was significant for the anterior (β = 0.32 (95% confidence interval (95% CI) = 0.24, 0.41) cm) and lateral thigh (β = 0.23 (95% CI = 0.17, 0.30) cm). The effect of RET on 1RM knee extension relative to Ctrl was significant (β: 7.02 (95% CI, 6.01 to 8.04) kg; P < 0.001). There was evidence of mediation for the anterior thigh model, where 10.6% of the changes in 1RM knee extension were explained by changes in the anterior thigh muscle thickness (average causal mediation effect (ACME) = 0.74 (95% CI = 0.20, 1.35) kg, P = 0.002). Conversely, there was no significant effect of mediation for the lateral thigh model, evident by the ACME (0.45 (95% CI = -0.03, 1.10) kg, P = 0.066). The findings indicate that part of the effect of RET on increases in knee extension strength is mediated by anterior thigh muscle growth.