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 regiment-ed resistance training, one hundred and seventy-five older women completed 12 weeks of no inter-vention 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 anteri-or 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 resistance training on increases in maximal knee extension strength is mediated by anterior thigh muscle growth.