Muscle Thickness Changes Do Not Mediate Changes In Muscle Strength

Abstract

The position that hypertrophy mechanistically increases muscle strength is currently debated among scientists. PURPOSE: To determine indirect (via hypertrophy) and direct (not hypertrophy) effects of training on muscle strength. METHODS: 151 participants were randomized into control, one-repetition maximum training, or traditional training. For 6 weeks control avoided resistance exercise; training groups performed elbow flexion 3x/week (dominant arm). One-repetition maximum participants had 5 attempts to lift the greatest load possible. Traditional participants performed 4 sets to task failure (load adjusted for ~8-12 repetitions). Attempts/sets were separated by 90 s. Anterior muscle thickness (B-mode ultrasound) at 50, 60, and 70% upper arm length, and strength (one-repetition maximum) were assessed pre- and post-training. Change-score mediation models (adjusted for sex, pre-muscle thickness, and pre-strength) were constructed for each muscle thickness site. Effects of each training were evaluated relative to control. Data presented as coefficient (95% CI). RESULTS: Relative direct effects on strength were greater for one-repetition maximum [50% = 1.89 (1.20, 2.58); 60% = 1.88 (1.19, 2.58); 70% = 1.81 (1.12, 2.50) kg] and traditional training [50% = 2.04 (1.28, 2.79); 60% = 1.98 (1.21, 2.74); 70% = 1.79 (1.04, 2.53) kg]. The relative effect of one-repetition maximum on muscle thickness was different in 60% [0.09 (0.01, 0.17) cm] and 70% [0.09 (0.00,0.17) cm] models [50% = 0.67 (-0.01,0.14) cm] while traditional was greater in all three: [50% = 0.24 (0.15, 0.32); 60% = 0.24 (0.16, 0.33); 70% = 0.22 (0.14, 0.31) cm]. The effect of muscle thickness on strength was not significant for 50% [-0.44 (-1.72, 0.84) kg], 60% [-0.15 (-1.48, 1.17) kg], or 70% [0.73 (-0.48, 1.96) kg] models. The relative indirect effect on strength was not significant for one-repetition maximum [50% = -0.02 (-0.16, 0.09); 60% = -0.01 (-0.17, 0.16); 70% = 0.06 (-0.09, 0.27), or traditional training [50% = -0.10 (-0.48, 0.29); 60% = -0.03 (-0.42, 0.40); 70% = 0.16 (-0.22, 0.58)]. CONCLUSIONS: One-repetition maximum and traditional training increase strength, however, there was no evidence that the increase in strength was mediated by hypertrophy, providing experimental and analytical evidence for the disconnect between variables.