Do Changes In Muscle Size Mediate Changes In Non-specific Strength?

Abstract

It has been suggested that increases in muscle strength occur due to morphological and neurological factors. However, neurological adaptations can be highly specific to the task trained. Testing non-specific strength (via unaccustomed task) may better elucidate the effects of hypertrophy on strength. PURPOSE: Determine the indirect (via hypertrophy) and direct (not hypertrophy) effects of training on muscle strength. METHODS: 151 participants were randomized into control, one-repetition maximum training (1RM), or traditional training. For 6 weeks the control avoided resistance exercise; training groups performed elbow flexion 3x/week. Participants in the 1RM training group had 5 attempts to lift the greatest load possible. The traditional group performed 4 sets of 8-12RM. Anterior muscle thickness at 50, 60, and 70% upper arm length, and maximal isokinetic torque at 60°/s 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 group were evaluated relative to control. Data is presented as coefficient (95% CI). RESULTS: There were no significant relative direct effects on non-specific strength for the 1RM training group or for traditional training group outside of the 60% site [1.7 (0.13, 3.27) Nm]. The relative effect of 1RM training on muscle thickness was different from control 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 non-specific strength was not significant for the 50% or 70% models but was for the 60% [-3.06 (-5.7, -0.35) Nm] model. The relative indirect effect on non-specific strength was not significant for the 1RM training group [50% = -0.11 (-0.44, 0.09); 60% = -0.28 (-0.74, 0.02); 70% = -0.07 (-0.39, 0.22), or traditional training [50% = -0.43 (-1.3, 0.3); 60% = -0.76 (-1.85, 0.06); 70% = -0.18 (-0.98, 0.41)]. CONCLUSIONS: Similar to previous findings on specific strength, increases in muscle thickness do not appear to mediate training induced increases in non-specific strength. The importance of growth for strength may need to be reconsidered.