Muscle Growth Is Very Strongly Correlated with Strength Gains after Lower Body Resistance Training: New Insight from Within-Participant Associations.

PURPOSE Quantitative mechanistic evidence for muscle size and agonist neuromuscular activation explaining the individual strength gains that occur after resistance training (RT) remains opaque and controversial, with non-significant or weak/moderate relationships reported. This study aimed to quantify the within-participant association of strength gains following 15-weeks of RT with muscle growth and the changes in agonist neuromuscular activation. METHODS Knee extension muscle strength (isometric maximum voluntary torque [iMVT] and single repetition maximum [1RM]), MRI-derived quadriceps muscle volume, and normalized (to Mmax) quadriceps surface electromyography (sEMG) amplitude were measured in 39 previously untrained healthy young males, pre and post 15-weeks of lower body RT (3 x per week). Data were analyzed using repeated measures (within-participant) correlations and linear mixed models. RESULTS Very strong repeated measures correlations were found between muscle growth and strength gains (iMVT, r = 0.92 P < .001; 1RM, r = 0.89 P < .001). Changes in sEMG amplitude were moderately correlated with the changes in strength (iMVT, r = 0.58 P < .001; 1 RM, r = 0.56 P < .001). Linear mixed models revealed muscle growth and changes in neuromuscular activation both had significant positive effects on strength gain, but muscle growth contributed >5-fold more than neuromuscular activation (Standardized beta coefficient 0.88-0.94 vs 0.13-017). CONCLUSIONS Using more powerful within-participant statistical techniques, these results show that muscle growth is, in fact, very strongly, and neuromuscular activation moderately associated with strength gains after RT. These findings suggest that muscle growth is a major adaptation that is highly relevant to the individual increases in strength of young healthy males after RT.