Muscle Volume Adaptations To Resistance Training Drive Changes In Dynamic Strength: A Causal Modeling Approach

PURPOSE: Progressive resistance training (RT) has many health benefits and is well known to cause increases in muscle size and function, though significant heterogeneity of response exists. Debate exists as the inter-relationship dynamics between muscle size and functional adaptations. The purpose of this study was to use statistical modeling to determine if a causal relationship between muscle size and strength progressive resistance training exists, testing the hypothesis that size changes would drive functional adaptations. METHODS: We used retrospective data from 600 participants in a large unilateral progressive 12-week RT study. Measurements including demographics, muscle size (volume) and muscle strength (one repetition maximum (1RM) and maximal voluntary contraction (MVC)) were collected before and after training. To model the qualitative causal structure among the variables, we applied the GFCI (Greedy Fast Causal Inference) algorithm. The GFCI can estimate the presence of causal relationships and hidden common causes up to statistical equivalence represented as a PAG (partial ancestral graph). We estimated the causal effects using Structure Equation Model based on the estimated PAG. RESULTS: Among the 22 variables, GFCI identified 21 edges, 10 represent causal relationships, 3 represent confounded relationships by hidden variables, and 8 represent potentially confounded relationships. Of particular interest, the change in the muscle volume was estimated as a cause of the change in 1RM (but not MVC) in the exercised arm. Age was estimated as a potential cause of change in 1RM in the exercised arm. These relationships are relatively stable, identified 64% and 49% times in 100 bootstrap samples. Assuming age is a cause, the estimated causal effect for change in muscle volume and age are 2.53+/-0.39 and -0.16+/-0.03, respectively (p < 0.01). The percent variance in the change of strength explained by the two variables is relatively small, with R-square = 0.10. CONCLUSIONS: Our results suggest that during 12 weeks of exercise, dynamic (but not static) muscle strength changes are caused by changes in muscle size and, possibly, age. Our results should help inform future RT programming for optimal outcomes. The Functional Polymorphisms Associated with Muscle Size and Strength (FAMuSS) Study was funded by the National Institutes of Health Grant no. 5R01NS040606