Resistance exercise-induced muscle hypertrophy

Resistance exercise training (RET) can lead to muscle hypertrophy; however, the relative contribution that exogenous (protein supplementation and specific training variables) versus endogenous (biology inherent to the individual) factors have on RET-induced muscle hypertrophy is controversial. In Study 1, we provided an evidence-based conclusion that protein supplementation during periods of RET results in a small but statistically significant increase in RET-induced muscle hypertrophy. In Study 2, we corroborate previous research and observed that the amount of mass lifted per repetition (load) did not determine RET-induced muscle hypertrophy in resistance-trained men when RET was performed to volitional fatigue. In Study 4, we observed similar muscle fibre activation following resistance exercise with lighter versus heavier loads when both were lifted until volitional fatigue. In Studies 2 and 3, we observed no relationship between circulating anabolic hormones (e.g., testosterone) and RET-induced muscle hypertrophy. Nonetheless, in Study 3, we found significantly greater muscle androgen receptor content in the top versus the bottom quintile of respondents for muscle hypertrophy following 12 weeks of RET indicating that androgen receptor content, and not circulating androgen concentration, may be an important determinant of hypertrophy. Finally, in Study 5, we observed that RET-induced muscle hypertrophy was an consistent within an individual (independent of load and limb) but considerably different between participants. Together, these data suggest that the exogenous factors we studied – protein supplementation and load (when RET was performed to volitional fatigue) – had a relatively small influence on RET-induced muscle hypertrophy. In contrast, we found that endogenous variables, such as intramuscular androgen receptor content and likely other genetic influences, appear to contribute more to the significant heterogeneity seen in RET-induced muscle hypertrophy. Future research in this area should prioritize understanding the biology that underpins the individual variability in RET-induced muscle hypertrophy.