Effect of Small and Large Energy Surpluses on Strength, Muscle, and Skinfold Thickness

Abstract Background A goal of many who perform resistance training (RT) is to increase muscle mass and strength. An energy surplus is commonly advised to support such gains; however, if too large, it could cause unwanted fat gain without augmenting adaptation. Thus, we randomized 21 trained lifters into maintenance energy (MAIN), 5% (MOD), and 15% (HIGH) energy surplus groups while performing RT 3d/wk for eight weeks to determine if skinfold thicknesses (ST), squat and bench one-repetition maximum (1-RM), or biceps brachii or quadriceps muscle thicknesses (MT) were influenced by group assignment. COVID-19 complications reduced our ability to recruit and retain participants, leaving 17 completers. Thus, in addition to Bayesian ANCOVA group comparisons, we regressed outcomes against body mass (BM) changes to analyze the relationship between BM gain, ST increases, and increase in 1-RM and MT. For all outcomes we reported Bayes Factors (BF 10 ) indicating the odds ratio of the likelihood of one hypothesis relative to another (e.g., a BF 10 = 2 indicates the hypothesis is twice as likely as another) and coefficients of determination (R 2 ) for regressions. Results ANCOVAs provided no evidence in support of the group model for MT or squat 1-RM. However, there was moderate (BF 10 = 9.9) and strong evidence (BF 10 = 14.5) that MOD increased bench 1-RM more than HIGH and MAIN, respectively. Further, there was moderate evidence (BF 10 = 4.2) MOD increased ST more than MAIN and weak evidence (BF 10 = 2.4) HIGH increased ST more than MAIN. The regression provided strong evidence that BM changes predicted changes in ST (BF 10 = 14.3, R 2 = 0.49) and weak evidence that BM changes predicted changes in biceps MT (BF 10 = 1.4, R 2 = 0.24), but no evidence any other variable was predicted by BM changes. Conclusions While some group-based differences were found, our larger N regression provides the most generalizable evidence. Therefore, we conclude faster rates of BM gain (and by proxy larger surpluses) primarily increase rates of ST gain rather than augmenting 1-RM or MT. However, the biceps, the muscle group which received the greatest stimulus in this study, may have been positively impacted by greater BM gain, albeit slightly. Our findings are limited to the confines of this study, where a group of lifters with mixed training experience performed moderate volumes 3d/wk for eight weeks. Thus, future work is needed to evaluate the relationship between BM gains, increases in ST and RT adaptations in other contexts.