Examining interindividual differences in select muscle and whole-body adaptations to continuous endurance training.

NEW FINDINGS: What is the central question of the study? Do interindividual differences in trainability exist for morphological and molecular skeletal muscle responses to aerobic exercise training? What is the main finding and its importance? Interindividual differences in trainability were present for some, but not all, morphological and molecular outcomes included in our study. Our findings suggest that it is inappropriate, and perhaps erroneous, to assume that variability in observed responses reflects interindividual differences in trainability in skeletal muscle responses to aerobic exercise training. ABSTRACT: Studies have interpreted a wide range of morphological and molecular changes in human skeletal muscle as evidence of interindividual differences in trainability. However, these interpretations fail to account for the influence of random measurement error and within-subject variability. The purpose of the present study was to use the standard deviation of individual response (SDIR ) statistic to test the hypothesis that interindividual differences in trainability are present for some but not all skeletal muscle outcomes. Twenty-nine recreationally active males (age: 21 ± 2 years; BMI: 24 ± 3 kg/m2 ; V̇O2peak ; 45 ± 7 ml/kg/min) completed 4 weeks of continuous training (REL; n = 14) or control (n = 15). Maximal enzyme activities (citrate synthase and ß-hydroxyacyl-CoA dehydrogenase), capillary density, fibre type composition, fibre-specific succinate dehydrogenase activity and substrate storage (intramuscular triglycerides and glycogen), and markers of mitophagy (BCL2-interacting protein 3 (BNIP3), BNIP3-like protein, parkin and PTEN-induced kinase 1) were measured in vastus lateralis samples collected before and after the intervention. We also calculated SDIR values for V̇O2peak , peak work rate and the onset of blood lactate accumulation for the REL group and a separate group that exercised at the negative talk test stage. Although positive SDIR values - indicating interindividual differences in trainability - were obtained for aerobic capacity outcomes, maximal enzyme activities, capillary density, all fibre-specific outcomes and BNIP3 protein content, the remaining outcomes produced negative SDIR values indicating a large degree of random measurement error and/or within-subject variability. Our findings question the interpretation of heterogeneity in observed responses as evidence of interindividual differences in trainability and highlight the importance of including control groups when analysing individual skeletal muscle response to exercise training.