Examining interindividual differences in select muscle and whole‐body adaptations to continuous endurance 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 (SD_IR ) 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 ; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mover><mml:mi>V</mml:mi><mml:mo>̇</mml:mo></mml:mover><mml:mrow><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>peak</mml:mi></mml:mrow></mml:msub></mml:math> ; 45 ± 7 ml/kg/min) completed 4 weeks of continuous training (REL; n = 14) or control (n = 15). Maximal enzyme activities (citrate synthase and β-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 SD_IR values for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mover><mml:mi>V</mml:mi><mml:mo>̇</mml:mo></mml:mover><mml:mrow><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>peak</mml:mi></mml:mrow></mml:msub></mml:math> , 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 SD_IR 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 SD_IR 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.