Inter-Individual Responses of Maximal Oxygen Uptake to Exercise Training: A Critical Review

It has recently been reported how to quantify inter-individual differences in the response to an exercise intervention using the standard deviation of the change scores, as well as how to appraise these differences for clinical relevance. In a parallel-group randomised controlled trial, the key trigger for further investigation into inter-individual responses is when the standard deviation of change in the intervention sample is substantially larger than the same standard deviation derived from a suitable comparator sample. ‘True’ and clinically relevant inter-individual differences in response can then be plausibly expected, and potential moderators and mediators of the inter-individual differences can be explored. We now aim to critically review the research on the inter-individual differences in response to exercise training, focusing on maximal oxygen uptake (VO₂max). A literature search through the relevant bibliographic databases resulted in the identification of six relevant studies that were published prior to the influential HEalth, RIsk factors, exercise Training And GEnetics (HERITAGE) Family Study. Only one of these studies was found to include a comparator arm. Re-analysis of the data from this study, accounting for random within-subjects variation, revealed an absence of clinically important inter-individual differences in the response of VO₂max to exercise training. The standard deviation of change was, in fact, larger (±5.6 mL/kg/min) for the comparator than the intervention group (±3.7 mL/kg/min). We located over 180 publications that resulted from the HERITAGE Family Study, but we could not find a comparator arm in any of these studies. Some authors did not explain this absence, while others reasoned that only inter-individual differences in exercise response were of interest, thus the intervention sample was investigated solely. We also found this absence of a comparator sample in on-going studies. A perceived high test–retest reliability is offered as a justification for the absence of a comparator arm, but the test–retest reliability analysis for the HERITAGE Family Study was over a much shorter term than the length of the actual training period between baseline and follow-up measurements of VO₂max. We also scrutinised the studies in which twins have been investigated, resulting in concerns about how genetic influences on the magnitude of general within-subjects variability has been partitioned out (again in the absence of a comparator no-training group), as well as with the intra-class correlation coefficient approach to data analysis. Twin pairs were found to be sometimes heterogeneous for the obviously influential factors of sex, age and fitness, thereby inflating an unadjusted coefficient. We conclude that most studies on inter-individual differences in VO₂max response to exercise training have no comparator sample. Therefore, true inter-individual differences in response cannot be quantified, let alone appraised for clinical relevance. For those studies with a comparator sample, we found that the inter-individual differences in training response were not larger than random within-subjects variation in VO₂max over the same time period as the training intervention.