Inter-Individual Heterogeneity in Aerobic Training Adaptations: Systematic Review of the Evidence Base for...

Personalized exercise prescriptions require identifying inter-individual differences in exercise intervention effects. This review examined whether true inter-individual differences exist after accounting for measurement error. PubMed, Web of Science, and EMBASE databases were systematically searched. Inclusion criteria included: (1) original randomized controlled trials or controlled trials using standardized, supervised aerobic exercise interventions, or meta-analyses examining inter-individual differences in aerobic training adaptations; (2) explicitly identifying the assessment of inter-individual response variability as a research objective; (3) Reporting sufficient statistical information to quantify inter-individual variability. The search yielded a total of 3203 studies, and ultimately 78 studies were included for systematic review. Main findings included: (1) Few studies appropriately considered random measurement error and within-individual variability when quantifying inter-individual differences. (2) Analysis of both primary studies and existing meta-analyses revealed a consistent lack of statistical evidence for true inter-individual differences. (3) Observed inter-individual variability was primarily attributed to uncontrolled measurement error or within-individual variability, not true individual response differences. The evidence analyzed in this systematic review indicates that there is insufficient support for true inter-individual differences in the effects of aerobic exercise training intervention. Given physiological complexity and methodological limitations, any conclusions on exercise intervention heterogeneity or individual responsiveness require caution.