Heterogeneity and incidence of non-response for changes in cardiorespiratory fitness following time-efficient sprint interval exercise training

Interindividual variability for training-induced changes in maximal oxygen uptake (V̇O_2max) is well described following continuous aerobic and high-intensity interval training. Whether similar variability is observed following time-efficient sprint interval training with minimal training volume (i.e., reduced-exertion high-intensity interval training; REHIT) is unknown. We conducted a pooled analysis of n = 117 (68 men) training participants (mean ± SD: age: 30 ± 10 y; V̇O_2max: 34.8 ± 7.5 mL·kg^-1·min^-1), who completed a V̇O_2max assessment before and 3 days after 6 weeks of REHIT comprising of two 10-20-s 'all-out' cycling sprints per session, and n = 40 no-intervention control participants (age: 30 ± 13 y; V̇O_2max: 31.5 ± 6.5 mL·kg^-1·min^-1) who completed repeated V̇O_2max tests over a comparable timeframe. Individual responses estimated using 50% confidence intervals derived from the technical error were interpreted against a smallest worthwhile change of 1.75 mL·kg^-1·min^-1. The standard deviation of individual responses was 2.39 mL·kg^-1·min^-1 demonstrating clinically meaningful heterogeneity in training-induced changes in V̇O_2max following REHIT that exceed the technical, biological and random within-subjects variability of V̇O_2max assessment. The likely (75% probability) non-response rate was 18% (21/117), and 49% (57/117) of individuals demonstrated increases in V̇O_2max likely higher than the smallest worthwhile change. We conclude that the well-described increase in V̇O_2max following REHIT at the group level is subject to substantial variability in magnitude at an individual level. This has important implications for exercise prescription and can be harnessed to elucidate mechanisms of adaptation. Novelty: There is substantial heterogeneity in V̇O_2max responses following time-efficient sprint interval training. Proportion of non-response was 18% and ∽50% of individuals show clinically meaningful increases in V̇O_2max.