Adaptive response heterogeneity induced by a strenuous programme of endurance exercise: a reanalysis of Hickson et al. (1977)

This study reanalysed the classic endurance training dataset by Hickson et al. (1977) using modern statistical methods to explore potential non-linear adaptation trajectories and inter-individual variability. Weekly maximal oxygen uptake (V̇O _2max ) data from 8 participants (31.5 ± 7.3 years, 182.0 ± 8.6 cm, 79.2 ± 17.1 kg) undertaking a high-intensity training programme over 10 weeks were modelled using mixed-effects models of varying functional forms (linear, quadratic, cubic, logarithmic, rectangular hyperbolic, and mono-exponential). Model selection was based on the Akaike information criterion corrected for small sample sizes (AICc), with preference for the most parsimonious model when AICc differences were small. Curvilinear models with random slopes provided superior fits, capturing plateauing effects not represented by linear models. In particular, the quadratic model with a random slope outperformed its fixed-slope equivalent (χ ² = 26.1, p < 0.001), indicating systematic inter-individual variability in V̇O _2max trajectories, with an estimated slope standard deviation of 0.029 L·min ^-1 ·week ^-1 (95%CI: 0.010 to 0.042). In summary, this updated analysis aligns the classic study by Hickson et al. (1977) with current physiological understanding of diminishing returns, in which V̇O _2max tends to plateau as individuals approach their “genetic ceiling”, while adaptive response variability remains after accounting for residual variability. The present investigation also reinforces the notion that adaptive heterogeneity persists even when the training stimulus is normalised on the basis of a maximal sustainable effort. These findings underscore the value of revisiting legacy datasets using contemporary analytical approaches to more accurately characterise human physiological adaptation to exercise training.