Author(s): BOSSI, A., Institution: EDINBURGH NAPIER UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 222

INTRODUCTION:
In 1977, Hickson and colleagues reported remarkably large and apparently linear increases in maximal oxygen uptake (V̇O2max) during ten weeks of strenuous endurance training, a finding that the authors themselves described as surprising given prevailing physiological expectations. Because this study combined weekly V̇O2max measurements with the publication of individual-level data, it provides a unique opportunity to re-examine both the assumed linearity of adaptation and the magnitude of inter-individual variability using contemporary statistical methods that can distinguish true heterogeneity from random variation. It was hypothesised that a non‑linear model that assumes diminishing returns would best describe V̇O2max change trajectories. Moreover, it was expected that the analysis would reveal appreciable heterogeneity in adaptive responses to training.
METHODS:
Weekly V̇O2max 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 differences were small (i.e. ΔAICc < 2).
RESULTS:
Curvilinear models with random slopes generally provided superior fits, capturing plateauing effects missed by linear models. Among these, the cubic, rectangular hyperbolic, mono-exponential, and quadratic models, all incorporating a random intercept and slope, received equivalent support (ΔAICc < 2). In particular, the most parsimonious quadratic model with a random slope outperformed its fixed-slope equivalent (χ2 = 26.1, p < 0.001), indicating true inter-individual variability in V̇O2max responses, with a standard deviation of 0.029 L·min-1·week-1.
CONCLUSION:
Mixed-effects modelling revealed a more nuanced adaptation trajectory than originally described by Hickson and colleagues. V̇O2max increased in a curvilinear fashion with diminishing returns, and individuals demonstrated considerable heterogeneity in their responses. This updated analysis aligns the classic findings with current physiological understanding, recognising that V̇O2max gains become less pronounced as individuals near their genetic ceiling. Moreover, it 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.