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

This study reanalyzed the classic endurance training dataset by Hickson et al. (1977) using modern statistical

Methods: to explore potential nonlinear adaptation trajectories and interindividual variability. Weekly maximal oxygen uptake ([Formula: see text]) data from eight participants (31.5 ± 7.3 yr, 182.0 ± 8.6 cm, 79.2 ± 17.1 kg) undertaking a high-intensity training program over 10 wk were modeled using mixed-effects models of varying functional forms (linear, quadratic, cubic, logarithmic, rectangular hyperbolic, and monoexponential). 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 -1 ·wk -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 [Formula: see text] tends to plateau as individuals approach their "genetic ceiling," whereas 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 normalized on the basis of a maximal sustainable effort. These

Findings: underscore the value of revisiting legacy datasets using contemporary analytical approaches to more accurately characterize human physiological adaptation to exercise training. NEW & NOTEWORTHY This reanalysis challenges the original interpretation that maximal oxygen uptake increased linearly during the 10-wk strenuous endurance program investigated by Hickson et al. (1977). Using mixed-effects modeling of weekly individual data, adaptations were better described by curvilinear trajectories with diminishing returns. Systematic interindividual variability in response magnitude remained despite a normalized, "maximal" training stimulus. These

findings demonstrate the value of revisiting legacy datasets to refine understanding of human adaptability and inform individualized exercise prescription.