Limited reproducibility of individual physiological adaptations to repeated endurance exercise training

Between-individual variability in physiological adaptations to endurance exercise training is often interpreted as reflecting stable differences in training responsiveness. However, whether the capacity for physiological adaptation itself represents a stable, individual-specific trait under repeated exposure to the same training stimulus remains unclear. This study investigated the reproducibility of within-individual adaptations to repeated endurance training. Forty-two middle-aged, untrained men and women completed two similar 8-wk endurance training periods (TP₁ and TP₂), separated by an 8-wk detraining period. Assessments spanned multiple physiological and performance outcomes, including hemoglobin mass (Hb_mass), skeletal muscle citrate synthase (CS) activity, capillaries·fibre^-1, maximal oxygen uptake (V̇o_2max), and 15-min maximal mean power output (MMPO_15-min). Within-individual reproducibility was evaluated using intraclass correlation coefficients with [95% confidence intervals]. Participants completed 23.9 (0.4) and 23.8 (0.7) training sessions in TP₁ and TP₂, respectively, with minimal within-individual variation. Baseline values were highly consistent between periods for V̇o_2max (0.98 [0.97, 0.99]), MMPO_15-min (0.96 [0.93, 0.97]), and most other outcomes, indicating effective detraining. Nevertheless, reproducibility of within-individual adaptations was poor for Hb_mass (0.00 [0.00, 0.30]), CS activity (0.15 [0.00, 0.33]), capillaries·fibre^-1 (0.00 [0.00, 0.36]), V̇o_2max (0.19 [0.00, 0.37]), and MMPO_15-min (0.20 [0.00, 0.37]). Within-individual variability in hematological and skeletal muscle adaptations accounted for little of the within-individual variability in V̇o_2max and MMPO_15-min responses. This study demonstrates that individual adaptations to endurance training exhibit limited reproducibility across multiple physiological and performance outcomes. These findings indicate that physiological adaptability to endurance training does not reflect a stable, individual-specific attribute but instead emerges from dynamic and context-dependent biological processes.NEW & NOTEWORTHY Repeating the same exercise training intervention within the same individuals after an adequate detraining period provides a direct test of whether physiological adaptations are reproducible within individuals. This study demonstrates that individual physiological adaptations to a repeated endurance training program are largely nonreproducible across multiple physiological and performance outcomes, despite highly standardized and closely supervised training conditions. The substantial within-individual response variability challenges the notion that physiological training responsiveness is a stable, trait-like characteristic.