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 eight-week endurance training periods (TP 1 and TP 2 ), separated by an eight-week 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-minute 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 TP 1 and TP 2 , 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 sub> (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 emerge from dynamic and context-dependent biological processes.