Responses to exercise at a given percentage of one's maximum rate of oxygen consumption (V̇o_2max), or percentage of the power associated with V̇o_2max during a graded exercise test (i.e., P_GXT), vary. The purpose of this study was to determine if differences in critical power (P_CRIT, maximum metabolic steady state) and work-prime (W', the amount of work tolerated above steady state) are related to training-induced changes in endurance. P_CRIT, W', V̇o_2max, and other variables were determined before and after 22 adults completed 8 wk of either moderate-intensity continuous training (MICT) or high-intensity interval training (HIIT) performed at fixed percentages of P_GXT. On average, P_CRIT increased to a greater extent following HIIT (MICT: 15.7 ± 3.1% vs. HIIT: 27.5 ± 4.3%; P = 0.03), but the magnitude of change varied widely within each group (MICT: 4%-36%, HIIT: 4%-61%). The intensity of the prescribed exercise relative to pretraining P_CRIT, not P_GXT, accounted for most of the variance in changes to P_CRIT in response to a given protocol (R² = 0.61-0.64; P < 0.01). Although P_CRIT and V̇o_2max were related before training (R² = 0.92, P < 0.01), the training-induced change in P_CRIT was not significantly related to the change in V̇o_2max (R² = 0.06, P = 0.26). Before training, time-to-failure at P_GXT was related to W' (R² = 0.52; P < 0.01), but not V̇o_2max (R² = 0.13; P = 0.10)_. Training-induced changes in time-to-failure at the initial P_GXT were better captured by the combined changes in W' and P_CRIT (R² = 0.77, P < 0.01), than by the change in V̇o_2max (R² = 0.24; P = 0.02). Differences in P_CRIT and W' account for some of the variability in responses to endurance exercise.NEW & NOTEWORTHY As the highest percentage of V̇O_2max at which steady state conditions can be achieved, a person's critical power (P_CRIT) strongly influences the metabolic strain of a given exercise. In this study we demonstrate that training-induced changes in endurance are more strongly related to the intensity of an exercise training program, relative to P_CRIT than relative to V̇o_2max. Thus, exercise may be more homogenously and effectively prescribed in relation to P_CRIT than traditional factors like V̇o_2max.

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