At the onset of exercise, horses exhibit O₂ uptake (O₂) kinetics that are qualitatively similar to those of humans. In humans, there is a marked dissymmetry between on- and off-kinetics for O₂. This investigation sought to formally characterize the off-transient (recovery) O₂ kinetics in the horse within the moderate (M), heavy (H), and severe (S) exercise domains. Six horses were run on a high-speed treadmill at M, H, and S exercise intensities (i.e., that speed which yielded ~50, 85, 100% peak O₂, respectively, on the maximal incremental test). The time courses for the recovery were modeled by using a three-phase model with a single-exponential (fast component) or double-exponential (fast and slow component) phase 2. The single-exponential phase 2 model provided an excellent fit to the off-transient data, with the exception of one horse in the H domain which was best modeled by a double exponential. The time delay elicited no domain dependency (M, 18.0 ± 1.0; H, 17.6 ± 1.1; S, 17.8 ± 2.0 s; P > 0.05), as was the case for the fast-component time constants (M, 16.3 ± 2.0 s; H, 13.5 ± 1.0 s; S, 14.6 ± 0.3 s; P > 0.05). In the H and S (but not M) domains, the O₂ following resolution of the fast component was elevated above the preexercise baseline (H, 3.0 ± 1.0 l/min; S, 5.7 ± 1.1 l/min). This additional postexercise O₂ was correlated to the end-exercise increase in lactate (r = 0.94, P < 0.001) but not the end-exercise pulmonary arterial blood temperature (r = 0.45, P > 0.05). These data indicate that the time delay and subsequent kinetic response of the primary (fast-component) phase of exercise O₂ recovery in the horse is independent of the preceding exercise-intensity domain. However, in the H and S domains, the fast component resolves to an elevated baseline.

oxygen uptake; horse; excess postexercise oxygen uptake; exercise energetics

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