VO2 kinetics in the horse during moderate and heavy exercise

The horse is a superb athlete, achieving a maximal O2 uptake (approximately 160 ml . min-1 . kg-1) approaching twice that of the fittest humans. Although equine O2 uptake (VO2) kinetics are reportedly fast, they have not been precisely characterized, nor has their exercise intensity dependence been elucidated. To address these issues, adult male horses underwent incremental treadmill testing to determine their lactate threshold (Tlac) and peak VO2 (VO2 peak), and kinetic features of their VO2 response to "square-wave" work forcings were resolved using exercise transitions from 3 m/s to a below-Tlac speed of 7 m/s or an above-Tlac speed of 12.3 +/- 0.7 m/s (i.e., between Tlac and VO2 peak) sustained for 6 min. VO2 and CO2 output were measured using an open-flow system: pulmonary artery temperature was monitored, and mixed venous blood was sampled for plasma lactate. VO2 kinetics at work levels below Tlac were well fit by a two-phase exponential model, with a phase 2 time constant (tau1 = 10.0 +/- 0.9 s) that followed a time delay (TD1 = 18.9 +/- 1.9 s). TD1 was similar to that found in humans performing leg cycling exercise, but the time constant was substantially faster. For speeds above Tlac, TD1 was unchanged (20.3 +/- 1.2 s); however, the phase 2 time constant was significantly slower (tau1 = 20.7 +/- 3.4 s, P < 0.05) than for exercise below Tlac. Furthermore, in four of five horses, a secondary, delayed increase in VO2 became evident 135.7 +/- 28.5 s after the exercise transition. This "slow component" accounted for approximately 12% (5.8 +/- 2.7 l/min) of the net increase in exercise VO2. We conclude that, at exercise intensities below and above Tlac, qualitative features of VO2 kinetics in the horse are similar to those in humans. However, at speeds below Tlac the fast component of the response is more rapid than that reported for humans, likely reflecting different energetics of O2 utilization within equine muscle fibers.