Method for quantifying net anaerobic power in exercising horses

Reason for performing study: There is no good method for measuring net anaerobic power in exercising horses to allow accurate estimates of total metabolic power. Hypothesis: The increase in VO_2max when breathing hyperoxic (HO) gas should be accompanied by a stoichiometrically equal (in terms of ATP turnover, i.e. energy equivalents) decrease in plasma lactate accumulation rate (M_lactate). Methods: Six 3-year-old Thoroughbreds were trained on an equine treadmill wearing a semi-open flow mask for measurement of VO₂. After 4 months the horses ran with reproducible specific VO_2max (VO_2max/kg bwt). The mask design allowed mixing of O₂ or N₂ with the inward bias flow of gas so that inspired O₂ concentration of the horse could be controlled. While the horse breathed either HO (25.1% O₂), normoxic (NO, 21% O₂) or hypoxic (LO, 19.5% O₂) gas, it ran at a speed sufficient to elicit VO_2max in NO while jugular venous blood was drawn at 15 sec intervals over a period of 2 min to determine M_lactate. Results: VO_2max/kg bwt was not significantly different between LO and NO conditions, and LO data could not be used in the comparison. The VO_2max/kg bwt increased from 2.59 ± 0.24 (s.d.) to 2.86 ± 0.24 mlO₂ (STPD)/sec/kg in NO and HO, respectively, while M_lactate decreased from 11.5 ± 4.2 to 9.0 ± 3.9 mmol/min as VO₂ increased. Conclusions: The ratio of Δ M_lactate to Δ VO_2max/kg bwt suggests that M_lactate of approx 11.1 ± 6.7 mmol/min is associated with net anaerobic power approximately equivalent to 1.0 mlO₂ (STPD)/sec/kg of aerobic power (20.1 W/kg^-1). The high variability in VO_2max/kg bwt observed in data from some runs, particularly in LO, suggests that caution must be used when comparing data from the same horse during different runs. Potential relevance: This study provides a tool for estimating net anaerobic power and, more accurately, evaluating total metabolic power of horses exercising at or above their aerobic capacities.