Reasons for performing study: Accumulated O2 deficit (AOD) and plasma lactate accumulation rate (PLAR) are alternative methods for estimating net anaerobic energy utilisation (NAEU) in exercising horses. How they compare or their accuracy is unknown.
Objectives: We hypothesised net anaerobic energy utilisation calculated by PLAR (NAUEPLAR) is equivalent to NAUE estimated by AOD (NAUEAOD).
Methods: Six Thoroughbred horses ran at identical supramaximal speeds (118% aerobic capacity) until exhaustion for 2 runs while breathing normoxic (NO, 21% O2) or hyperoxic (HO, 26% O2) gas. Jugular blood was sampled at 15 s intervals to measure plasma lactate concentration. Horses also ran at incremental submaximal speeds from 1.7–11.0 m/s to determine the linear relationship between speed and O2 consumption to estimate O2 demand for AOD calculations.
Results: Maximum O2 consumption of horses increased 11.6 ± 2.3% in HO and NAEUPLAR and NAUEAOD decreased 38.5 ± 8.0% and 46.2 ± 17.7%, respectively. The NAEUPLAR in NO was 114.5 ± 27.4 mlO2 (STPD) equivalent/kg bwt contributing 23.5 ± 3.7% to total energy turnover and in HO was 70.9 ± 19.8 mlO2 (STPD) equivalent/kg bwt contributing 14.6 ± 3.8% to total energy turnover. The NAUEAOD in NO was 88.6 ± 24.3 mlO2 (STPD) equivalent/kg bwt contributing 19.9 ± 2.1% to total energy turnover and in HO was 56.2 ± 19.1 mlO2 (STPD) equivalent/kg bwt contributing 10.9 ± 4.3% to total energy turnover. Overall, NAEUAOD was systematically biased -23.5 ± 16.8 mlO2 (STPD) equivalent/kg bwt below NAEUPLAR. Total energy demand estimated by PLAR was 11.1 ± 5.4% greater than that estimated by AOD and was higher in every horse.
Conclusions: The NAUEPLAR estimates average 40.0 ± 29.6% higher than NAUEAOD and are highly correlated (r2= 0.734), indicating both indices are sensitive to similar changes in NAEU. Accuracy of the estimates remains to be determined. Multiple considerations suggest NAUEAOD may underestimate total energy cost during high-speed galloping, thus biasing low the AOD estimate of NAEU.