Hyperbolic relationship between time-to-fatigue and workload.

The power:time-to-fatigue relationship for high-intensity exercise in man is useful in determining anaerobic work capacity. The purpose of this study was to determine the nature of this relationship in horses. Eight Standardbred horses performed 5 or 6 run-to-fatigue trials on a treadmill. Exercise intensities were chosen to induce fatigue in 30 to 240 s. The order of trials was randomised for each horse, but balanced overall for the first 4 trials. The data for power (independent variable) and time-to-fatigue (dependent variable) were tested for goodness of fit to hyperbolic, linear and exponential equations by nonlinear regression. The best fit to the data was obtained using the hyperbolic relationship t = W'(P- phi PA) where t is the time to fatigue, W' is the anaerobic work capacity, P is the power and phi PA is the critical power value. The values for W' and phi PA were 47,000 +/- 500 J and 2490 +/- 150 watts, respectively. We conclude that the power:time-to-fatigue relationship of horses is hyperbolic and that this relationship may be useful in assessing anaerobic capacity of horses.