Stride length and respiratory tidal volume in exercising thoroughbred horses.

The purpose of this study was to test the hypothesis that locomotor-respiratory coupling in horses may provide a basis for the appropriate matching of lung ventilation to running speed and metabolic power. This was achieved by determining the effect on respiratory frequency and tidal volume in thoroughbred horses of changing running speed while maintaining a constant metabolic demand by also varying the incline of the treadmill. This procedure was performed at three different speeds at two different levels of metabolic rate. It appears that, in response to metabolic demands which are not directly related to speed, horses can control tidal volume independently of stride length but they do not vary respiratory frequency independently of stride frequency. When running at 0 degrees incline, however, they probably do not operate an elevated tidal volume, because of the increased energetic costs such an elevated tidal volume would incur.