THE EFFECT OF EXERCISE ON DIAPHRAGMATIC ACTIVATION IN HORSES

Horses chronically-instrumented with costal diaphragmatic electromyogr aphic electrodes were studied during exercise while unencumbered by a breathing mask. Exercise-associated changes in esophageal (Pes), gastr ic (Pga) and transdiaphragmatic (Pdi) pressures were measured and rela ted to diaphragmatic electromyographic activity (CS EMG) and to left f orelimb impact. In all breaths examined, CS EMG always coincided with decrements in Pes. For all exercise trials, linear increases in CS EMG , Pga and Pdi and linear decreases in Pes, as a function of exercise i ntensity always occurred. During all gaits, breathing frequency (fR) w as entrained with stride frequency (fs) one for one. However, a consta nt phase-coupling relationship between fR and fs, observed when horses cantered and galloped, was absent when horses walked or trotted. We c onclude that biomechanical forces contribute minimally to ventilation in exercising horses, that the diaphragm is always phasically active d uring each breath and its total electrical activity and mechanical out put are proportional to the exercise hyperpnea.