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.