Pulmonary-locomotory interactions in exercising dogs and horses

In exercising quadrupeds, limb movement is often coupled with breathing frequency. This finding has lead some investigators to conclude that locomotory forces, associated with foot plant, abdominal visceral displacements or lumbo-sacral flexion, are the primary determinants of airflow generation. Analysis of respiratory muscle electrical activation (EMG) and contraction profiles in chronically instrumented dogs and horses, along with measurements of esophageal pressure (Pes) changes and limb movements, provide evidence that each breath during the exercise hyperpnea is determined by respiratory neuromuscular events. Specifically: (1) Phasic diaphragmatic EMG and tidal shortening are always synchronous with decreases in Pes; (2) decrements in Pes are always associated with inspiratory flow generation; and (3) strict phase coupling between breathing and stride frequency is not obligatory. Thus, although locomotory-associated forces may minimally assist with flow generation, they are not the primary determinants of breathing during exercise.