The effects of locomotion on respiratory muscle activity in the awake dog

Using 6 chronically-instrumented awake dogs, we characterized the response of the respiratory muscles to mild and moderate treadmill exercise in terms of (1) the magnitude and timing of the EMGs of the costal and crural diaphragm, triangularis sterni and transverse abdominal muscles, and (2) the concomitant changes in esophageal (PE) and gastric (PG) pressures during treadmill exercise. Dogs wore a bivalved breathing mask which constrained breathing frequency and prevented some of the exercise-induced hypocapnia. Inspiratory and chest expiratory muscle EMGs increased linearly with a 1.5-fold tidal volume (VT) change during exercise. Abdominal muscle recruitment occurred during most exercise levels and exhibited (1) phasic activity coincident with footplant and (2) tonic activation evidenced by a baseline shift in PG and EMG activities. During control, inspiratory and expiratory flow preceded the onset of respiratory muscle EMG activity, but during exercise, electrical activation of these muscles coincided with the onset of mechanical flow. We conclude that phasic and tonic activation of expiratory muscles during exercise in the dog is significantly influenced by both respiratory and locomotory requirements. These patterns of expiratory muscle recruitment may assist inspiration by lengthening inspiratory muscles through decrements in end-expiratory lung volume and by passive recoil of the rib cage and abdominal compartments.