Respiratory action of the canine deep pectoral muscles

The potential respiratory action of the pectoral muscles is unclear. We studied the effect of electrically stimulated contraction of the deep pectoral muscles on intrathoracic pressure (Pes) and lung volume in 15 supine anesthetized dogs. Changes in Pes and lung volume were also compared with the forelimbs held at the animal's side and elevated 45 degrees to the horizontal plane to alter the mechanical advantage of the deep pectoral muscles. In all animals, bilateral supra-maximal stimulation of the deep pectorals produced negative changes in Pes (-5.9 +/- 0.6 cmH2O) and inspiratory changes in lung volume (324 +/- 44 ml) with the forelimbs elevated. Conversely, in all animals, stimulation of the deep pectorals with the forelimbs at the side produced positive changes in Pes (+ 2.6 +/- 0.2 cmH2O, Plt;0.001) and expiratory changes in lung volume (-113 +/- 16 ml, Plt;0.001). Furthermore, forelimb elevation increased deep pectoral muscle fiber length (mean change 39.9 +/- 7.5% of resting length, Plt;0.001) and reoriented fiber alignment with the rib cage more rostrally. These data indicate that the pectoral muscles exert a respiratory action on the rib cage. Moreover, forelimb posture affects the direction (i.e., inspiratory vs. expiratory) and magnitude of pectoral muscle mechanical action by altering muscle fiber alignment and precontraction length.