Locomotor function of the pectoral girdle 'muscular sling' in trotting dogs

In therian mammals, gravitational and locomotor forces are transferred between the forelimb and trunk primarily, or entirely, through the muscles that connect the limb and trunk. Our understanding of this force transmission is based on analyses of shoulder anatomy and on a handful of descriptive electromyographic studies. To improve our understanding, we manipulated the locomotor forces of trotting dogs and monitored the resulting change in recruitment of five extrinsic muscles of the forelimb: m. serratus ventralis thoracis, m. serratus ventralis cervicis, m. pectoralis superficialis transversus, the anterior portion of the m. pectoralis profundus, and m. rhomboideus thoracis. Locomotor forces were modified as the dogs trotted at constant speed on a motorized treadmill by (1) adding mass to the trunk, (2) inclining the treadmill so that the dogs ran up and down hill, (3) adding mass to the wrists and (4) applying horizontally directed force to the trunk through a leash. These experiments indicate that the thoracic portion of the serratus ventralis muscle is the main antigravity muscle of the shoulder during trotting in dogs. Its activity increased when we added mass to the trunk and also when we ran the subjects downhill. In contrast, the cervical portion of the serratus ventralis did not show a consistent increase in activity in response to added mass. Instead, its activity increased when we ran the subjects up hill and added mass to their wrists, suggesting that it functions to stabilize the fulcrum of the forelimb in the cranial-caudal direction during active retraction of the forelimb. The thoracic portion of the rhomboideus muscle also appears to provide this cranial-caudal stabilization during active retraction of the forelimb. The force manipulations indicate that the transverse pectoralis muscle acts to both protract and retract the forelimb, depending on the position of the limb. In contrast, the anterior portion of the pectoralis profundus muscle acts as a retractor of the forelimb during the end of swing phase and the beginning of support phase. We found that adding mass to the trunk did not increase the activity of forelimb retractor muscles, suggesting that the ground reaction force vector passes through, or very near, the fulcrum of the shoulder during a trotting step. Whether or not the functions of these extrinsic appendicular muscles in dogs characterize therian mammals or represent specializations for highspeed, economical running remains to be determined.