DEBAN, S/M*; FISCHBEIN, T; STAKEBAKE, E/F; CARRIER, D/R; University of Utah; University of Utah; University of Utah; University of Utah: Function of the Forelimb Extrinsic Appendicular Muscles in Trotting Dogs

The forelimbs of mammals are attached to the axial skeleton by a complex system of extrinsic appendicular muscles. In this study, we elicited changes in the activation of these muscles with controlled manipulations of locomotor forces during trotting in dogs. The changes in activation, as measured by EMG, were interpreted to reflect the function of each muscle during trotting. To increase activation of the muscles responsible for retraction and protraction of the limb we 1) ran the dogs up and down hill and 2) added rear- and forward directed forces as the dogs trotted. To increase activation of the muscles that produce swing phase we added mass to the wrists of the dogs. To increase activation of muscles responsible for support of the body against gravity we added mass to the trunk of the dogs. Our results indicate that the m. serratus ventralis thoracis, m. serratus ventralis cervicis, and the cranial portion of the m. pectoralis profundus contribute to support against gravity. Most of the extrinsic muscles, however, function to produce retraction or protraction of the forelimb. Several muscles (e.g., m. pectoralis transversus and m. trapezius pars thoracica) function in both protraction and retraction of the limb, depending on the position of the shoulder and the activity of other muscles. Although all of the protractor and retractor muscles have roles in swing phase, most play little or no role in support phase when dogs trot at steady speed on the level. The lack of protractor and retractor activity during the limb support phase of level trotting suggests that there is little or no moment at the fulcrum of the shoulder. This may increase the economy of running and facilitate lung ventilation by reducing muscle forces on the axial skeleton.