Function of the extrinsic hindlimb muscles in trotting dogs

The extrinsic appendicular muscles of mammals have been suggested to impose parasagittal torques on the trunk that require recruitment of the oblique hypaxial muscles for stabilization. To determine if the recruitment of the protractors and retractors of the hindlimb are compatible with this hypothesis, we monitored changes in the recruitment of eleven muscles that span the hip joint to controlled manipulations of locomotor forces in trotting dogs. The results indicate that the primary retractor muscles of the hindlimb produce a small retraction moment at the hip joint early in the support phase during trotting at constant speed on a level surface. Thus, although the forelimb of dogs appears to function as a compliant strut, the hindlimb functions as a lever early in stance phase. Nevertheless, our results indicate that when dogs run at constant speed on a level surface a primary function of both the retractor and protractor muscles of the hindlimb is to produce swing phase of the limb. When the trotting dogs did net work in the fore-aft direction, by running uphill or downhill or by resisting a horizontally directed force, recruitment of the protractor and retractor muscles of the hip joint increased or decreased in the anticipated fashion. These observations are consistent with the hypothesis that recruitment of the oblique hypaxial muscles in trotting dogs function to stabilize the trunk against torques produced by protractor and retractor muscles of the hindlimb.