Analysis of ground reaction forces and sagittal trunk kinematics led us to the hypothesis that the epaxial muscles do not directly stabilize the trunk against the vertical and horizontal components of the ground reaction force. Instead, the epaxial muscles appear to counteract the tendency of the trunk to rebound (flex) in the sagittal plane during the latter half of the support phase.

Dale A. Ritter, Peter N. Nassar, Epaxial muscle function in trotting dogs The Journal of Experimental Biology 204, 3053–3064 (2001)

The primary protractor muscles of the forelimb exhibited activity during the last part of the ipsilateral support phase and the beginning of swing phase, a pattern that is consistent with the initiation of swing phase but not with active protraction of the limb during the beginning of support phase. The m. pectoralis profundus and the m. latissimus dorsi were completely silent during the support phase of the ipsilateral limb when the dogs ran unencumbered and exhibited little or no increase in activity when the dogs carried added mass on their backs. The most likely explanation for these observations is that the ground force reaction vector is oriented very close to the fulcrum of the forelimb such that the forelimb functions as a compliant strut at the shoulder when dogs trot at constant speed on level surfaces.

David R. Carrier, Stephen M. Deban, Locomotor function of forelimb protractor and retractor muscles of dogs: evidence of strut-like behavior at the shoulder The Journal of Experimental Biology 211, 150-162 Published by The Company of Biologists 2008