Activity of extrinsic limb muscles in dogs at walk, trot and gallop.

The extrinsic limb muscles perform locomotor work and must adapt their activity to changes in gait and locomotor speed, which can alter the work performed by, and forces transmitted across, the proximal fulcra of the limbs where these muscles operate. We recorded electromyographic activity of 23 extrinsic forelimb and hindlimb muscles and one trunk muscle in dogs while they walked, trotted and galloped on a level treadmill. Muscle activity indicates that the basic functions of the extrinsic limb muscles - protraction, retraction and trunk support - are conserved among gaits. The forelimb retains its strut-like behavior in all gaits, as indicated by both the relative inactivity of the retractor muscles (e.g. the pectoralis profundus and the latissimus dorsi) during stance and the protractor muscles (e.g. the pectoralis superficialis and the omotransversarius) in the first half of stance. The hindlimb functions as a propulsive lever in all gaits, as revealed by the similar timing of activity of retractors (e.g. the biceps femoris and the gluteus medius) during stance. Excitation increased in many hindlimb muscles in the order walk-trot-gallop, consistent with greater propulsive impulses in faster gaits. Many forelimb muscles, in contrast, showed the greatest excitation at trot, in accord with a shorter limb oscillation period, greater locomotor work performed by the forelimb and presumably greater absorption of collisional energy.