Cat hindlimb muscles exert substantial torques outside the sagittal plane.

1. We studied the contributions of several hindlimb muscles to ankle torque in adult cats deeply anesthetized with pentobarbital sodium. Isometric torques were measured with a multiaxis, force-moment sensor connected to the plantar surface of the foot. 2. Individual muscle torques were provoked by using a combination of muscle nerve stimulation and selective denervations and tenotomies. Torques were represented by three orthogonal components; defined as dorsiflexion/plantarflexion, inversion/eversion (rotation about the long axis of the foot), and toe-in/toe-out (rotation about the axis of the tibia). 3. Most of the muscles tested exerted substantial torques about more than one of the orthogonal axes, each of which shared a common origin centered midway between the medial and lateral malleoli. The lateral and especially the medial head of the gastrocnemius muscle exhibited large toe-out torques and eversion torques as well as the classical plantarflexion components. 4. The torque exerted by tibialis anterior was seen to oppose that of both medial and lateral gastrocnemius in each of the three directions. The toe-in and inversion torques exerted by tibialis posterior was opposed in these directions by both peroneus brevis and peroneus longus. Flexor hallucis longus exerted approximately 10 times more plantarflexion torque than did flexor digitorum longus; therefore, these two muscles cannot be considered pure synergists. 5. The major plantarflexors and dorsiflexor of the cat ankle joint contribute substantial torques outside the sagittal plane. Their opposing torques lead to increased joint stiffness; the net effect of coactivation of these muscles causes ground reaction forces oriented so as to maintain stability during quadrupedal stance.