CONTROL OF TORQUE DIRECTION BY SPINAL PATHWAYS AT THE CAT ANKLE JOINT

To study the biomechanics of the calcaneal tendon's complex insertion onto the calcaneus, we measured torque-time trajectories exerted by th e triceps surae and tibialis anterior muscles in eight unanesthetized decerebrate cats using a multi-axis force-moment sensor placed at the ankle joint. The ankle was constrained to an angle of 110-degrees plan tarflexion. Muscles were activated using crossed-extension (XER), flex ion (FWR), and caudal cutaneous sural nerve (SNR) reflexes. Torque con tributions of other muscles activated by these reflexes were eliminate d by denervation or tenotomy. In two animals, miniature pressure trans ducers were implanted among tendon fibers from the lateral gastrocnemi us (LG) muscle that insert straight into the calcaneus or among tendon fibers from the medial gastrocnemius (MG) that cross over and insert on the lateral aspect of calcaneus. Reflexively evoked torques had the following directions: FWR, dorsiflexion and adduction; SNR, plantarfl exion and abduction; and XER, plantarflexion and modest abduction or a dduction. The proportion of abduction torque to plantarflexion torque was always greater for SNR than XER; this difference was about 50% of the magnitude of abduction torque generated by tetanic stimulation of the peronei. During SNR, pressures were higher in regions of the calca neal tendon originating from MG than regions originating from LG. Simi larly, pressures within the MG portion of the calcaneal tendon were hi gher during SNR than during XER, although these two reflexes produced matched ankle plantarflexion forces. Selective tenotomies and electrom yographic recordings further demonstrated that MG generated most of th e torque in response to SNR, while soleus, LG, and MG all generated to rques in response to XER. Previous studies have shown that interneuron s processing afferent information from both XER and SNR differentially excite the MG and LG motoneuron pools. Further, our data demonstrate that forces produced by this differential activation are preserved thr oughout the calcaneal tendon. We conclude that selective activation of the gastrocnemei permits the animal to take advantage of the complex mechanical insertion of MG and LG at calcaneus and, specifically, to g enerate different torques at the ankle joint in response to different reflex activations.