Adaptive Changes in Locomotor Activity Following Botulinum Toxin Injection in Ankle Extensor Muscles of Cats

The present study investigated the adaptations made in motor behavior following a temporary reduction in ankle extensor activity in the walking cat. Temporary muscle weakness was induced by injecting botulinum toxin into the lateral gastrocnemius (LG), plantaris (PL), and soleus (SOL) muscles, or SOL alone. The medial gastrocnemius (MG) muscle was not injected. Adaptations in the level of muscle activity were recorded using chronically implanted electromyographic (EMG) electrodes. Serial recordings were made prior to botulinum toxin injections and for several days following the injections. Kinematic analysis of ankle joint movements was made from video records to assess the impact of the botulinum toxin injections on the function of the ankle joint during walking. Following injection of the LG, PL, and SOL muscles with botulinum toxin, the amplitude of the MG burst increased over a period of a few days to a week. This increase was similar to the previously reported changes produced in MG following transection of the nerves serving LG, PL, and SOL. Following the weakening of the ankle extensor muscles, there was a temporary deficit in ankle function during walking as evidenced by a marked increase in the amount of ankle flexion that occurred at stance onset. This functional deficit recovered relatively quickly and was not associated with a return of the EMG pattern to the preinjection pattern. After recovery from the initial injections, a second injection of botulinum toxin into SOL alone was performed. No functional deficits were observed in the ankle movements during walking following this second injection. However, weakening SOL produced increases in the burst amplitudes of the MG, LG, and PL muscles over a period of a few days. This suggests that normal movements at the ankle during walking can be generated with more than one pattern of ankle extensor activity and that there is flexibility in how the necessary torque is produced. A final procedure, transection of the nerves serving LG, PL, and SOL, failed to produce any functional deficits in ankle movements. The implication is that adaptations to the neural control of ankle extensor activity that were induced by the initial procedure persisted after the recovery of the injected muscles and were sufficient to compensate for the subsequent challenges.