The Modulation of Locomotor Speed is Maintained Following Partial Denervation of Ankle Extensors in Spinal Cats

Speed modulation requires spatiotemporal adjustments and altered neural drive to different muscles. The loss of certain muscles produces changes in the locomotor pattern and functional compensation. However, how the loss of specific muscles affects speed modulation has not been specifically investigated. Here, we denervated the lateral gastrocnemius and soleus muscles unilaterally in seven cats that had recovered hindlimb locomotion following complete spinal transection (spinal cats). Hindlimb locomotion was tested at 10 speeds, from 0.1 to 1.0 m/s, before, 1-2 days, and 1-8 wk after denervation. Six of seven cats performed hindlimb locomotion 1-2 days postdenervation at all speeds, with the exception of two out of those six cats that did not perform stable stepping at 0.9 and 1.0 m/s. All seven cats performed hindlimb locomotion 1-8 wk postdenervation at all speeds. In some cats, at 1-2 days postdenervation, the ipsilateral hindlimb performed more steps than the contralateral hindlimb, particularly at slow speeds. This 2:1 coordination disappeared over time. In three cats, the linear increase in the amplitude of the electromyography of the ipsilateral medial gastrocnemius was reduced with increasing speed early after denervation before recovering later on. Overall, the results indicate that spinal circuits interacting with sensory feedback from the hindlimbs compensate for the partial loss of ankle extensors, retaining the ability to modulate locomotor speed. NEW & NOTEWORTHY We investigated speed modulation after denervating 2 ankle extensors unilaterally at 10 treadmill speeds in spinal-transected cats. Although we observed new forms of left-right coordination and changes in muscle activity of a remaining synergist, modulation of spatiotemporal variables with increasing speed was largely maintained after denervation. The results indicate that spinal locomotor centers interacting with sensory feedback compensate for the loss of ankle extensors, allowing speed modulation.