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A cinematographic comparison of treadmill and overground performances by the cat revealed considerable flexibility in the neural control program for locomotion. For the single limb's step cycle, swing duration (time foot is off the surface) was approximately equal in both situations, as has been found previously. Subcomponents of the swing (flexion duration and timings between joints during the transition from flexion to extension) differed, however. Interlimb timings also responded to situation. The interval between touchdown of one hindlimb and the ipsilateral forelimb was reduced for treadmill stepping. An ipsilateral coupling interval also differed that had been previously reported to involve propriospinal activity, the time for onset of extension during the swing phase of the hindlimb to the onset of flexion at the beginning of the swing phase in the forelimb. Segmental afferent input, visual and other suprasegmental inputs, and motivational variables probably all contribute to the separation of treadmill and overground timing profiles.

1. Cinematographic measurements were made of stepping by cats on a motor-driven treadmill, both normally and 2-3 wk after deafferentation of the LH (left hind) limb. 2. After surgery, rhythmic cycling of the LH limb was blurred whether the leg was dragged, as by some cats, or if it was lifted from the surface, as by others. 3. Interlimb coordination was also blurred with respect to normal, although distinct rhythms were still seen. The RH (right hind) limb descended prematurely and, in the walk, had a prolonged stance phase. The interval between touchdowns of hind- and forelimb on the left side no longer equaled that interval on the right side. 4. As is true for a normal cat, if the LH-deafferented animal stumbled, relatively normal single and interlimb cyclings were regained after several strides. 5. By kinematic analysis, force deficits were found in the deafferented LH limb both during the stance, when extensors should be most active, and the swing, when the limb failed to attain a normal position above the surface of the belt. Weight bearing by the LF (left fore) limb was altered in some animals. 6. At high speed, mean LH stance duration failed to decrease in the normal fashion. Inter-limb timings were reset to greater extent than in low-speed walking, as if the LH limb was being used only minimally. 7. It was concluded that both rhythm and force were impaired in the deafferented limb and also in the three intact limbs, whose weight bearing had to compensate for LH weakness. The changed mechanical demands after surgery were probably met by interactions between the remaining afferent input and central pattern generators so as to secure fairly effective and expedient locomotion.

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