Simultaneous control of two rhythmical behaviors. I. Locomotion with paw-shake response in normal cat.

We investigated the ability of normal cats, trained to maintain a constant position while walking on a treadmill, to combine the paw-shake response with quadrupedal locomotion. Hindlimb paw-shake responses were elicited during walking after the right hindpaw was wrapped with tape. To assess intralimb and interlimb coordination of the combined behaviors, electromyographic (EMG) recordings from forelimb extensor muscles and from selected flexor and extensor muscles at the three major hindlimb joints were correlated with joint motion by using high-speed, cinefilm analysis. When paw shaking was combined with walking, the response occurred during the swing phase of the taped hindlimb. To accommodate the paw-shake response, swing duration of the shaking hindlimb and of the homolateral forelimb increased and was followed by a brief recovery step. Concurrently, to compensate for the response, stance durations of the contralateral forelimb and hindlimb increased. The magnitude of these adjustments in interlimb coordination was influenced by the number of paw-shake cycles, which ranged from one to four oscillations. Transitions between the muscle synergies for the paw-shake response and swing were smooth in the shaking limb. Early in the swing phase, when the flexor muscles were still active (F phase), the paw shake was initiated by an early onset of knee extensor activity, which preceded extensor activity at the hip and ankle. This action provided a transition from the general reciprocal synergy between flexor and extensor muscles of locomotion to the mixed synergy that is typical of the paw shake (30). Following the last paw-shake cycle, an extensor synergy initiated the E-1 phase of swing, and the resultant joint motion was in-phase extension of the hip, knee, and ankle to lower the paw for stance. Average cycle period and burst duration for muscles participating in the paw-shake response were similar to those reported for normal cats assuming a standing posture (28, 30). The average number of paw-shake cycles, however, decreased from eight to three when the response occurred during walking, suggesting that the response was truncated to provide for continued locomotion. Further, hip motion was variable when the paw shake was combined with swing, and sometimes the hip failed to oscillate and its trajectory was similar to that of an unperturbed swing phase. When hip joint oscillations occurred during the paw-shake response, they were in-phase with ankle motions.(ABSTRACT TRUNCATED AT 400 WORDS)