Intralimb coordination of the paw-shake response: a novel mixed synergy

Intralimb coordination of the paw-shake response (PSR) was studied in five normal and eleven spinal adult cats. Representative extensor and flexor muscles that function at the hip, knee, and ankle joints were recorded, and in six spinal cats the kinematics of these joints were determined from high-speed cinefilm. The PSR was characterized uniquely by mixed (flexor-extensor) synergies. Knee extensor (VL) and ankle flexor (TA) coactivity constituted one synergy, while the second synergy included hip extensors (GM, BF), knee flexors (BF, LG), and ankle extensor (LG). Joint displacements reflected the mixed synergy. Motions at the knee and ankle were out of phase, while motions at the hip were in phase with movements of the knee. Electromyographic burst durations and onset latencies were similar for normal and spinal cats, and in all cycles of a given PSR, the recruitment pattern was consistent for all muscles, except VL. High variability and missing bursts marked the activity of VL in some spinal cats. In PSRs with missing VL bursts, oscillations at the knee joint were not coordinated with cyclic actions at the hip and ankle. From the kinematic records three distinct phases of the PSR were identified: start-up consisted of the initial four to six cycles during which hip, knee, and ankle actions progressively became organized; steady-state included the middle three to five cycles that were characterized by consistent displacement at all three joints; and slow-down comprised the last three to four cycles during which the rate of oscillations slowed, and joint excursions decreased. During steady-state cycles, muscle contractions acted to reverse joint motions at the knee and ankle joints. Thus, knee and ankle extensor recruitment coincided with joint flexion, while joint flexors were recruited during joint extension. Muscle activity at the hip, however, was in phase with displacement. While neural input to muscle is consistent throughout the three phases of the PSR, segment motions can become progressively organized during start-up to achieve stable oscillations. Whether the PSR attains steady-state or not may hinge on the sensitive interplay that occurs between muscle activities and intersegmental mechanical interactions. That kinetic interplay is detailed in the following paper.