Responses of elbow extensors to landing forces during jump downs in cats.

Forces and displacements at the elbow joint have been related to EMG responses of flexor and extensor muscles during landing from jump downs at heights of 1.2 m to 0.6 m in five cats. Prelanding EMG activity consisted of two prelanding extensor bursts. Onset of both bursts was constant across all jump heights with reference to landing and not to take-off, occurring on average 73 +/- 12 ms and 17 +/- 8 ms prior to ground contact for the lateral triceps. Post-landing EMG activity was less than prelanding activity and was often packaged in three bursts, occurring on average at 18 +/- 6, 34 +/- 8 and 50 +/- 9 ms after touchdown. Other measurements from extensor EMG including burst duration and integrated activity pre- and post-landing were also invariant. Across jump heights, maximum flexion angular velocity and elbow displacement were reached on average 28 +/- 5 and 85 +/- 7 ms postlanding, respectively. Although vertical (y) and horizontal (x) ground reaction forces increased with jump height, torque values at the elbow joint were not significantly different and were small in magnitude. At landing an animal typically experienced a 20 ms flexor torque (0.3 Nm/kg b.wt.) followed by an extensor torque (0.4 Nm/kg b.wt.) that continued for the major portion of elbow flexion. The temporal constancy of the kinematic and kinetic data and EMG activity across jump heights suggests that a generalized motor program can be used to activate extensor muscles at the elbow joint during the prelanding phase of self-initiated jumps. Since the onset of extensor activity is related to landing rather than to take-off, it is hypothesized that this activity is triggered by visual cues rather than by vestibular reflexes. After impact, adjustments for slight differences in post-landing torque about the elbow may be accomplished by intrinsic properties of the activated muscle as well as through segmental reflexes.