MUSCLE ACTIVATION PATTERNS DURING 2 TYPES OF VOLUNTARY SINGLE-JOINT MOVEMENT

We examined the systematic variations in the EMG patterns during two t ypes of single joint elbow movements. These patterns may be interprete d as exhibiting rules by which the CNS controls movement parameters. N ormal human subjects performed two series of fast elbow flexion moveme nts of 20-100 degrees in a horizontal plane manipulandum. The first se ries consisted of pointing movements (PMs) from an initial position to a target; the second series consisted of reversal movements (RMs) to the same targets with an immediate return to the starting position. Bo th series showed kinematic and electromyographic (EMG) patterns that f ollowed our previously described speed-insensitive strategy for contro lling movement distance. Kinematic patterns of PMs and RMs were identi cal to about the time of peak PM deceleration. Agonist EMG bursts were also initially the same, but RM bursts ended abruptly in a silent per iod, whereas PM bursts declined more gradually. Antagonist EMG bursts of RMs were later than those of PMs but were not larger, contrary to o ur prior expectation and despite the larger net extension torque durin g RMs. The increase in net RM extension-directed torque that takes the limb back to its initial position appears to be a consequence of redu ced flexor muscle torque rather than increased extensor muscle torque. We propose that rules for movement control may be similar for differe nt kinds of movements as long as they are functionally sufficient for the task. However, even in a single-joint movement paradigm, physics a lone, that is, the knowledge of net muscle torque and limb kinematics, is not adequate to fully predict those rules or the muscle activation patterns they produce. These must be discovered by experiment. The si mplest expression of such rules may not be in terms of torque or kinem atic variables but rather explicitly in terms of muscle activation pat terns.