The neural control of single degree-of-freedom elbow movements. Effect of starting joint position.

It is known that muscle activation patterns are changed when the force requirements of a task are increased (e.g., moving a heavier inertial load) or when the available muscle force is reduced (e.g., by inducing muscle fatigue). It is not known whether this is true when the torque-producing capability of a muscle is altered. Eight neurologically healthy subjects performed flexion and extension maximum voluntary isometric contractions (MVC) at five different joint positions (10 degrees, 40 degrees, 70 degrees, 100 degrees, and 130 degrees, where 0 degrees is full elbow extension). Flexion MVC increased by 138% and extension MVC increased by 74% as the elbow joint position changed from the most extended to the most flexed position tested. The same subjects then made rapid, 30 degrees elbow flexion movements from each of four starting elbow positions (10 degrees, 40 degrees, 70 degrees, and 100 degrees). Muscle activation patterns for movements made from the more extended positions showed an increased first agonist burst duration and increased latency of the antagonist burst. There was no change in the initial rate of rise of the agonist burst across starting joint positions. Movements made from the most extended starting position were significantly slower and had longer acceleration and deceleration times than did movements made from the more flexed starting positions. The changes in muscle activation patterns were consistent with those seen when the force requirements of a task are increased or the available muscle force is reduced. We hypothesize that a fall in the ratio of available to required muscle forces causes the nervous system to change muscle activation patterns, to increase the ratio. Our results are consistent with this hypothesis.