Control of single-joint movements with a reversal

We studied the kinematic and electromyographic (EMG) patterns during single-joint elbow flexion movements with a reversal and tested two hypotheses. First, that the amplitude of the second phase of the movement (M2) will be controlled by two different means, a drop in the second flexor burst for a small M2 and an increase in the integral of the extensor burst for larger M2. Second, based on the muscle stretch-shortening cycle (SSC), that movements reversing without a delay will show a larger extensor burst, as compared to movements that reverse after a delay. Changes in EMG patterns with M2 amplitude supported the first hypothesis and could be interpreted within the framework of the equilibrium-point hypothesis. The observations also corroborate a hypothesis that discrete movements represent outcomes of an oscillatory control process stopped at a particular phase. In Experiment-2, even the shortest delay at the target led to a significantly larger extensor burst. However, there were no differences in the peak velocity of M2 with and without the delay. These observations do not support a major role of stretch reflexes in the SSC effects during such movements. However, they are compatible with the idea of peripheral factors, such as peripheral muscle and tendon elasticity, playing a major potentiating role in the SSC.