Quantification of the timing of continuous modulated muscle activity in a repetitive-movement task.

The timing of muscle activity is commonly measured in studies of motor control. In repetitive-movement tasks, muscle activity may be continuous, and no defined onset or offset of activity may be measured. This does not imply that no timing of muscle activity occurs. Where activity is continuous, this timing will typically be exhibited by modulation of the amplitude of the signal in specific movement phases. The existence of this electromyographic (EMG) timing is dependent upon the existence of EMG amplitude modulation. This paper investigates this relationship in developing a quantification algorithm of EMG timing in a repetitive-movement task. A frequency domain quantification algorithm involving EMG linear-envelope generation is used. An EMG simulation algorithm is used to test this algorithm and determine the minimal amplitude-modulation threshold for timing detection. At five repetitive-movement speeds (25, 50, 75, 100 and 125 cycles of movement per minute), thresholds between 1.558 and 2.326 times maximal to minimal linear-envelope amplitude are required for reliability of timing detection. Analysis of variance indicates that the robustness of the quantification algorithm was not significantly affected by burst width (F = 3.69, p = 0.055) or the underling input timing parameter (F = 0.52, p = 0.992). The phase-lead/lag quantification algorithm represents a useful tool for the analysis motor control via EMG during repetitive-movement tasks.