Muscle activation patterns for reaching: the representation of distance and time

Abstract

1. The timing and intensity of phasic muscle activation were related to the distance of reaching movements of the human arm. We dissociated phasic components of muscle activation from complete muscle activation waveforms by subtracting waveforms obtained during very slow movements. 2. We recorded electromyographic (EMG) activity from elbow and/or shoulder muscles as standing subjects reached forward and upward to targets at four distances. Accuracy was deemphasized and no terminal corrections were allowed. In the first part of the experiment subjects were asked to move at their preferred speed. In the second part of the experiment they were asked to move using a range of speeds. 3. In the first part of the experiment subjects moved faster to more distant targets but they also increased movement time as a nearly linear function of target distance. The slope of this function was very similar across subjects. The phasic EMG waveforms for different distances appeared to be similar in shape but of variable duration. EMG time base was quantified using a correlation technique that identified the time base scale factor that best superimposed a given trace with a template. This technique revealed that the slope of the relation between EMG time base and target distance was not the same for all muscles. 4. In the second part of the experiment, where subjects moved to each target at a range of specified speeds, time base scaling was again significantly different for different muscles. The scaling differed most dramatically between anterior deltoid and medial head of triceps. 5. EMG intensity was more strongly related to movement time than to distance. We quantified the correspondence of distance and movement time to phasic EMG intensity using a multiple regression analysis of all distances and speeds, assuming a power relation. Distance exponents were positive and movement time exponents were larger and negative. This implies that movement time is more important than distance in its relation to EMG intensity.