MUSCLE ACTIVATION PATTERNS FOR REACHING - THE REPRESENTATION OF DISTANCE AND TIME

The timing and intensity of phasic muscle activation were related to t he distance of reaching movements of the human arm. We dissociated pha sic components of muscle activation from complete muscle activation wa veforms by subtracting waveforms obtained during very slow movements. We recorded electromyographic (EMG) activity from elbow and/or shoulde r muscles as standing subjects reached forward and upward to targets a t four distances. Accuracy was deemphasized and no terminal correction s were allowed. In the first part of the experiment subjects were aske d to move at their preferred speed. In the second part of the experime nt they were asked to move using a range of speeds. In the first part of the experiment subjects moved faster to more distant targets but th ey 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 simil ar in shape but of variable duration. EMG time base was quantified usi ng a correlation technique that identified the time base scale factor that best superimposed a given trace with a template. This technique r evealed that the slope of the relation between EMG time base and targe t distance was not the same for all muscles. In the second part of the experiment, where subjects moved to each target at a range of specifi ed speeds, time base scaling was again significantly different for dif ferent muscles. The scaling differed most dramatically between anterio r deltoid and medial head of triceps. EMG intensity was more strongly related to movement time than to distance. We quantified the correspon dence of distance and movement time to phasic EMG intensity using a mu ltiple regression analysis of all distances and speeds, assuming a pow er relation. Distance exponents were positive and movement time expone nts were larger and negative. This implies that movement time is more important than distance in its relation to EMG intensity.