BASIC FEATURES OF PHASIC ACTIVATION FOR REACHING IN VERTICAL PLANES

The purpose of this study was to fully characterize the timing and int ensity of the phasic portion of the electromyographic (EMG) waveform f or reaching movements in vertical planes. Electromyographic activity w as simultaneously recorded from nine superficial elbow and/or shoulder muscles while human subjects made rapid arm movements. Hand paths com prised 20 directions in a sagittal plane and 20 directions in a fronta l plane. In order to focus on the more phasic aspects of muscle activa tion, estimates of postural EMG activity were subtracted from the EMG traces recorded during rapid reaches. These postural estimates were ob tained from activity recorded during very slow reaches to the same tar gets. After subtraction of this postural activity, agonist or antagoni st burst patterns were often observed in the phasic EMG traces. For ne arly all muscles and all subjects, the relation between phasic EMG int ensity and movement direction was a function with multiple peaks. For all muscles, the timing of phasic EMG bursts varied as a function of m ovement direction: the data from each muscle exhibited a gradual tempo ral shift of activity over a certain range of directions. This gradual temporal shift has no obvious correspondence to the mechanical requir ements of the task and might represent a neuromuscular control strateg y in which burst timing contributes to the specification of movement d irection.