Invariability of the privileged direction of the maximal EMG activity during speed-related activation for reaching in...

Spatial tuning of goal-directed movements is a critical phenomenon for the central nervous system. The use of reliable models is a necessary step for the understanding of the neural mechanisms governing reaching movements. We analysed phasic electromyographic (EMG) activities associated with upper limb reaching movements in the vertical plane (12 targets) in seven healthy subjects during three successive sessions. We tested the hypothesis that one of the fundamental parameters of the directional tuning of phasic EMG activities is the peak of EMG activity. To characterize the tuning of phasic EMG activities for the seven recorded muscles (brachioradialis, biceps, medial and long head of triceps, anterior and posterior deltoid, latissimus dorsi), we digitally compressed the EMG activities corresponding to slow reaches to the same targets into the time frame of the fast EMG traces. Estimates of gravity-related components were subtracted. Peak of EMG activities in the resulting phasic traces were identified for each muscle and each target. The maximal peak EMG amongst the 12 peaks of EMG activity in the sagittal plane was called M Peak EMG. We defined the directional dominance as the ratio of the M Peak EMG divided by the Peak EMG in the opposite direction. The M Peak EMG for each muscle was always found in the same privileged direction of the movement, except for the latissimus dorsi. The directional dominance remained unchanged across recording sessions for the brachioradialis, anterior deltoid and posterior deltoid. However, the directional dominance increased across sessions for the biceps, medial head of triceps, long head of triceps and latissimus dorsi. The invariability of the privileged direction of the M peak EMG was thus observed for six of the seven muscles investigated. Therefore, we suggest that the selection of the privileged direction of the M Peak EMG represents a robust parameter to study the neuromuscular control strategy underlying the specification of movement direction.