Goal-directed arm movements: I. Analysis of EMG records in shoulder and elbow muscles.

The coordinated control of shoulder and elbow muscles during goaldirected arm movements has been studied. Timing and amplitude of the electromyographic activity of 13 muscles and muscle parts have been analyzed. Triphasic alternating agonist-antagonist activity commonly associated with acceleration and deceleration of the limb has been found in eight shoulder muscles or muscle parts. Of these muscles, the pectoralis major (pars clavicularis), the deltoideus (pars anterior and posterior), and the latissimus dorsi act on the humerus and can be regarded as prime movers of the upper arm. The serratus anterior and the trapezius (pars descendens, transversalis, and ascendens) act on the scapula. In these scapular muscles, agonist activity similar to that in the prime movers has been found. This indicates an important role of scapular muscles during acceleration of the arm. Significant diferences in timing between synergist muscles have been demonstrated: Activity of the latissimus dorsi precedes the deltoideus pars posterior by up to 62 ms. Only minor differences in timing were observed between scapular muscles and prime movers. In biarticular (shoulder and elbow) muscles and in muscles acting only on the elbow, continued activity has been observed throughout the movement. This activity yielded coactivation of functional antagonist muscles. The movement direction in which the largest activity occurred is consistent with the function of overcoming a resistance in the elbow resulting from the cocontraction found.