THE RELATIVE ACTIVATION OF MUSCLES DURING ISOMETRIC CONTRACTIONS AND LOW-VELOCITY MOVEMENTS AGAINST A LOAD

Surface electromyographic (EMG) and motor unit activity were measured in human arm muscles during isometric contractions and during movement s against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizon tal plane. During isometric contractions the direction in which the la rgest EMG activity was measured corresponded to the direction in which motor units had the smallest recruitment threshold, for each muscle. The same was found for movements against an elastic load. However, thi s direction was different for isometric contractions and for movements . Because the magnitude and sign of these changes varied for different muscles, this resulted in a different relative activation of muscles for the two conditions. The amplitude of the surface EMG during contra ctions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachio radialis isometric conditions yielded occasionally increased EMG activ ity. The change in EMG activity could be attributed completely to chan ges in motor unit recruitment thresholds leading to proportionate chan ges in the number of recruited motor units. However, the initial firin g rate of motor units at recruitment was the same under both condition s and, therefore, did not contribute to changes in amplitude of surfac e EMG activity.