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 movements against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizontal plane. During isometric contractions the direction in which the largest 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, this 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 contractions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachioradialis isometric conditions yielded occasionally increased EMG activity. The change in EMG activity could be attributed completely to changes in motor unit recruitment thresholds leading to proportionate changes in the number of recruited motor units. However, the initial firing rate of motor units at recruitment was the same under both conditions and, therefore, did not contribute to changes in amplitude of surface EMG activity.