Human involuntary postural aftercontractions are strongly modulated by limb position

Involuntary muscle activations called aftercontractions occur in skeletal muscles following sustained voluntary contractions. They are strongest following high-force voluntary contractions in proximal muscles. Their mechanism is unknown. Some authors have hypothesised that they are dependent on proprioceptive feedback; others believe that they are independent of such influences. These experiments tested this hypothesis by examining the effect of shoulder joint excursion magnitude and direction on aftercontraction amplitude. A 1-min maximal isometric voluntary abduction of the shoulder joint was used to evoke a postural involuntary aftercontraction in the deltoid muscle. During the 20-s aftercontraction which followed the arm was allowed to abduct in the coronal plane and active muscle shortening took place. The maximum amplitude of EMG activity during the aftercontraction of the deltoid muscle was equal to 20-50% of the EMG amplitude of the maximal voluntary contraction. The aftercontraction EMG amplitude grew as the angle of shoulder joint abduction increased. This growth ceased and the activity levelled off if arm movement was blocked. The results showed that the final EMG amplitude reached depended linearly on the final shoulder angle allowed-it did not grow purely as a function of time. Forcible adduction of the arm by the experimenter and consequent lengthening of the muscle caused the EMG of the aftercontraction to fall with decreasing shoulder joint angle. It is concluded that the neural centres controlling the involuntary aftercontraction are strongly modulated by proprioceptive feedback. Results are given as mean (SD) unless otherwise stated.