Interactions between unloading reflex and voluntary muscle activity under different instructions

Changes in the instantaneous firing frequency of a separate motor unit (MU) in the unloading reflex and voluntary movement according to instruction, triggered by external mechanical disturbance in the position of the arm, were studied. The activity of m. biceps brachii and m. pronator teres was recorded using surface bipolar electrodes with small area and small interelectrode distance. The correct position of the electrode with respect to the MU was controlled vectorelectromyographically. The probabilities of the appearance of an impulse in successive 10 ms intervals before and after the unloading were determined. The effect of the voluntary movement performed by the subject according to instruction on the reflex response is demonstrated, as well as the opposite effect, i.e. of the type of the external disturbing action on the voluntary movement. Certain problems related to the transcortical reflex response are discussed. It is assumed that some maxima in the summated electromyogram (EMG) which correspond in time to transcortical reflex influences are not due to a higher excitability of the alpha-motoneurones, but they are obtained by grouping of the impulses of many activated tonic MUs whose activity is recorded by recording the summated EMG.