Motor unit differentiation and integrated surface EMG in voluntary isometric contraction.

The role of the differentiation of motor units (MU's) in the relation between integrated surface electromydgram and force was studied here for the biceps brachii, in the course of static contraction in man. The global EMG of the biceps brachii was recorded by two silver electrodes fixed to the skin. The MU's activity was simultaneously lead off by three bipolar wire electrodes inserted in three points of the muscle. MU firing frequency and recruitment were estimated by counting the action potentials on the three elementary records. The comparison between the increase of the integrated surface EMG and MU recruitment as a function of force shows that MU recruitment in itself cannot account for the increase in the integrated global EMG, particularly for high values of force. The difference between MU recruitmentand integrated global EMG, which can neither come from a possible MU's synchronisation as was shown in the discussion, is being suggested to be connected to the progressive firing of "phasic" MU's. This type of recruitment may also play an important role in the gradation of isometric force in normal man.