A SIMULATION STUDY OF THE ELECTROMYOGRAPHIC VOLLEY AT INITIATION OF RAPID ISOMETRIC CONTRACTIONS IN THE FIRST DORSAL INTEROSSEOUS MUSCLE

Monopolar surface electromyograms (EMGs) of rapid isometric abduction of the first dorsal interosseous muscle (FDI) were initiated from an E MG Volley that was characterized by a negative potential lasting over several tens of milliseconds. An EMG model was developed to study how the EMG volley was generated. EMGs were defined as the linear summatio ns of surface-recorded action potential trains originating from single motor units (MUs). All action potential trains had the same discharge pattern but different recruitment thresholds, depending on the potent ial amplitude. Real action potentials in single MUs in FDI were record ed with a monopolar surface electrode, one of which was used as a prot otype wave in simulation. The model predicted an initial negative pote ntial comparable to that of the EMG volley observed in rapid contracti ons of FDI. Results from our simulation studies suggest that the EMG v olley is caused by at least two independent factors: (1) the negative phase of the action potential is greater in area than the positive one , in which the effect is enhanced by the high discharge rate of many M Us; (2) many MUs are recruited within a short time in an orderly fashi on starting from those with small action potentials to those with larg e ones. (C) 1997 Elsevier Science Inc.