A STUDY OF THE REPRODUCIBILITY OF 3 DIFFERENT NORMALIZATION METHODS IN INTRAMUSCULAR DUAL FINE WIRE ELECTROMYOGRAPHY OF THE SHOULDER

The purpose of this study was to determine the most appropriate method of normalisation for dual fine wire electromyography of shoulder musc les. Five healthy subjects were studied, with one muscle investigated in each subject (2 supraspinatus, 2 infraspinatus, 1 subscapularis). T hree dual fine wire electrodes were inserted 1 cm apart around the rec ognised insertion points. Each subject performed five types of cyclic exercise on an isokinetic muscle dynamometer with an isometric maximal voluntary contraction (MVC) being performed before and after the exer cise protocol. The EMG signal was normalised using each of the MVC vol tage, the peak voltage and the whole-cycle mean voltage. There was a c onsiderable difference (5-143%) between the MVC signals pre- and post- protocol, although no systematic trend was demonstrable. The overall m ean between electrode variation in the normalised signal measured at t he peak of the cycle ranged from 48-71% when normalised to pre-protoco l MVC, but only 4-13% when normalised to the peak voltage and 9-17% us ing the whole-cycle mean voltage. However the pattern of activation wi thin the movement cycle, which was preserved by normalisation using th e peak or mean signal, was consistent between different electrode posi tions. It was concluded that the EMG signal depended on electrode posi tion even when near the recognised insertion point, and that the MVC s ignal is highly variable in magnitude between electrodes and between p re- and post-protocol measurements. (C) 1998 Elsevier Science Ltd. All rights reserved.