CORRELATION BETWEEN ELECTROMYOGRAPHIC ACTIVITY AND STRENGTH OF THE FLEXOR MUSCLES OF THE WRIST

Abstract

INTRODUCTION: In analysing the electromyographic (EMG) signal, researchers are often led to interpret variations of the signal as variation in strength of muscle contraction. However, the relationship between the force exerted and the electrical activity of a muscle are not well established. This is due to the considerable variation of data acquisition and processing techniques, the detection site of the muscles analysed, as well as the alterations in their length and the type of contraction. The object of this paper was to evaluate the correlation between electrical activity and muscular strength. METHODS AND PROCEDURES: The flexor muscles of the wrist of the non dominant member of 21 female volunteers (mean age = 21 years, sd = ± 1.5) with no history of skeletal muscle dysfuntion, were analysed. The EMG signal was captured by active surface electrode (DELSYS), over the belly of the muscle in the longitudinal sense of the fibres, having a gain of 10 Hz. The electrode was coupled to a 16 channel 12/36 electromyograph (AqDados - LYNX TECNOLOGIA ELETRÔNICA LTDA), which permitted a gain of 50 Hz. The system bandwidth was 10 to 500 Hz, with an overall gain of 1,000. The data acquisition rate was 1,000 Hz. In order to eliminate possible interferences, a ground electrode was coupled to the distal segment of the forearm. Muscular strength was measured by means of a load cell (model MM/50 kg, KRATOS), placed perpendicularly in the center of the palm surface of the hand. The electrode and the load cell were connected to a 16 channel signal acquisition system (AQDADOS 4.0, LYNIX). The temperature of the room was kept at 23ºC, ± 1. The EMG registers and the strength were collected simultaneously. Result were obtained from the average of three repetitions, shown in RMS and Kgf respectively. The data was collected with the hand fixed to a device which limited the wrist to 45º of flexion and 45º of extension, providing an isometric contraction. RESULTS AND DISCUSSION: The results demonstrated a positive correlation between electromyographic activity and strength of contraction (r = 0.76) in the 45º flexion position, whereas, for the 45º extension position, the correlation was not significant (r = 0.23), considering the sample analysed. The data suggest that there is a positive correlation between EMG and strength, depending on the position of the muscle analysed.