Investigação das propriedades do sinal eletromiográfico do músculo bíceps braquial em diferentes níveis de força isométrica voluntària

Abstract

Introduction: Central nervous system (CNS) uses the motor unit recruitment and the rate coding of the recruited units to control the voluntary force exerted by a muscle. Surface electromyography (sEMG) can be used to capture the electrical signal resulting from the summation of the action potentials of the muscle fibers recruited during the force production. The analysis of the variable root mean square (RMS) and median frequency (Fm) has been used to extract information about the control mechanisms of force used by the CNS. Objectives: To investigate the properties of the electromyographic signal through RMS and Fm variables and analyze the power density distribution in frequency bands in isometric force levels equal to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% of maximal voluntary isometric contraction (MIVC). Methods: The signals were obtained bilaterally from the isometric contraction of the biceps brachii muscle of twenty young healthy volunteers in submaximals force levels. An algorithm was developed to identify the ideal section of the electromyographic signal to be analyzed. The variable RMS, Fm and power density distribution in frequency bands of 5 Hz and 10 Hz were calculated. Results and discussion: The variable RMS increased linearly with increasing force, as expected. The variable Fm had increased between 10% and 30% MVIC and than decreased toward high force levels. This behavior can be related to the experimental protocol as described in the literature. The analysis of the power density distribution in frequency bands showed a concentration of power density in the range between 40 Hz and 50 Hz in 10% MIVC. This power density concentration progressively reduced in 20% to 30% of the MIVC and occurs again on the same frequency range in 40% MVIC. This behavior may be related to increased fibers type IIa and IIb to the force production process. From 40% to 90% of the MIVC the concentration in power density in range between 40 Hz and 50 Hz decreases. Conclusions: The variables extracted from the EMG signal showed behavior compatible with the protocols described in the literature containing submaximal contractions at different levels of MIVC. Their variations may be related to differences in experimental protocol. The analysis of power density distribution in frequency bands can provide additional information being related to the recruitment and rate coding of motor units during the increase in voluntary force.