Biomechanical and electromyographic analyses of muscle strengthening

Abstract

Eleven male subjects completed a six-week strengthening program on their right quadriceps muscles, which consisted of 30 maximal voluntary contractions (MVC) per day, 3 days a week, for 6 weeks. The maximal isometric strength of the quadriceps muscles was measured by a computerized dynamometer (Cybex "Norm"), with the hip and knee at about 60 flexion. Surface electromyography (EMG) of the rectus femoris muscle was acquired at MVC and at two pre-set submaximal torques of 100 Nm and 150 Nm. The EMG signal was band-pass filtered at 5 to 350 Hz, and sampled at 5 kHz. EMG was measured initially before the strengthening and subsequently at the 2nd, 4th and 6th weeks of the strengthening program. Integrated EMG (IEMG) and root mean square (RMS) voltage corresponding to MVC and torque levels at 100 Nm and 150 Nm were analyzed. The EMG data at the pre-set submaximal contractions were normalized twice to facilitate comparison between sessions. Spectral analysis was performed by the use of Fast Fourier Transform and the median frequency (MF) was obtained from the power density spectrum. There was a significant (p=0.001) increase in the strength of the right quadriceps muscles (mean is 22%). However there was no significant change in the EMG at MVC. Therefore the role of neural adaptation in the process of muscle strengthening was not substantiated in this study. Both IEMG and RMS voltage at 150 Nm, after double normalization, had significantly decreased after the strengthening program (p = 0.024). Moreover, there was a right shift of the torque-EMG relationship. It implied that the muscle became more efficient after the six-week strengthening program. It also showed that the decrease in EMG under a reasonably strong submaximal contraction at predetermined torque level in the course of strengthening was an indication of improvement in the quadriceps' mechanical output. This result can be used to develop a test protocol that uses submaximal torque level to monitor the improvement of muscle strength in the quadriceps during strengthening. This protocol is safer for persons undergoing rehabilitation when compared with the commonly used test protocol using percentage of MVC because the test of MVC may be difficult or dangerous for persons with lower limb injury. This test protocol has the potential to be applied in rehabilitation especially in monitoring the progress of muscles whose strength cannot be easily measured by mechanical means. Furthermore, the double normalization technique can be applied in future studies if the test protocol with predetermined torque level is used.