Physiological Mechanisms Of SEMG Signal Responses To Muscle Fatigue During Isometric Contractions

SEMG signal is the sum of bioelectrical activities that recorded from the skin of working muscle by surface electrodes. The change in SEMG signal could reflect motor units activation patters and the status of muscle fatigue in some certain degree. Because it was non-invasive and local sensitive, the technique of SEMG signal analysis had become very important method in examining and evaluating human muscle function. Numerous investigators confirmed a progressive spectral compression toward lower frequencies during sustained contraction. It has been named as "spectrum shift". These spectral changes are generally attributed to a progressive decrease in muscle fiber conduction velocity with the accumulation of H+ in muscle in the development of muscle fatigue. However, there are a few other experimental studies revealed it was not all.The goal of our study was to explore the physiological mechanisms of SEMG signal responses to muscle fatigue during isometric contractions. In this research, four experiments had been designed to study the patterns and characteristics of SEMG signal by linear and nonlinear signal analysis methods during the short period of recovery, ischemic forearm exercise, sustained and intervallic maximal voluntary contraction of biceps brachii in twenty-eight human subjects.The results indicated that MPF restituted rapidly in the short recovery period after muscular fatigue. It recovered 26.5% in 2 sec and 87.7% in 30 sec after the exercise. The change velocity of MPF was not significant (P>0.05) during ischemic forearm exercise by artificially hold fully back blood flow at brachial artery in different load tensions. During sustained maximal voluntary contraction, MPF, Kolmogorov Entropy and Lempel-Ziv Complexity decreased rapidly in the beginning of exercise and leaved off at 30sec, which was discovered firstly. But iEMG rose speedily in the start and fell in a short time. The change of MVC was also rapid decline in the begin and became slow after 40 sec. There was a great difference between the changes of these indexes of SEMG signal and H+ accumulation during sustained maximal voluntary contractions. Whereas, the change pattern of MPF most resembled the firing pattern of single motor units during sustained maximal voluntary isometric contractions that been detected by Marsden and Bigland-Ritchie et al. in the past time. The decreasing of Kolmogorov Entropy and Lempel-Ziv Complexity, indexes of nonlinear analysis, indicted the synchronization of motor units. It validated the results of the study of Wangjian et al in the lumbar paraspinal muscle.Our results suggested the accumulation of FT in muscle was not the only factor that affected "spectrum shift" during muscle fatigue. The decline of the firing rate of motor units, the synchronization of motor units and an inadequate CNS drive may be the important mechanisms that attributed to the change of SEMG signal during isometric contractions.