Changes in surface EMG parameters during static and dynamic fatiguing contractions
Introduction
Surface electromyographic (EMG) signals normally show random waveforms, because they represent a sum of action potentials from many independently activated motor units. It has only been possible to estimate a degree of muscular activity based on the average amplitude of surface EMGs in spite of the efforts of many researchers who tried to extract information on muscle functions from EMG waveforms [1]. With the development of computer technology, it has become feasible to apply the technique of frequency analysis to EMG signals. As a result, the power spectrum of the EMG signal was found to shift toward the lower band during prolonged muscle contraction This phenomenon has further been used to estimate localized muscular fatigue [1].
The shift of the spectrum toward the lower band is caused by a decrease in muscle fiber conduction velocity (MFCV) 2, 3, 4, 5. The decrease in MFCV is then due to an accumulation of metabolic byproducts such as lactic acid, which reduces intracellular pH and decreases the excitability of the muscle fiber membrane.
In recent years it has become easy to measure MFCV using a surface electrode array 5, 6, 7, 8, which enables us to directly examine the relationship between MFCV and the power spectrum 2, 3, 9. Many researchers observed a positive correlation between the EMG parameters during static prolonged contraction 9, 10, 11, 12, 13.
Zwarts et al. [12] further reported that the correlation between MFCV and the power spectrum disappears under ischemia. They measured the surface EMG parameters during the recovery period after an isometric prolonged contraction and compared the normal and ischemic conditions. The results showed that the median frequency (MDF) of the power spectrum recovered with time, but MFCV did not go back to the normal level before the ischemia. This finding indicates that MDF does not always change in accordance with MFCV and suggests that the blood flow in a muscle determines the relationship between MFCV and MDF.
Except for the study by Zwarts et al. [12], the relationship between MFCV and the power spectrum has been investigated only under static contraction. It is important to clarify the changes in EMG parameters during dynamic contraction for quantitative analyses of fatigue in sport and labor. Dynamic contraction, which includes the stretching and shortening of a muscle, should enhance the blood flow by the enhanced venous return from the contracting muscle. The enhanced blood flow removes metabolic byproducts and contributes to the inhibition of the decrease in intracellular pH. The difference in the intracellular state may affect the changes in MFCV and MDF. Therefore, in the present study we compared the changes in MFCV, MDF and other EMG parameters during both static and dynamic fatiguing contractions.
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Subjects
The subjects were 19 healthy male adults aged 19–73 years. They were informed of the content and risk of the experiment in advance and gave written agreement to voluntarily participate in the experiments. The body height and body mass are 174.0±4.6 cm and 66.9±4.3 kg (mean±SD), respectively.
Experimental protocol
Prior to the experiment, the maximum voluntary contraction (MVC) force of the knee extensor muscles at 90° of knee joint was determined using an instrumentation chair (Musculator GT-30, OG-Giken, Okayama,
Endurance time
The endurance time of the static contraction ranged from 54–136 s and had a mean 75.7±20.2 s, while that of the dynamic contraction ranged from 84 to 258 s with a mean 149.7±50.9 s. Because the endurance time varied between subjects, we normalized the time in the contraction by making the endurance time as 100%. The endurance time of each subject was divided into 10 equal segments. The values of MFCV, MDF and AMP were resampled at 11 points at every 10% time including the initial and last period of
Variability in the endurance time
Differences in endurance time among the subjects are considered to be caused by individual differences in resistance to fatigue due to contraction properties of the skeletal muscles represented by muscle fiber composition, enzyme activities and differences in metabolic systems. Since changes in properties of these factors depend on time elapsed, it may be better to show the changes in the EMG signals in absolute time. However, it was more convenient to normalize time in order to show the
Conclusions
MDF decreased and AMP increased during both the static and dynamic contractions. These results agreed with the previous studies. On the other hand, MFCV significantly decreased during the static contraction, but did not decrease during the dynamic contraction. This discrepancy is caused by the difference in blood flow, which is maintained during the dynamic contraction and removes the metabolic byproducts. The dissociation between MFCV and MDF during the dynamic contraction indicates that MFCV
Kazumi Masuda received the master's degree in health and sport sciences from the University of Tsukuba, Japan, in 1996. He is currently in the final year of the doctoral program at the University of Tsukuba. His major research interests focus on exercise-induced skeletal muscle plasticity, especially in mechanisms of oxygen transport into muscle tissue and energy production.
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Kazumi Masuda received the master's degree in health and sport sciences from the University of Tsukuba, Japan, in 1996. He is currently in the final year of the doctoral program at the University of Tsukuba. His major research interests focus on exercise-induced skeletal muscle plasticity, especially in mechanisms of oxygen transport into muscle tissue and energy production.
Tadashi Masuda received the ME degree in information engineering in 1978, and the PhD degree in mathematical engineering in 1987 from the University of Tokyo, Japan. In 1978 he joined Industrial Products Research Institute, Tsukuba Science City, Japan, which is reorganized as the National Institute of Bioscience and Human Technology in 1993. He is currently a chief of the Physiological Informatics Division. His research interests are in the instrumentation and analysis of myoelectric and myomagnetic signals for the diagnosis of human motor functions.
Tsugutake Sadoyama was born in Nagoya, Japan in 1941. He received the BPhysEd degree in 1966 from Tokyo University of Education and the PhD degree in 1993 of from Tsukuba University, Japan. He is currently an associate professor of applied physiology in the Department of Kansei Engineering in the Faculty of Textile Science and Technology at Shinshu University. His current research and teaching interests concentrate on the area of psychophysiology in ergonomics.
Mitsuharu Inaki received the PhD degree in health and sport sciences from the University of Tsukuba, Japan, in 1994. After working as an assistant at the University of Tsukuba from 1994 to 1997, he has been affiliated with Seinan Jo Gakuin University in Kyushu, Japan. His major interests include the energy metabolism using 31P-NMR during exercise.
Shigeru Katsuta received the PhD degree in medicine from Kyushu University, Japan, in 1974. He worked as a guest professor in Washington State University from 1974 to 1975. After working as an associate professor at Kyushu University, he has been affiliated with the Exercise Physiology Laboratory at the Institute of Health and Sport Sciences, the University of Tsukuba, since 1979. His major research interests include the plasticity of skeletal muscle with exercise. He is currently the chairperson of the board of trustees in the Japanese Society of Physical Education.