The aim of this study was to quantitatively investigate the effects of force load, muscle fatigue, and extremely low-frequency (ELF) magnetic stimulation on surface electromyography (SEMG) signal features during side arm lateral raise task. SEMG signals were recorded from 18 healthy subjects on the anterior deltoid using a BIOSEMI ActiveTwo system during side lateral raise task (with the right arm 90 degrees away from the body) with three different loads on the forearm (0 kg, 1 kg, and 3 kg; their order was randomized between subjects). The arm maintained the loads until the subject felt exhausted. The first 10 s recording for each load was regarded as nonfatigue status and the last 10 s before the subject was exhausted was regarded as fatigue status. The subject was then given a five-minute resting between different loads. Two days later, the same experiment was repeated on every subject, and this time the ELF magnetic stimulation was applied to the subject’s deltoid muscle during the five-minute rest period. Three commonly used SEMG features, root mean square (RMS), median frequency (MDF), and sample entropy (SampEn), were analyzed and compared between different loads, nonfatigue/fatigue status, and ELF stimulation and no stimulation. Variance analysis results showed that the effect of force load on RMS was significant (
Surface electromyography (SEMG) is a noninvasive technique to measure muscle electrical activity during muscle contraction, which can reflect the functional status of muscles. It has been widely used by clinicians as a diagnostics tool to identify neuromuscular diseases and disorders of motor control and to evaluate and monitor rehabilitation program [
SEMG is composed of action potentials from groups of muscle fibers organized into motor units (MUs) and therefore contains information about the characteristics and physiology of the active MUs [
Many physiological properties of the muscle, including the number of MUs, the peak discharge rates, and MU synchronization, are also affected by fatigue and peripheral stimulation [
Muscle fatigue occurs after a prolonged or repeated muscle activity with a failure to maintain the required or expected force [
Low-intensity low-frequency magnetic stimulation has been shown to induce neuromodulation in humans without causing any pain [
Previous studies have investigated the separate relationships between SEMG signal and force and between SEMG features and neurophysiology of muscle fatigue [
To analyze the changes of SEMG signal with muscle force and fatigue, various SEMG signal characteristics, including amplitude-based features, spectral features, time-frequency features, and nonlinear features of SMEG, have been analyzed during muscle contraction [
This study therefore aimed to quantitatively investigate the effects of different force loads on the RMS, MDF, and sample entropy derived from SEMG signals during the side arm lateral raise task and to compare the different effects between fatigue and nonfatigue status and between ELF magnetic stimulation and no ELF magnetic stimulation. The experiment will be conducted on healthy adults in this study to provide preliminary evidence for future development of alternative rehabilitation programs for alleviating physical fatigue.
18 healthy male subjects (aged 25 ± 3 years) without any known history of neurological or psychiatric disorders were recruited. All subjects were right-handed, according to Oldfield’s Edinburgh Inventory (Oldfield, 1971). Informed and written consent was obtained from each of the subjects after the aims, potential benefits, and risks were explained. The study was carried out according to the Declaration of Helsinki (1989) of the World Medical Association and approved by the Local Ethics Committee of Beijing University of Technology.
During the experiment, the subjects were asked to sit comfortably with the right arm side lateral raise (90 degrees away from the body) as shown in Figure
Illustration of lateral raise task with a subject sitting on a chair.
It has been suggested by clinicians that the anterior deltoid plays an important role in maintaining the lateral raise [
Measurement protocol, timing diagram of the recorded SEMG signal, and stimulus signal.
Measurement protocol
Timing diagram
Stimulus signal
Two days later, the same experiment was conducted with additional 9 SEMG signals. This time, an ELF magnetic stimulation was applied to the subject’s deltoid muscle during the five-minute resting period.
The magnetic stimulation device was developed in our lab with a four-circular coil. The stimulus signal was generated and driven by an ARM microprocessor and power amplifier. The intensity and frequency of stimulation were adjustable between 10 and 40 mT and between 1 and 10 Hz, respectively. In this study, their corresponding values were 30 mT and 6 Hz. There were three 50 Hz pulses within each simulation cycle, and their duty cycle was 50%, as shown in Figure
The recorded SEMG signals from a pair of electrodes were differentially processed. For the SEMG signal recorded at a certain load on the arm, the first 10 s recording was regarded as nonfatigue status and the last 10 s period before the subject was exhausted as fatigue status, as shown in Figure
The interference (raw) EMG contains main frequency of 10~300 Hz [
Three commonly used SEMG features (RMS, MDF, and SampEn) were calculated in this study.
RMS was calculated as
MDF is the frequency value that separates the power spectrum in two parts of equal energy [
Power spectra density
Entropy is a nonlinear measurement of the complexity of SEMG signal. For a given embedding dimension
For the time-series SEMG of length
The value of
The mean, standard deviation (SD), or standard error of the mean (SEM) of lateral raise task duration (the endurance time with a load until the subject felt exhausted) and the SEMG signal features (RMS, MDF, and SampEn) were calculated across all the subjects, separately for different force loads, for the fatigue/nonfatigue status, and for ELF magnetic stimulation/no ELF magnetic stimulation. Analysis of variance was performed using SPSS 22 (SPSS Inc.) to assess the measurement repeatability and the effect of force, fatigue, and magnetic stimulation on SEMG features, with their difference between forces, fatigue/nonfatigue, and stimulation/no stimulation compared. A
The raise duration varied between subjects and with different force loads. As shown in Figure
Lateral raise task duration with different force loads, separately for ELF stimulation and no ELF stimulation. The data was presented as mean ± SD.
ANOVA analysis showed that there was no significant difference between the three repeated measurements for all the SMEG features derived in this study (all
ANOVA analysis showed that the effect of force loads on RMS was significant (
Mean
The differences of RMS, MDF, and SampEn of SEMG signals between fatigue status and nonfatigue status are shown in Figure
Differences (mean ± SEM of difference) of RMS, MDF, and SampEn of SEMG between fatigue and nonfatigue, separately for different force loads and for ELF magnetic stimulation and no ELF magnetic stimulation (
More importantly, the RMS difference between fatigue and nonfatigue gradually and significantly became larger with increasing load forces (both
The three SEMG features (RMS, MDF, and SampEn), their changes with force and their differences between fatigue and nonfatigue were not significantly different between ELF magnetic stimulation and no ELF magnetic stimulation (all
This study investigated the effect of force, fatigue, and ELF magnetic stimulation on SEMG signal features (including RMS, MDF, and SampEn) from the SMEG signals recorded with different force loads applied on the forearm during the lateral raised task. To the best of our knowledge, this is the first comprehensive study to quantify these effects.
As expected, the lateral raise task duration decreased with increased force loads on the arm. Although 60–70% of the subjects improved their endurance after ELF stimulation, the raise duration was not significantly different between magnetic stimulation and no magnetic stimulation. One of the possible reasons might be the fact that a larger sample size is needed in this experiment.
An objective and noninvasive assessment of muscle activity can be indicated by SMEG feature changes with different force loads. It is known that SEMG consists of the weighted sum of the electrical contributions of active MUs and therefore contains information about the characteristics and physiology of the active MUs including their activation and firing rates. During voluntary muscle contractions, the modulation of the firing rates of existing active MUs and the recruitment of new MUs are the two main mechanisms responsible for the maintenance of a specific level of force. Both the force exerted by a muscle and the amplitude of the SEMG depend on the number of recruited MUs and the discharge rate of each active MU. A higher muscle contraction level requires the recruitment of more MUs, resulting in higher RMS of the EMG signal [
Muscle fatigue occurred when the subject was unable to maintain force during a sustained muscle contraction. Our work showed that RMS was larger at fatigue, and MDF and SampEn were smaller in comparison with nonfatigue status. This agreed with a published study [
This study also showed that there was no significant difference in SEMG features and their changes with force between magnetic stimulation and no magnetic stimulation, which corresponded to the nonsignificant difference of raise duration with and without ELF stimulation. Some possible reasons could include the following: the intensity of ELF stimulation was too weak, the duration of stimulation was not long enough, or the ELF magnetic stimulation itself did not have delay effect on the SEMG signal recorded after the magnetic stimulation. Nevertheless, this study has provided preliminary evidence for future development of alternative rehabilitation programs for alleviating physical fatigue.
It is noticed that there was large interindividual variability due to different muscle strength between individuals. However, this preliminary study mainly focused on the within-subject comparison between loads, fatigue/nonfatigue, and stimulation/no stimulation. Besides, ANOVA analysis showed that there was no significant difference in all the SMEG features between three repeated measurements, separately for each load (all
One of the limitations of this study is that the task order (with and without simulation) should be randomized between subjects in the study design. However, it should be acceptable that the effect of the task order (with and without simulation) could be neglected in this particular study because there was a 2-days interval between the tasks.
Due to the variability of muscle characteristics between individuals, there is no simple way to define a precise muscle fatigue threshold. It is known that the amplitude of muscle contraction is often compared to maximum voluntary contraction (MVC), which can be rescaled to % of MVC. However, considering the potential clinical applications of raising arm, it may not be easy and completely safe to obtain the MVCs from patients with movement disorder. Therefore, to simplify the experimental procedure and reduce the study risks, as a preliminary study, the absolute forces were applied in this study. The absolute forces would impose different challenges between individuals, resulting in different duration of the lateral raise, as shown in Figure
In addition, the effect of using different stimulus modes including the waveform, intensity, and frequency could be investigated, as well as the comparison with simultaneous SEMG recording during magnetic stimulation. Finally, as a pilot study, only male subjects were used. In the future, a comparison between male and female subjects is also worthy of further investigation.
In conclusion, our study comprehensively analyzed the effects of force, fatigue, and the ELF magnetic stimulation on SEMG features, which may facilitate better understanding of the underlying physiological mechanisms of muscle activities associated with force, fatigue, and SEMG response to ELF magnetic stimulation.
All procedures performed in studies involving human participants were in accordance with the ethical standards of Local Ethics committee of Beijing University of Technology and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards.
Informed consent was obtained from all individual participants included in the study.
All the authors declare that they have no conflicts of interest.
This study was funded by National Natural Science Foundation of China (81441053) and Beijing Natural Science Foundation (7172015 and 7132028).