Evaluation of the Effects of AC Magnetic Field Exposure on Muscle Fatigue Using Surface Electromyography

Although magnetic field therapy has been applied for various physical conditions, clinical evidence and physiological mechanisms are not sufficiently clear to support the effectiveness of the therapeutic approach of alternating current magnetic fields (AC MFs) for muscle fatigue recovery. This study investigated the acute effects of AC MF exposure (50 Hz, Bmax = 180 mT for 15 min) on muscle fatigue in healthy humans. Healthy right-handed subjects (12 males, 20–27 years, mean = 22.83 years, SD = 1.57 years) participated after signing an informed consent form approved by the university’s institutional review board. Sham control (sham) and MF exposure were conducted using a randomized, double-blind, crossover study design. Handgrip training was performed by repeated 30 kg loading with a handgrip device up to the limit for each trial, and handgrip strength was measured before and after handgrip training. MF was applied to the left forearm for 15 min before handgrip training in a seated position. The conditions of localized muscle fatigue in the left forearm were analyzed using surface electromyography (EMG). Four electrodes were attached to the distal part of the flexor carpi ulnaris muscle (intensity 1), flexor carpi radialis muscle (intensity 2), proximal part of the flexor carpi ulnaris muscle (intensity 3), and brachioradialis muscle (intensity 4). The EMG sampling rate was 1,000 Hz. The raw data were bandpass filtered between 15 and 500 Hz. The energy inclusion line of the EMG waveform during repeated handgrip training was obtained, and the EMG signal during handgrip training was extracted for approximately 2 s. The Welch spectrum was obtained from the EMG signals measured during the handgrip exercise, and the intermediate frequencies for estimating muscle fatigue were calculated using frequency analysis. The results showed that the intermediate frequencies in the EMG power spectrum, which indicate muscle fatigue, were significantly reduced after handgrip training from the baseline values for intensities 1–4. MF exposure significantly suppressed the reduction of these frequencies in intensity 1, and there were significant differences between MF and sham exposure were observed intermittently. However, there was no significant difference between the MF and sham exposure for intensities 2-4. Thus, the effect of MF on intermediate frequencies varied depending on the muscle region. We speculate that the MF-induced electric fields caused eddy currents and thereby induced changes in EMG, suppressing the reduction of intermediate frequencies. Moreover, the maximum number of handgrip repetitions with MF exposure was significantly larger than that with sham exposure. Furthermore, the mean values of handgrip strength immediately after handgrip training were significantly reduced from the baseline values after both MF and sham exposure. Although the mean values of handgrip strength in the MF exposure group were higher than those in the sham exposure group, there was no significant difference between them. For both exposures, these values did not return to the baseline 5 min post-training. Overall, these results indicate that MF may prevent muscle fatigue.