Author(s): ANDO, S., TERADA, H., SATO, R., NINOMIYA, W., IMAI, H., SUDO, M., Institution: UNIVERSITY OF ELECTRO-COMMUNICATIONS, Country: JAPAN, Abstract-ID: 1353

INTRODUCTION:
Electrical muscle stimulation (EMS) is an effective alternative to voluntary exercise for enhancing muscular function. However, the associated electrical stimuli often induce pain, which can act as a significant psychophysiological stressor for novice users. While pain habituation through repeated exposure is generally expected, the underlying neurophysiological mechanisms and systemic stress responses during this adaptation process remain poorly understood. This study investigated how different intensity protocols affect the adaptation process in EMS-naive individuals.
METHODS:
Twelve healthy males (Exp. 1) and ten healthy males (Exp. 2) participated in three 20-minute EMS sessions. EMS was applied bilaterally to the abdomen, glutes, thighs, and lower legs (4 Hz, pulse width: 0.25 ms). In Exp. 1, intensity was adjusted to the maximal tolerable level in each session. In Exp. 2, intensity was fixed at the level determined during the first session. Pain intensity was assessed via a visual analogue scale (VAS). Physiological markers, including electroencephalogram (EEG: alpha and beta power), heart rate variability (HRV: LF/HF ratio), and salivary cortisol (Exp. 1 only), were measured pre- and post-session. Data were analyzed using repeated measures ANOVA.
RESULTS:
In Exp. 1, beta power at Pz significantly increased post-EMS (p = 0.034). VAS scores remained high and unchanged across the three sessions, indicating that pain habituation does not occur when intensity is consistently pushed to the tolerable limit. Salivary cortisol showed no significant changes. In Exp. 2, both alpha and beta power increased post-EMS (p = 0.042, p = 0.074), while VAS scores significantly decreased over the sessions (p < 0.001), demonstrating clear pain habituation. The LF/HF ratio remained stable in both experiments.
CONCLUSION:
Our findings suggest that psychophysiological adaptation to EMS is highly dependent on the intensity protocol. Fixed-intensity stimulation (Exp. 2) facilitates pain habituation and increases alpha/beta power, likely reflecting a reduction in perceived threat and stabilized cortical arousal. Conversely, maintaining maximal tolerable intensity (Exp. 1) prevents habituation and selectively enhances beta activity, suggesting sustained nociceptive processing or higher cognitive load. Importantly, the stability of cortisol and LF/HF ratios suggests that EMS-induced pain does not trigger a significant systemic endocrine or autonomic stress response, even in novices. We conclude that while EMS is a low-stress modality for systemic functions, intensity progression should be managed carefully to balance the training stimulus with sensory adaptation.
REFERENCES: [1] Akagi et al. (2023) J Sports Sci [2] Ando et al. (2024) J Physiol