Background & Aims
Exercise-induced hypoalgesia (EIH) is a phenomenon characterized by a generalized reduction in pain sensitivity during a single exercise session. The mechanisms underlying EIH involve the activation of the pain modulatory system, encompassing both peripheral and central processes. Motor imagery, which stimulates brain activity in motor areas, and passive exercise, which generates afferent input from joint movements, can induce analgesia. However, the specific impact of muscle contraction alone on EIH remains unclear. Electromyostimulation (EMS) has been investigated for its potential in treating muscle atrophy, suggesting it might also induce systemic analgesia through myokine production. This study aimed to examine the systemic analgesic effects of EMS-induced muscle contraction, without involving brain activity in motor areas or afferent input from joint movements, in healthy young adults, and to explore the relationship between these effects and the peripheral and central processes.
Methods
This cross-over study involved 27 healthy individuals aged 18 to 23 years. All participants were assessed demographic data (age, sex, and body mass index [BMI]), skeletal muscle mass index (SMI) by bioelectrical impedance analysis, and conditioned pain modulation (CPM), an indicator of the pain inhibitory system, before the first intervention. Participants underwent both EMS and sham interventions on different days. EMS was applied on the non-dominant quadriceps with a 30Hz frequency, 300?s pulse duration, duty cycle of 5 seconds on followed by 10 seconds off, for 20 minutes. The amplitude was adjusted to induce muscle contraction without causing joint movement or discomfort. The pressure pain threshold (PPT) was measured at the quadriceps (Quad) and the extensor carpi radialis longus (Arm) before and after interventions to evaluate changes in pain sensitivity. We analyzed differences between and within interventions and examined the correlations between PPT change and SMI or CPM.
Results
PPT significantly increased at the Quad following the EMS intervention but not at the Arm. The sham intervention did not produce significant changes in PPT at either site. Comparing post-intervention PPT values revealed significant differences between the EMS and sham interventions at the Quad, but not at the Arm. Additionally, a strong positive correlation was observed between PPT changes at the Quad and SMI (r = 0.81) following the EMS intervention, with no significant correlation noted with CPM (r = 0.18).
Conclusions
This study elucidated the localized EIH effect induced solely by muscle contraction using EMS in the stimulated muscle. A significant correlation was observed between PPT changes in the stimulated muscle and SMI, but not with CPM, suggesting the possibility of a peripheral analgesic mechanism, such as the secretion of myokines from skeletal muscle. These results suggest that muscle contractions, in the absence of afferent input from joint movements or brain activity related to voluntary movements, produce a localized peripheral analgesic effect confined to the area of muscle contraction.
References
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Presenting Author
Satoshi Ohga
Poster Authors
Topics
- Treatment/Management: Pharmacology: Psychological and Rehabilitative Therapies