100 Section - Mechanisms of reduced immobilization-induced muscle pain under different electrical muscle stimulation intensity

Thirty-eight Wistar rats were divided into the four groups: Control (CON; normal housing without intervention), Immobilization (Im; bilateral ankle joint immobilization ), High-intensity stimulation (HS; 60% of maximal muscle contraction induced during immobilization), and Low-intensity stimulation (LS; 30% of maximal muscle contraction during immobilization). Electrical stimulation parameters were as follows: intensity were 4.7 mA for HS and 2.5mA for LS, 50 Hz frequency, and a 1:1 duty cycle (2 seconds of contraction/2 seconds of rest), was applied for 15 minutes, 6 days per week, for 2 weeks. Before immobilization, and after 1 and 2 weeks of immobilization, behavioral tests were performed. Then, the gastrocnemius muscles were used for histological and immunohistological analyses.

Results:

After 1 and 2 weeks of immobilization, the pressure pain thresholds in the IM, HS and LS were significantly lower than in the CON. However, the HS and LS both showed significantly higher thresholds compared to the IM. The number of myonuclei and cross-sectional area (CSA) in the HS were significantly higher than those in the IM and LS. The number of macrophages in the HS was significantly lower than in the IM and LS, with no noticeable difference compared to the CON.

Conclusion: Muscle contraction induced by electrical muscle stimulation appears to be effective in preventing immobilization-induced muscle pain; however this effect may be independent of stimulation intensity. High-intensity stimulation partially supports our hypothesis, suggesting that it may prevent muscle fiber atrophy, muscle nuclei apoptosis and macrophage accumulation.