Effects of Clenching Intensity and Duration on Pressure Pain Threshold and Masseter-Muscle Stiffness Under Standardised EMG-Based Contraction Exposure.

BACKGROUND: Low-intensity, long-duration clenching is considered more harmful to the stomatognathic system than high-intensity, short-duration clenching; however, their physiological effects under standardised EMG-based contraction exposure remain unclear. OBJECTIVE: To compare the effects of low-intensity, long-duration (LL) and high-intensity, short-duration (HS) clenching on pressure pain threshold (PPT) and masseter muscle stiffness under standardised EMG-based contraction exposure. METHODS: Thirty-six participants completed a randomised crossover study. They performed Task LL (10% maximum voluntary contraction [MVC] for 75 s) and Task HS (50% MVC for 15 s) on separate days, with a washout period of ≥ 7 days. Each task comprised 30 sets with 15-s rests, and equivalent prescribed contraction exposure (intensity × duration × sets). PPT and muscle stiffness were assessed at baseline and 0, 10, 20 and 30 min post-task. Data were analysed using mixed-effects models with Bonferroni-adjusted post hoc comparisons and baseline-adjusted ANCOVA; p < 0.05 was considered significant. RESULTS: All participants (mean age 29.0 ± 6.6 years; 50% female) completed the study. PPT immediately after LL was significantly lower than after HS (baseline-adjusted mean difference, -11.5 kPa; p = 0.026). Muscle stiffness showed a significant time effect only after HS (p < 0.001). Immediately after HS, muscle stiffness was lower than baseline (mean difference, -22.4 N/m; p < 0.001) and lower than after LL (baseline-adjusted mean difference, -20.0 N/m; p < 0.001). CONCLUSION: Under equivalent prescribed EMG-based contraction exposure, PPT was lower after LL than after HS, whereas HS caused transient reductions in muscle stiffness, indicating pattern-dependent responses.