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

ABSTRACT 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.