Manual Diaphragm Therapy and Masseter Muscle Activity in Healthy Young Women: An Exploratory Randomized Study of sEMG, Mandibular Motor Function, and Perceived Stress

This exploratory randomized study examined whether manual diaphragm therapy is associated with masseter muscle activity, mandibular motor function, and perceived stress in healthy women. Forty-two asymptomatic women aged 20–30 years were randomized to an intervention group (n = 20), receiving six sessions over two weeks, or a no-intervention control group (n = 22). Before and after the intervention period, bilateral masseter surface electromyography (sEMG) was recorded at rest and during standardized mandibular motor tasks; mandibular range of motion was measured with an electronic caliper, and perceived stress was assessed using the Perceived Stress Questionnaire. Non-parametric tests with Bonferroni correction were applied. No significant within-group changes were observed in resting masseter sEMG. The only task-related within-group result that remained significant after correction was a reduction in normalized right masseter mean RMS amplitude during protrusion in the intervention group (p = 0.001; adjusted p = 0.032; r = 0.685). Mandibular range of motion increased in both groups, but change scores did not differ between groups. Stress-related outcomes and correlations with sEMG or mandibular motion were not significant after adjustment. These findings do not demonstrate consistent group-specific effects. The isolated within-group finding should be regarded as preliminary and hypothesis-generating and cannot be attributed specifically to manual diaphragm therapy.