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Abstract
Objective: The aim of this study was to analyze the immediate effect of post-isometric stretching of the hamstring muscles on active mouth opening and pressure pain threshold [PPT] in subjects presenting with trigger points [TrPs] in the masseter muscle.
Methods: Fifty subjects, 31 men and 19 women, aged 24 to 57 years old, participated in this study. Subjects underwent a screening process to establish the presence of TrPs in the masseter muscle as described by Simons et al. (1). Subjects were divided randomly into two groups. Group A was treated with a post-isometric relaxation technique to the hamstring muscle ipsilateral to the masseter TrP and group B was a control group, without any type of intervention. The outcome measures were the PPTs at the TrP and maximum mouth opening. These were assessed pre-treatment and two minutes post-treatment by an examiner blinded to the treatment allocation of the subject.
Results: Within-group changes showed a significant improvement in the PPT [P = 0.01] and in active mouth opening [P < 0.01] in the treatment group, but not in the control group [P < 0.3]. Pre-post effect size was large in the treated group [d > 1]. The experimental group obtained a greater improvement than the control group in both outcomes [P < 0.01 for PPT, P = 0.02 for active mouth opening]. A positive correlation [r = 0.46; P = 0.01] was found between the improvement in the PPT and the improvement in mouth opening in the experimental group, but not in the control group. Within the experimental group, active TrPs showed a greater improvement in both outcomes than latent TrPs.
Conclusions: The present study demonstrated an increase in active mouth opening and a decrease in TrP sensitivity in the masseter muscle in response to the stretch of the hamstring muscles. The hypothesis of a functional relationship between the masticatory and hamstring muscles is presented. Activation of descending inhibtation pathways as a result of the stretching is also discussed.