Introduction: This study assessed the effect of polarized low-level laser therapy (PLLLT) on the treatment of myofascial trigger points (MTrPs) in the trapezius muscles. Its effectiveness in pain reduction was compared to low-level laser therapy (LLLT). Methods: Sixty-four patients with MTrPs were randomly divided into 2 groups, namely PLLLT and LLLT. Each patient received treatment for a period of 2 weeks, 5 sessions a week. The intensity of laser irradiation to the skin surface was 6 J/cm 2 . The system exit power was 160 mw at a 755 nm wavelength. The visual analog scale (VAS) for the 1st, 5th and 10th sessions was analyzed through two-way repeated measures ANOVA. Results: Increasing the number of treatment sessions was effective in reducing pain intensity (P > 0.05). The effect of LLLT on pain reduction was significantly greater than that of PLLLT (P > 0.05). Referred pain (RP), the limitation of neck movement (LNM), the presence of muscular taut band (PMTB) and the sensitivity of muscular taut band (SMTB) were reduced significantly by the end of the 10th session, compared with participants' condition at the beginning of the 1st session in both groups. Conclusion: PLLLT and LLLT can effectively treat MTrPs in the trapezius muscles and they reduce RP, LNM, PMTB, and SMTB in particular. However, the effect of the LLLT was significantly greater than that of PLLLT. In accordance with the observed results, LLLT is recommended as an effective method for treating MTrPs.

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