The MyotonPRO: a reliable and valid tool for quantifying the viscoelastic properties of a trigger point on the infraspinatus in non-traumatic chronic shoulder pain

Introduction: Clinicians rely on palpation for locating and diagnosing trigger points in muscles. Measuring a trigger point with clinical palpation remains a challenge. There are currently no validated tools available in clinical practice to objectively measure a trigger point.

Method: The presence of a trigger point within the infraspinatus muscle was identified on thirty-five individuals with non-traumatic chronic shoulder pain via palpation according to Travell and Simons criteria. Trigger and non-trigger points were marked within the same muscle and the viscoelastic properties of both points were independently measured twice with the MyotonPRO by two evaluators on two days.

Results: Significant differences were observed when the trigger and non-trigger point (discriminant validity) were compared. The trigger points showed greater tone and stiffness compared to the non-trigger points (tone: 15.30 ± 1.99 Hz vs 13.57 ± 1.76 Hz; stiffness: 270.20 ± 46.96 N/m vs 227.86 ± 43.44 N/m; p < 0.05) and less elasticity (decrement of 1.13 ± 0.21 vs 1.06 ± 0.27; p < 0.05). The reliability of the three viscoelastic properties was found to be excellent for intra- and inter-evaluator reliability (ICC: 0.925-0.984 and 0.918-0.972, respectively) and good to excellent for test-retest reliability (between days) (ICC: 0.770-0.875).

Conclusion: The MyotonPRO can differentiate the viscoelastic properties of a trigger point from a non-trigger point. Our findings support the reliability of this myotonometer. This affordable and portable tool can be used to objectively measure viscoelastic properties of trigger points in the infraspinatus.