Evaluation of human muscle hardness is useful for improving muscle condition and avoiding injuries. Recently, it has become possible to evaluate human muscle hardness with ultrasound elastography (shear wave or strain). However, its reliability has not been fully established, for some muscles involved in shoulder injuries like infraspinatus and teres minor muscles. In the present study, we investigated both relative and absolute reliability of ultrasound strain elastography (ultrasound real-time tissue elastography) for measuring hardness of infraspinatus and teres minor muscles using 21 shoulders of 11 healthy volunteers having no history of shoulder pain. To obtain the intraobserver reliability, the measurement of muscle hardness was repeated five times at each condition for each muscle. To examine the change in muscle hardness induced by changing the position, the muscle hardness was measured at two different positions, 30 degree and 90 degree of shoulder horizontal flexion angle. The muscle hardness was defined as the strain ratio of a region of interest in the muscle tissue to a reference material (acoustic coupler). A higher strain ratio indicates grater muscle hardness. Relative reliability was assessed using an intraclass correlation coefficient (ICC(1,3)). Absolute reliability was assessed using the standard error of the measurement (SEM) and the sensitivity of the measurement to true changes was assessed using the minimal detectable change (MDC95). The strain ratio calculated for each muscle at each angle was found to have high intraobserver reliability (ICC range 0.89-0.98, SEM range 0.03-0.08, MDC95 range 0.09-0.23). The strain ratio of both infraspinatus and teres minor muscles was significantly higher at 90 degree than at 30 degree of flexion angle. From these results, we conclude that ultrasound real-time tissue elastography is useful for evaluating the condition of infraspinatus and teres minor muscles.
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