Layer-Specific Shear Wave Elastography Reveals the Tissue Origin of MyotonPRO-Derived Muscle Stiffness

Background: Shear wave elastography (SWE) and myotonometry are commonly used to assess muscle stiffness, but they reflect different aspects of tissue mechanics. SWE allows layer-specific evaluation, whereas myotonometry captures the response of a broader tissue complex. The contribution of individual layers to MyotonPRO-derived stiffness remains unclear. This study examined the relationship between MyotonPRO stiffness and layer-specific SWE measurements in the tibialis anterior muscle. Methods: In this cross-sectional study, 56 healthy adults were evaluated. Muscle stiffness was assessed using MyotonPRO and SWE across three regions of interest: superficial tissues including skin, subcutaneous tissue, and deep fascia (ROI 1); muscle tissue excluding fascia (ROI 2); and a composite region including all tissues from the skin surface to a depth of 2 cm (ROI 3). Relationships were analyzed using Spearman correlation, multiple linear regression, and Bland–Altman analysis. Results: Moderate but consistent correlations were observed between MyotonPRO stiffness and SWE values in ROI 2 (ρ = 0.516, p < 0.001) and ROI 3 (ρ = 0.550, p < 0.001), whereas no significant association was found in ROI 1. SWE values in ROI 2 and ROI 3 were significant predictors of MyotonPRO stiffness, explaining 38% of the variance. Bland–Altman analysis demonstrated moderate agreement with a systematic bias toward higher MyotonPRO values. Conclusions: MyotonPRO-derived stiffness reflects a composite mechanical response of muscle and surrounding tissues, with the strongest associations observed in regions encompassing both muscle and fascia. These findings highlight the importance of tissue composition and ROI selection in stiffness assessment and may facilitate more accurate interpretation of MyotonPRO measurements in musculoskeletal diagnostics and rehabilitation.