Shear wave elastography provides reliable assessments of stiffness in the skin, fascia, and both superficial and deep muscle tissues

Shear wave elastography (SWE) is a reliable method of estimating stiffness of superficial rectus femoris (RF) muscle at various muscle lengths. Its reliability has not been assessed in deeper muscle regions or its overlying fascia and skin tissue. 20 healthy participants completed two visits in which RF shear wave velocity (SWV) was measured twice, separated by 30mins, at four depths (skin-SKIN, fascia-FAS, superficial muscle-SUP, and deep muscle-DEEP) in three positions (relaxed-REL, neutral-NEU, and stretched-STR). Relative reliability was determined by intraclass correlation coefficients and absolute reliability was assessed by standard error and bias of measurements. SWE reported poor to good relative intra-day reliability (ICC, 0.33-0.82) but good to excellent absolute intra-day reliability (SEM, 0.14-0.25 m/s; bias, -0.12-0.07). Relative SWE inter-day reliability was moderate to excellent (ICC, 0.54-0.91) and absolute reliability was good to excellent (SEM, 0.12-0.21 m/s; bias, -0.05-0.06). Reliability was highest in the order of STR, NEU, REL with regards to position, with SKIN and FAS reliability greater than in muscle. SUP measurements reported greater reliability than DEEP. Therefore, SWE is a reliable method of estimating SKIN, FAS, SUP and DEEP RF tissue stiffness. To optimise repeatability, measurements should be taken from stretched superficial tissue.