Elasticity mapping of gastrocnemius fascia and myotendinous junction of the medial gastrocnemius during passive ankle and knee dorsiflexion

Background and purpose Muscle strain is a common side-effect of exercise. In this context, many scholars have focused primarily on the biological characteristics of the muscle, generally overlooking the importance of myofascial elasticity. Therefore, the aims of the present study were: (1) to evaluate the intra- and inter-operator reliability of elasticity measurements in the myofascial and myotendinous junction (MTJ) using ultrasound shear wave elastography (USWE); (2) to establish the association between myofascial elasticity and passive joint angles. Methods Twenty-one healthy and physically active adults participated in this study. The elasticity of the medial gastrocnemius muscle fascia (MGF), lateral gastrocnemius muscle fascia (LGF), and MTJ were quantified at different angles using USWE. Results USWE demonstrated excellent intra-operator (ICC = 0.893–0.980) and inter-operator (ICC = 0.907–0.981) reliability for quantifying the elastic modulus of the gastrocnemius fasciae and MTJ. The elasticity of these structures was significantly influenced by joint angles: it decreased with knee flexion and increased with ankle dorsiflexion ( p < 0.05). Across all postures, the MTJ exhibited consistently higher elasticity than the fascial layers ( p < 0.001). When the knee joint was flexed at 90°, there was no significant difference in the elasticity of the MGF compared to the LGF, regardless of the ankle joint angle ( p > 0.05). However, when the knee joint was fully extended, the elasticity of the LGF at plantar flexion (PF) angles of 0° and 50° was significantly greater than that of the MGF ( p < 0.01), while the elasticity was higher in the MGF than in the LGF at dorsiflexion (DF) 20° ( p < 0.01). Conclusions This study confirms the high reliability of USWE for assessing myofascial elasticity and reveals its dynamic regulation by joint posture. The findings provide crucial imaging-based biomechanical evidence that enhances the understanding of myofascial mechanics in vivo , which may guide future evaluation and rehabilitation strategies for muscle strain-related injuries.