Background <p>“Tennis leg” injuries originate predominantly at the medial gastrocnemius (MG) musculotendinous junction (MTJ). Whether posture-dependent mechanical asymmetries between the MG and lateral gastrocnemius (LG) explain this susceptibility remains unclear. To quantify posture-specific stiffness of gastrocnemius muscle and MTJ with shear-wave elastography (SWE) and to identify mechanical signatures that may predispose the MG to strain.</p> Methods <p>Ultrasound data were obtained from 22 calves, resulting in 44 samples, across three ankle positions: neutral, 15° plantarflexion, and 15° dorsiflexion, with the knee in an extended position. Young’s modulus was sampled at the muscle belly, the MTJ, and proximal 1&#xa0;cm and distal 1&#xa0;cm on both side; each value represented the mean of three trials. Independent t-tests were employed to assess side-to-side differences, while one-way ANOVA (Bonferroni-adjusted, α = 0.05) was utilized to evaluate posture effects.</p> Results <p>Across neutral postures, the MG was stiffer than the LG at the muscle belly (<i>P</i> &lt; 0.05), MTJ (<i>P</i> &lt; 0.001), and at the distal 1&#xa0;cm of MTJ. Dorsiflexion induced the highest stiffness values on both sides, with the MG demonstrating significantly greater stiffness than the LG (<i>P</i> &lt; 0.05). Notably, despite its higher baseline stiffness, the MG showed a smaller contraction-induced percentage increase in stiffness compared to the LG at the MTJ in both plantarflexion (<i>P</i> &lt; 0.05) and dorsiflexion (<i>P</i> &lt; 0.05), indicating a high-stiffness/low-compliance (HNC) profile.</p> Conclusion <p>Dorsiflexion markedly increases the stiffness of the MG while decreasing its proportion to enhance stiffness during contraction. This HNC behavior emphasizes stress at the in tennis-leg ruptures. SWE-derived HNC measurements may assist in identifying at-risk athletes and customizing preventive measures.</p> Trial registration <p>Clinical trial number: not applicable.</p>