Mechanical asymmetries in gastrocnemius stiffness: shear-wave elastography insights into the biomechanics and injury susceptibility of calf muscle strain injuries.

BACKGROUND

"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.

METHODS

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 cm and distal 1 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.

RESULTS

Across neutral postures, the MG was stiffer than the LG at the muscle belly (P < 0.05), MTJ (P < 0.001), and at the distal 1 cm of MTJ. Dorsiflexion induced the highest stiffness values on both sides, with the MG demonstrating significantly greater stiffness than the LG (P < 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 (P < 0.05) and dorsiflexion (P < 0.05), indicating a high-stiffness/low-compliance (HNC) profile.

CONCLUSION

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.

TRIAL REGISTRATION: Clinical trial number: not applicable.