Medial gastrocnemius muscle and aponeurosis shear wave velocity and morphological changes after Achilles tendon rupture: A 1-year follow-up study

Achilles tendon rupture (ATR) alters stiffness of the tendon and other structures within the triceps surae muscle-tendon unit. Although stiffness of the tendon has been studied after rupture, regional adaptations of the medial gastrocnemius (MG) muscle and aponeurosis mechanical properties are unknown. Therefore, we assessed changes in MG muscle and aponeurosis shear wave (SW) velocity and morphology during a 1-year follow-up after unilateral ATR. Twenty-three (17 males, 6 females) participants were assessed for SW velocity of MG muscle and aponeurosis and morphological properties at 2, 6 and 12 months at rest. Linear mixed models were used to investigate the differences between limbs at different time points, and partial correlations controlled for age to explore associations between SW velocity and morphological properties. Regional SW stiffness of the injured MG muscle and aponeurosis were lower at 2 months but recovered by 6 months after ATR. When comparing limbs, MG muscle and aponeurosis SW velocity were lower in the injured limb at 2 months with a mean difference of -0.34 m/s (-0.48 to -0.21 m/s, t = -5.10), and -1.6 m/s (-2.39 to 0.89 m/s, t = 4.38). SW velocity did not differ at the muscle or aponeurosis between limbs at 6 or 12 months. Fascicle length of the MG muscle was negatively correlated with SW velocity of the MG muscle (r = -0.25, p = 0.041) and positively correlated with aponeurosis SW velocity (r = 0.29, p = 0.018). The remodelling of the MG muscle to shorter fascicles might help to enhance stiffness and maintain tension at the muscle.