Angle-specific stiffness profiles of the achilles tendon and Triceps surae muscles: a continuous characterization

The stiffness of the triceps surae (TS) muscles during passive stretch has been increasingly studied using shear wave elastography (SWE). However, the continuous behavior of the Achilles tendon (AT) and its interaction with the TS muscles remain poorly understood, limiting understanding of tissue mechanics. This study aimed to characterize the angle-specific stiffness profiles of the individual TS muscles-the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL)-and the AT during passive ankle dorsiflexion under identical experimental conditions. In this cross-sectional study, the ankles of 24 healthy university track and field athletes (age, 20.2 ± 1.3 years) were passively moved by a dynamometer from 30° of plantar flexion (PF) to 80% of maximum dorsiflexion (DF) at 1°/s (up to 25° DF). Shear wave velocity (SWV), an index of tissue stiffness, was measured at four sites (MG, LG, SOL, and AT). The percentage change in SWV from the -30° baseline was analyzed at 5° increments up to the neutral (0°) position using two-way analysis of variance. Significant SWV increases from baseline were observed at 10° PF for the MG, 5° PF for the LG, and 25° PF for the SOL. In contrast, the AT showed significant increases from 25° PF onward, with a rate of change consistently greater than that of all TS muscles across all measured angles. These findings indicate that the AT and TS exhibit asynchronous stiffening patterns: the tendon provides immediate resistance from the earliest range of motion, while the muscles remain compliant until further stretched. Thus, the tendon, rather than the muscle, primarily governs the initial resistive behavior of the muscle-tendon unit during passive stretch. Understanding this angle-specific behavior is essential for optimizing load management in rehabilitation and injury prevention.