Shear wave propagation in the Achilles subtendons is modulated by helical twist and non-uniform loading

The triceps surae is composed of the medial gastrocnemius, lateral gastrocnemius and soleus muscles. Each muscle inserts onto a subtendon that undergoes helical twist prior to insertion onto the calcaneus. Shear wave tensiometry is a non-invasive technique to gauge Achilles tendon loading, yet it is unknown whether subtendon loading can be resolved using subtendon-specific shear wave speeds. The objective of this study was to examine shear wave propagation in the lateral gastrocnemius, medial gastrocnemius and soleus subtendons of the free Achilles tendon. We expected to show that the helical arrangements of subtendons within the Achilles would modulate wave propagation, and that non-uniform loading between subtendons would elicit nonuniform wave propagation. We created a finite element model of the Achilles tendon and simulated shear wave propagation. We found that helical subtendon twist had little effect on wave propagation speed. When a two-fold stress differential was applied to the gastrocnemius and soleus subtendons, the shear wave speed-axial stress relationship was modulated by adjacent subtendon tension and the amount of overall subtendon twist. These findings enhance the basis for tensiometry in the Achilles tendon and inform causes for variability in shear wave speeds measured using shear wave tensiometry or elastography.