In-vivo identification of subject-specific Achilles subtendon twist

The three subtendons of the Achilles tendon (AT) that originate from the muscles of gastrocnemius medialis (MG), gastrocnemius lateralis (LG), and soleus (SOL) exhibit a twisted conformation towards the calcaneal insertional site. The twisted structure has been classified into Type I, II, and III ranging from lowest to highest twist. Currently, there is no in-vivo quantification system for classifying the global axial twist of the AT subtendons. We aimed to utilize the previously validated Ultrasound and electrical STIMulation (USTIM) method to quantify the subtendon twist in-vivo. Fourteen ATs from 12 healthy participants underwent the USTIM assessments at proximal and distal free AT. The twist angles from the subtendon centroids were calculated and compared to those identified from cadaveric studies. In our sample, the mean twist angle were 84° ± 76° for MG, 74° ± 105° for LG, and 50° ± 28° for SOL. Type I twist was the most prevalent, with individual variations of twist magnitude amongst the different subtendons. In high twist tendons, typically MG and LG twisted more than SOL. MG was found to occupy the largest area both at proximal (34% of whole tendon area) and distal locations (35%), with LG occupying 24% proximally and 21% distally, and SOL 23% proximally and 22% distally, leaving approximately 20% of the total tendon area unidentified. The individual variations in subtendon twist and cross-sectional area highlight the complex interactions between subtendons' geometries, and that the twist variations exist on a continuum rather than conforming to three distinct classification categories.