In Vivo Classification of Achilles Tendon Sub-Tendon Twist Patterns: A Novel Ultrasound-Based Method Revealing...

The Achilles tendon (AT) comprises three distinct sub-tendons from the medial gastrocnemius, lateral gastrocnemius, and soleus muscles (MG_AT, LG_AT, SOL_AT), exhibiting variable twist patterns. However, standardized and accessible in vivo methods for classifying individual-level sub-tendon torsion patterns remain not fully explored. To examine the distribution of the non-uniform twist structure of Achilles sub-tendons by the combined approach with high-resolution ultrasonography and optical flow analysis, and to investigate the asymmetric twisting structures of the Achilles sub-tendon. Fifty healthy adults underwent bilateral AT ultrasonography. High-resolution transverse AT images were captured from 40 to 30 mm proximally to the calcaneal insertion. Optical flow analysis tracked internal speckle patterns, and spectral clustering segmented the AT into three sub-tendons. Twist angles were calculated for each sub-tendon, and k-means clustering classified torsion patterns. Three clusters with moderate separation were identified: Cluster A (15%) showed SOL_AT-dominant twist, Cluster B (74%) showed MG_AT-dominant twist, and Cluster C (11%) showed LG_AT-dominant twist. Twist angles differed significantly among sub-tendons [F_(2,198) = 19.845, p < 0.001]. Notably, 62% of participants exhibited asymmetric patterns between limbs. The elliptical AT cross-section influenced twist distribution, with sub-tendons having shorter rotational radii showing greater twist angles. Our classification strongly correlated with conventional morphological types [χ² ₍₄₎ = 91.81, p < 0.001]. This novel method enables non-invasive classification of AT sub-tendon torsion patterns, revealing significant individual variations and bilateral asymmetry with potential implications for personalized injury prevention strategies.