Morphology-Mechanics Links of the Free Achilles Tendon Revealed by Three-Dimensional Magnetic Resonance Imaging and Ultrasound Texture Analysis.

This study aimed to elucidate the relationship between morphological and mechanical characteristics of the free Achilles tendon (ATFree) in vivo. The morphological features were characterized in terms of overall dimensions (length and cross-sectional area) and internal fascicular organization (longitudinal alignment and transverse structure). Their associations with ATFree mechanical characteristics, including stiffness and elastic modulus (E-modulus), were examined under low-load passive conditions. Eighteen healthy adults participated. ATFree length and cross-sectional area (CSA) were quantified from MRI-based three-dimensional reconstructions with the ankle positioned at 20° plantar flexion and 20° dorsiflexion. Passive ankle torque was measured to estimate ATFree stiffness and E-modulus. B-mode ultrasound images acquired at a standardized site of the ATFree were analyzed digitally using a two-dimensional Fourier transform-based framework, from which the mean horizontal and vertical peak spatial frequency components (PSF-H and PSF-V; reflecting fascicle longitudinal alignment and transverse structure respectively) were extracted as textural parameters. Simple linear regression revealed that ATFree stiffness (515 ± 119 N·mm-1) was positively predicted by CSA (β = 0.526, p = 0.025) but showed no significant association with tendon length or E-modulus. In contrast, E-modulus (485 ± 78 MPa) was negatively associated with PSF-H (β = -0.540, p = 0.021), whereas PSF-V was not predictive and exhibited minimal interindividual variation. These findings suggest two distinct morphology-mechanics links in the ATFree under low tensile load: greater CSA is associated with higher stiffness, whereas more longitudinally aligned fascicles (reflected by lower PSF-H) correspond to a higher apparent material property (E-modulus).