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

This study aimed to elucidate the relationship between morphological and mechanical characteristics of the free Achilles tendon (AT_Free) 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 AT_Free mechanical characteristics, including stiffness and elastic modulus (E-modulus), were examined under low-load passive conditions. Eighteen healthy adults participated. AT_Free 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 AT_Free stiffness and E-modulus. B-mode ultrasound images acquired at a standardized site of the AT_Free 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 AT_Free 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 AT_Free 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).