Location-Dependent Biomechanical Characterization of the Human Achilles Tendon in Diabetic and Nondiabetic Patients

Although diabetes is associated with alterations in the structural and functional properties of soft tissue, the response of the human Achilles tendon to location-dependent variations in both quasi-static and dynamic loading is unclear. This study aimed to characterize the elastic, viscoelastic, hysteresis, and failure properties of the distal, midsubstance, and proximal Achilles tendons in diabetic and nondiabetic patients and to investigate the relationship between biomechanical and clinical observations. Tendons were obtained from patients who underwent above- or below-knee amputation. Dumbbell-shaped specimens were harvested from the three sites. Relaxation tests were performed to determine viscoelastic characteristics. Cyclic loading tests at various frequencies were deployed to determine the dynamic modulus and phase angles. Incremental cyclic loading tests were carried out to investigate the backbone curve and energy dissipation due to hysteresis. Additionally, monotonic loading tests were performed to determine the elastic and failure properties. The results show that biomechanical parameters are not significantly different among the three sites. However, the midsubstance site exhibits significantly higher energy dissipation compared to other sites. Additionally, an increase in cyclic frequency enhances the phase angle, indicating that higher energy dissipation may protect the tendon from high loading rates. Furthermore, an increase in body mass index (BMI) and hemoglobin A1c (HbA1c) is significantly and negatively correlated with stiffness and viscoelasticity, suggesting that improving metabolic health may prevent tendon impairment. These findings may assist in creating more effective therapeutic strategies for tendon repair.