In vivo passive mechanical properties estimation of Achilles tendon using ultrasound

A methodology is proposed for estimating Achilles tendon tangent modulus in vivo, to account for its large deformations and non-linear behavior. True stress is found dividing the axial force by the tendon true cross-sectional area (CSA), whose shrinking caused by axial tension is estimated with Poisson׳s coefficient. The true strain is calculated as the integral of incremental deformations along the tendon length change. Triceps surae tendon CSA and ankle moment arm, with the foot at relaxed equilibrium position, are estimated from subject-personalized data. Healthy males (N=19) volunteered for the study. The test consisted of passive ankle mobilization at the dynamometer with 5°/s velocity, from 30° of plantar flexion to the limit of dorsiflexion. Ultrasound was used to track myotendinous junction (MTJ) and tendon elongation, with the probe oriented over the medial gastrocnemius. Non-linear tendon stiffness pattern was observed during the joint range of motion, reaching 200N/mm peaks for the subjects with greater amplitudes of maximum dorsiflexion. The maximum values of modulus of elasticity, calculated from usual engineering stress and strain, (188.56±99.19MPa) were smaller than those reported in the literature for active maximum voluntary contractions tests. Maximum values for tangent modulus from true stress and strain were 312.38±171.95MPa. Such differences are likely to increase in large deformations.