Musculoskeletal models and simulations allow for the estimation of forces acting on muscles and joints during human movement and athletic performance. In order to improve the accuracy of these models for a specific application, knowledge about subject-specific in vivo properties of human muscle and tendon is needed. This study presents a method for estimating in vivo mechanical properties of human tendon in the lower leg, using a combination of ultrasound imaging and motion capture. Key mechanical parameters — such as tendon stiffness, moment arm, slack length and force-strain relationship — and the contribution of tendon elongation to ankle mobility of the medial gastrocnemius (MG) and soleus (SOL) aspects of the Achilles tendon were obtained in vivo in 8 typically-developed adults, and the applicability of the method on the tibialis anterior (TA) tendon was investigated. In contrast to previous studies using a comparable method, variable tendon moment arm lengths during passive movement of the ankle joint was taken into consideration. As a novelty, the passive mechanical properties of the Achilles tendon were obtained in vivo in 4 hemiplegic post-stroke subjects and compared to the 8 typically-developed subjects. The estimated mechanical parameters of the MG and SOL aspects of the Achilles tendon were consistent with findings in the literature. In order to estimate stiffness of the TA tendon, it was shown that a larger range of motion (ROM) of the foot during the passive rotation experiments is needed. The comparison between typically-developed and hemiplegic post-stroke subjects revealed significantly lower tendon stiffness and slack angle, and significantly higher contribution of tendon elongation to ankle mobility in the post-stroke group. The developed method enables estimation of in vivo mechanical properties of tendon in the lower leg and contributes to improving the accuracy of subject-specific musculoskeletal models and simulations. / Person-specific biomechanical models

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