Non-uniform deformation in Achilles tendon is not influenced by a change in knee angle or level of force production during isometric contractions

Abstract

Mechanical loading has become the golden standard in managing achilles tendinopathy. The goal of this study was to evaluate the impact of different levels of force production and knee angle on the non-uniform behaviour in the achilles tendon during an isometric contraction. It was hypothesized that a flexed knee position would lead to a more non-uniform behaviour, due to greater differential loading of soleus versus gastrocnemius in this position, but that this effect would be attenuated by higher levels of force production. Nineteen healthy subjects participated in this study. A high-spatial and high-temporal resolution ultrasound system was used to acquire 2D + time ultrasound images during an isometric contraction at 25%, 50% and 75% MVC in the extended and flexed knee position. Local tendon tissue displacement and normalized displacement ratio of the different tendon layers were calculated. Contrary to the hypotheses, it was found that the non-uniform deformation, i.e. superficial-to-deep variation in displacement with highest displacement in the deep layer, is consistently present, irrespective of the level of force production and knee angle. From clinical perspective, this might indicate the absence of a mechanical rationale for a change in knee angle during eccentric heel drops. Additionally, it was found that despite reaching high-levels of force, the contribution of the Achilles tendon might sometimes be relatively smaller, potentially due to compensatory actions. This is relevant for rehabilitation as the goal is to reach specific local tendon loading. Therefore, the tool used in this study might be ideally placed to monitor local Achilles tendon loading during rehabilitation and individually tailor exercises to reach that goal.