The structural and mechanical recovery of soft tissues define joint-level function after acute injury: The healing of Achilles tendon rupture as a model system

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Muscle and tendon have an adaptive, symbiotic biomechanical relationship that is drastically altered following acute tendon injury. Such injuries, e.g. Achilles tendon rupture (ATR), do not only lead to impaired mechanical function in the resultant tendinous tissue, but also to irrecoverable atrophy in the connected muscle in series. As a result, a new homeostatic relationship between muscle and tendon is established during ATR rehabilitation, leading to lasting functional deficits in the lower limb. It remains unclear how this functional and structural develops after ATR, particularly since this imbalance may be influenced by the dependent relationship of the two tissues to each other. Therefore, we aim to assess the function and structure of both constituent tissues and the entire muscle-tendon unit (MTU) in the acute rehabilitation period post-ATR. In doing so, we would be able to determine the underlying biomechanical and structural reasons for the development of the vicious cycle of functional and structural maladaptations that lead to long-term functional deficits. Therefore, the goal of this study is to identify changes in functional and structural properties at the tissue level of the MTU after ATR that best correlate with changes in overall MTU and ankle function during active movement. By developing a technological basis to assess both the three-dimensional mechanical properties of the muscle and the dynamic tissue properties in MTU tissues during gait, this study aims to elucidate the specific adaptations occurring in each MTU constituent tissue during rehabilitation and their contribution to overall functional deficits. The results from this project may offer a basis for the development of new therapies and treatments that attempt to recover pre-injury MTU function and properties. The technologies developed within this project would also serve as a foundation (that is currently unavailable) for an objective, quantitative assessment of the effect of therapies on ATR rehabilitation. Furthermore, the newly developed combination of material characterization by elastography coupled with functional tissue analysis may be relevant for other application, such as rotator cuff tears.