Abstract
The unique structure of the Achilles tendon, combining three smaller sub-tendons, enhances movement efficiency by allowing individual control from connected muscles. This requires compliant interfaces between sub-tendons, but compliance decreases with age and may account for increased injury frequency. Current understanding of sub-tendon sliding and its role in the whole Achilles tendon function is limited. Here we show changing the degree of sliding greatly affects the tendon mechanical behaviour. Our in vitro testing discovered distinct sub-tendon mechanical properties in keeping with their mechanical demands. In silico study based on measured properties, subject-specific tendon geometry, and modified sliding capacity demonstrated age-related displacement reduction similar to our in vivo ultrasonography measurements. Peak stress magnitude and distribution within the whole Achilles tendon are affected by individual tendon geometries, the sliding capacity between sub-tendons, and different muscle loading conditions. These results suggest clinical possibilities to identify patients at risk and design personalised rehabilitation protocols.
Highlights
The Achilles tendon is fundamental to human locomotion, and its mechanical properties are an important contributor to athletic ability
The tendon used for Model 3 was the thinnest (CSA: 42.3 mm2), while tendons used in Models 1 and 2 were substantially thicker (89.1 and 90.4 mm2)
Our results demonstrate that the mechanical behaviour of the Achilles tendon is highly complex and affected by a combination of factors including different sub-tendon mechanical properties, individual tendon morphology, and age-related changes in sub-tendon sliding
Summary
The Achilles tendon is fundamental to human locomotion, and its mechanical properties are an important contributor to athletic ability. The Achilles tendon has a spring-like function (Lichtwark and Wilson, 2007), and the timely release of elastic energy is important to facilitate forward body propulsion. The tendon spring operates optimally at relatively high strains as this increases the utilisation of elasticity (Lichtwark, 2005; Lichtwark and Wilson, 2006), but as a consequence, the risk of developing tendon injuries, such as chronic painful tendinopathy or acute tendon ruptures, increases (Magnan et al, 2014; Martin et al, 2018). Allowing for a high degree of elongation during numerous cycles of loading and unloading is challenging for a biological material, given the complex twisted structure of the Achilles tendon.
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