Non-uniformity in pre-insertional Achilles tendon is not influenced by changing knee angle during isometric contractions

Achilles tendinopathy remains a prevalent condition among recreational and high-level athletes. Mechanical loading has become the gold standard in managing these injuries, but exercises are often generic and prescribed in a “one-size-fits-all” principle. The aim of this study was to evaluate the impact of knee angle changes and different levels of force production on the non-uniform behaviour in the Achilles tendon during isometric contractions. It was hypothesized that a flexed knee position would lead to a more distinct non-uniform behaviour, due to greater differential loading of soleus versus gastrocnemius, and that this effect would be attenuated by higher levels of force production. 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 (n = 19; mean normalized displacement ratio 6,32, 4,88 and 4,09 % with extended knee versus 5,47, 2,56 and 6,01 % with flexed knee, at 25, 50 and 75% MVC respectively; p > 0,05). From tendon perspective, aside from the influence on muscle behaviour, this might question the mechanical rationale for a change in knee angle during eccentric heel drops. Additionally, despite reaching high levels of plantar flexion force, the relative contribution of the AT sometimes appears to be decreased, potentially due to compensatory actions by agonist muscle groups. These results are relevant for optimizing AT rehabilitation as the goal is to reach specific local tendon loading.