Reduced intra-tendinous sliding in Achilles tendinopathy during active plantarflexion regardless of horizontal foot position

The Achilles tendon consists of three subtendons with the ability to slide relative to each other. As optimal intra-tendinous sliding is thought to reduce the overall stress in the tendon, alterations in sliding behavior could potentially play a role in the development of Achilles tendinopathy. The aims of this study were to investigate the difference in intra-tendinous sliding within the Achilles tendon during isometric contractions between asymptomatic controls and patients with Achilles tendinopathy and the effect of changing the horizontal foot position on intra-tendinous sliding in both groups. 29 participants (13 Achilles tendinopathy, 16 controls) performed isometric plantarflexion contractions at 60% of their maximal voluntary contraction (MVC), in toes-neutral, and at 30% MVC in toes-neutral, toes-in and toes-out positions during which ultrasound images were recorded. Intra-tendinous sliding was estimated as the superficial-to-middle and middle-to-deep relative displacement. Our results indicate that patients with Achilles tendinopathy present lower intra-tendinous sliding compared to asymptomatic controls. Regarding the horizontal foot position in both groups, the toes-out foot position resulted in increased sliding compared to both toes-neutral and toes-out foot position. We provided evidence that patients with Achilles tendinopathy show lower intra-tendinous sliding compared to asymptomatic controls. Since intra-tendinous sliding is a physiological feature of the Achilles tendon, the external foot position holds promise to increase sliding in patients with Achilles tendinopathy and promote healthy tendon behavior. Future research should investigate if implementing this external foot position in rehabilitation programs stimulates sliding within the Achilles tendon and improves clinical outcome.