ABSTRACT Purpose The Achilles tendon consists of three subtendons with the ability to slide relative to each other. As optimal intratendinous 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 intratendinous 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 intratendinous sliding in both groups. Methods Twenty‐nine participants (13 Achilles tendinopathy and 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. Intratendinous sliding was estimated as the superficial‐to‐middle and middle‐to‐deep relative displacement. Results Patients with Achilles tendinopathy present lower intratendinous sliding than asymptomatic controls. Regarding the horizontal foot position in both groups, the toes‐out foot position resulted in increased sliding compared with both toes‐neutral and toes‐out foot position. Conclusion We provided evidence that patients with Achilles tendinopathy show lower intratendinous sliding than asymptomatic controls. Since intratendinous 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.