Background <p>Retrocalcaneal bursitis is often accompanied by insertional Achilles tendinopathy. However, the mechanical effects of bursal effusion remain unclear. This study aimed to elucidate the mechanical impact of retrocalcaneal bursal effusion using Thiel-embalmed cadavers, which are known to exhibit mechanical properties comparable to those in vivo.</p> Methods <p>Ten lower limbs from seven Thiel-embalmed cadavers were examined. Ultrasound-guided injection of the preservation fluid was performed to simulate retrocalcaneal bursal effusion. Ultrasonographic evaluation was conducted under three conditions based on bursal thickness: Baseline, Mild (2 ± 0.5&#xa0;mm), and Severe (4 ± 0.5&#xa0;mm). The dynamics of the Achilles tendon insertion during passive ankle dorsiflexion were recorded using ultrasonography. Average curvature and thickness of the Achilles tendon insertion were measured and compared between the conditions and ankle angles.</p> Results <p>In Baseline and Mild conditions, the average curvature increased from plantarflexion to dorsiflexion but remained unchanged in the severe condition. The Severe condition demonstrated a significantly greater average curvature at plantar flexion than the other conditions. The thickness significantly decreased with dorsiflexion only in the severe condition, resulting in a significantly thinner insertion at maximum dorsiflexion compared with the other conditions.</p> Conclusion <p>Our findings suggested that Mild effusion of the retrocalcaneal bursa may not exacerbate the mechanical load of Achilles tendon insertion. In contrast, in the presence of severe effusion, the retrocalcaneal bursa may compress the Achilles tendon insertion even in the plantarflexion position, and during dorsiflexion the tendon showed deformation compatible with increased compression.</p>