Using a cadaveric model, we evaluated the effect of knee and ankle position on the displacement of the severed ends of an Achilles tendon transected at three different points. In six cadaveric legs the Achilles tendon was severed transversely, then marked with radiopaque wire suture. The distance between the wire markers was measured on radiographs taken in different positions of ankle and knee flexion. Ankle plantar flexion had a statistically significant effect on decreasing the gap between the severed ends of the Achilles tendon. This effect was clinically significant as, on average, the tendon edges were separated more than 20 mm when the ankle was in the neutral position and were apposed when the ankle was in 60 degrees of plantar flexion. With the ankle fixed in 60 degrees of plantar flexion, knee position had no significant effect on the displacement of the severed ends of the Achilles tendon. Overall, the effect of knee flexion was neither statistically significant nor clinically significant, as the increase in displacement of the severed ends of the Achilles tendon was only 3 mm from 0 degrees to 120 degrees of knee flexion. These results suggest that the nonoperative treatment of Achilles tendon ruptures requires immobilization in maximal ankle plantar flexion, and that immobilization of the knee may not be necessary to achieve tendon-edge apposition.