What causes an achilles tendon rupture?

Most commonly it: Is from a foreceful motion with a sudden change in direction which applies a torque to the tendon. Examples are a sudden change in direction when playing a sport such as tennis or basketball.

Direct trauma: Achilles tendon injuries are very catastrophic. Usually occurs when the foot is in a fixed plantargrade position and the tibia (shinbone) externally rotates rapidly, and the foot does not release from its flexed state. Injuries for achilles tendon ruptures also take the longest to resolve, which can be from 8-20 weeks, depending on the amount and time of damage to the tendon.