The mechanism of injury is a force that increases the tensile force in the tendon beyond its tensile strenght. This may be visualized as a dorsiflexion force at the foot or concommitantly a forward motion of the tibia over the foot while the calf muscles are contracting. As stated the force must exceed the tensile strength of the tendon. A forceful stretch of the tendon or a contraction of the muscles may create this force. Most often it is a combination of the two forces.
Many researchers feel that some degeneration is present in the tendon prior to rupture.
The usual site of rupture is approximately 2 to 6 centimeters proximal to the insertion in the calcaneus. This is also the portion of the tendon that has the poorest blood supply. Since vascularity decreases with age, this frequently occurs in the ageing athlete. A weakening of the Achilles tendon has been observed following intra- tendinous steroid injection. Therefore, injections of steroids are not recommended at this location.