When muscles aren't used, they tend to get weaker and shrink in size. Bones that aren't used become less dense and weaker. When tendons aren't used, they become weaker too. When tendons are weak, they are more likely to tear.

The Achilles tendon, which runs down the back of the lower leg and connects to the heel, is one of the most commonly torn tendons. It usually tears while jumping or cutting during sports activities. A torn Achilles tendon typically causes much pain and disability, and it almost always requires surgery.

These researchers tested the effects of immobilizing the Achilles tendon in rabbits. The rabbits wore a cast on one leg for up to eight weeks. Tendon function was then compared to the legs that had not been immobilized. The idea was to study any changes in the tendon and find out how it had weakened while in the cast.

The immobilized tendons looked the same as the healthy tendons under the microscope. However, they functioned much differently. The immobilized tendons had less range of motion. When researchers tested them under conditions of jumping, the tendons often broke at the point where they join the heel. Sometimes the heel bone actually broke, too. These tears most likely happen because of the reduced bone density that comes with immobilization. The researchers were surprised to find that the main part of the tendons did not rupture, which is usually how Achilles tendon tears happen in sports injuries.

The authors conclude that immobilization is not as damaging to the Achilles tendon as doctors may have thought. They suggest that rehabilitation after immobilization of the leg may need to focus more on regaining bone density than on strengthening the tendons, although more research is needed.