A study from East Germany shows that athletes who tear the anterior cruciate ligament (ACL) in their knees will have permanent knee damage if they return to competitive sports ( Arthroscopy, June 2005). The ACL runs from the top bone of the knee to the bottom one and prevents the top bone from sliding forward when the foot hits the ground during running and walking. If it is torn, the knee becomes so unstable that a person will have difficulty walking, so all torn anterior cruciate ligaments must be repaired.

In this study, East German Olympic athletes who tore their knee ligaments in 1963-1965 returned to competition after having their ligaments repaired and were examined 10 and 20 years later. Virtually all had severe knee cartilage damage and more than half had total knee replacement surgery.

Athletes with repaired torn anterior cruciate ligaments probably should never run or jump again, although they may be able to pedal a bicycle. Another study showed that people who have broken cartilage in their knees can walk and cycle, but should not run or jump.

To keep bones from wearing down at joints, their ends are covered with a thick white gristle called cartilage. Even one bleed into a joint damages cartilage forever. Doctors cannot replace or heal broken cartilage, they can only replace entire knee joints. Operating and removing broken cartilage probably increases a person's chances of needing a knee replacement, particularly if the exerciser continues to run and jump. Sheering forces on the knee are very great during walking downhill and running, and minimal during walking on level ground and cycling. So people who have ever damaged cartilage in their knee should walk on level ground, swim or cycle, and avoid running and jumping.