Many sports injuries cause a progressive permanent osteoarthritis that will prevent a person from exercising to cause the very diseases that a regular exercise program is supposed to prevent. Sports medicine surgeon James Garrick, writing in the medical journal Lancet (December 2005), explains why. You are supposed to exercise. It makes you stronger, faster, healthier and may even prolong your life. However, every time you exercise, you risk injury and many sports injuries last forever. Depression, heart attacks, strokes, obesity and diabetes are all associated with a sedentary lifestyle. A twisted ankle can change an active person into a sedentary one. A torn anterior cruciate ligament or meniscus of the knee has a greater than 50 percent chance of causing permanent pain within five years, regardless of the treatment.
If you tear your anterior cruciate ligament of you knee, you must have it repaired as soon as possible. After it is repaired at surgery, you have an almost certain chance of tearing it again if you try to return to a competitive sport. When your heel hits the ground during running, your foot stops moving suddenly, forcing the upper femur bone to slide forward on the lower tibia bone at the knee. The anterior cruciate ligament prevents the femur from sliding too far forward and shearing off the cartilage in your knee. Once you tear your anterior cruciate ligament (ACL), even if it is successfully repaired, you have a weaker ACL that still lets the top femur slide forward on the tibia and shear off cartilage. So running causes you to lose cartilage until you lose it all, bone rubs against bone and you hurt 24 hours a day. The only treatment then is a knee replacement. So anyone who has broken cartilage or ACL in his or her knee should never run again for the rest of his or her life.
On the other hand, strengthening the muscles of your upper leg stabilizes the knee and helps to delay and prevent a knee replacement. Anyone with broken cartilage or a torn anterior cruciate ligament in the knee can try to pedal a bicycle every day. Pedaling is done in a smooth rotary motion without sudden stopping, so it does not cause sudden forward movement of the femur on the tibia and does not shear off additional cartilage from the knee joint. If you pedal against increasing resistance you will strengthen the muscles around the knees and increase their stability so there is less wear on the cartilage.