Running seems like a safe sport, so many people are surprised to learn that it causes frequent injuries to muscles, tendons, bones and joints. During running, your foot strikes the ground with a force equal to three times body weight, a force more than double that of walking. Many common running injuries have bio-mechanical causes. For example, pain in the side of the right knee is often treated just by getting the runner to stop running on the roads. To facilitate drainage, roads slope a few degrees from the middle to the sides. Running on the side of a road, facing traffic, causes the right leg to be higher than the left and a larger proportion of the force of the foot strike to be transmitted up to the lateral part of the knee of the higher leg.
Pain behind the kneecap is often treated with special inserts in shoes. When you run, you land on the outside bottom part of your foot and roll toward the inside. This causes the lower leg to twist inward at the same time that the kneecap is pulled by the quadriceps muscle in the opposite direction. This causes the kneecap to rub against the long bone of the upper leg. Special inserts can be placed in running shoes that limit rolling in of the foot and prevent kneecap pain. For more on Runner's Knee see report #9639.
Bio-mechanics can explain injuries in other sports as well. Low back pain in bicycle riders is often treated just by raising the handlebars. Bending over excessively places excessive stress on the lower back muscles. Raising the handlebar stem can decrease the forces on the lower back and cure the pain. If something hurts when you exercise, ask yourself what you can do bio-mechanically to eliminate the excessive stress on that part of your body.