Runners are very fit people, their legs are very strong. Despite this fact, many are surprised when they realize that some of their muscles are actually weak due to biomechanical imbalances. The fact is tight muscles are weak ones; weak muscles increase chances of pain and possible injury. To be fully functional, muscles need to be able to do two things: contract when they are being used then relax and lengthen at other times. Healthy muscles allow you to move through movements and do not cause you to overuse the joints in certain ways. Doctors believe that knee problems are a direct result of weak hip, thigh, and leg muscles. "A lot of these injuries result from motion or mobility problems in the hip or low back," explains Dr. Aaron LeBauer, a physical therapist based in Greensboro, North Carolina. "Or it can be an instability issue because of lack of core engagement. If you have an imbalance that causes the leg to be unstable, it may be a hip control issue." One of the best ways to protect your knees and other joints is to actively engage in stretching and focus on realignment. Yoga can be a beneficial activity for runners to engage in because it increases flexibility, provides symmetry and balance, opens up the hips, and strengthens the stabilizing muscles that surround the joints. Many runners who participate in yoga declare that it helped strengthen muscles they did not even know that they had. One of the greatest benefits that comes with yoga is a strengthened core. A strong core is vital for runners because it creates a better running form, reduces weight and impact that is placed on the legs, allows legs to move more efficiently, and reduces the risk of injury.