The quadriceps and hamstring muscle sets work together to straighten (extend) and bend (flex) the leg. The adductor muscles pull the legs together. The hamstring and quadriceps muscle sets are particularly at risk for muscle strains because they cross both the hip and knee joints.
Muscle strains usually happen when a muscle is stretched beyond its limit, tearing the muscle fibers. This can also occur with a direct blow to the muscle. Muscle strains in the thigh can be quite painful. If the blood vessels are broken, some bruising may evolve. It is very important to allow the muscle to heal properly to prevent re-injury.