The thigh has three sets of strong muscles: the hamstring muscles in the back of the thigh, the quadriceps in the front and the adductor muscles on the inside. The quadriceps and hamstring muscles work in combination to straighten and bend the leg. The adductor muscles pull the legs together.
In cross section, these three sets of muscles can be found in three compartments that are divided by fascia, a layer of fibrous tissue. These three compartments are the medial, anterior and posterior fascial compartments. These compartments use the femur as an axis and are separated by tough connective-tissue membranes. Each of these compartments has its own blood and nerve supply and contains a different set of muscles.
The hamstring and quadriceps are at high risk from muscle strains because they cross both the hip and the knee joints. Also, these muscles are used for high-speed activities, such as track and field events and basketball. These muscles, particularly the quadriceps, experience strain when the fibres in them are overstretched. Once the fibres are overstretched to a certain point, muscle tear occurs. Muscle strains frequently occur near the point where the muscle joins the tough, fibrous connective tissue of the tendon. A similar injury occurs if there is a direct blow to the muscle.