The muscle, tendon and attached bone comprise a unit. The unit stabilizes the thigh and allows its motion. A strain occurs at the weakest part of a unit. There is no loss of strength. Strength is diminished.
Severe strain requires surgical repair.
Chronic strains are caused by overuse. Acute strains are caused by direct injury or overstress. Quadriceps femoris muscle or its various tendons. Pain when moving, stretching or flexing the thigh. Muscle spasm of the quadriceps femoris.