Injury to the sartorius muscle can occur when from overstretching or placing excessive strain on it, resulting in a muscle strain. Strains can range in severity from a partial tear of a few muscle fibers to a complete tear of the muscle.

Standing Hip Flexor Stretch

Performing this exercise regularly will help restore range of motion in your hip as well as stretch and facilitate healing in your injured sartorius muscle. Stand facing a bench or elevated platform so that you are about 3 feet away from the bench. Transfer your weight to your affected leg and place the foot of your other leg on the platform. Take a deep breath in. As you exhale, slowly bend your elevated leg and press your hips forward until you feel a mild stretch in the front of your affected thigh. Stretch only as far as possible without causing pain. Hold the stretch for 30 seconds then carefully relax. Keep your torso straight and upright throughout the movement.
When you attempt to cross your knee when in a sitting position, you usually lean back, thus raising the origin to lengthen this muscle; making it more effective in flexing and crossing the knee.