(HealthDay News) -- The hamstring is a group of three muscles that extends down the back of the thigh. A "pulled" hamstring is a common sports injury, especially among track, soccer or basketball athletes, the American Academy of Orthopaedic Surgeons says.

The academy says these factors increase the risk of injuring a hamstring:

• Having a sudden stretch of the hamstring, or "overloading" the muscles.
• Having a tightening of the muscles, often from lack of stretching before exercise.
• Having one set of muscles that is much stronger than its "opposing" set. For example, having stronger quadriceps at the front of the thigh than hamstrings, at the back of the thigh.
• Being in poor physical shape.
• Having muscle fatigue.