The hamstrings is made up of three distinct muscle/tendon groups. They are: 1) semimenbranosus, 2) semitendinosus, and 3) biceps femoris. Two other muscles near the hamstrings help flex the knee. These are the gracilis and sartorius muscles.

The semitendinosus and gracilis tendons lay one on top of the other as they insert into the bone along the side of the knee. The semitendinosus-gracilis (STG) tendons can be harvested together, braided, and folded over to make a stronger graft to replace the ACL.

Patients tend to like the hamstring graft better but surgeons often prefer the quadriceps tendon graft. The hamstring graft is more technically difficult for the surgeon, whereas the quadriceps tendon graft often leaves the patient with knee pain, especially if any pressure is applied to the kneecap.

Loss of knee flexor strength is another possible downside to the STG graft. Researchers are studying ways to best rehab each type of graft. As more and more people participate in sports, it's likely that ACL injuries will continue to rise. The need for a strong graft with good function afterwards is important, especially to get athletes back in action as soon as possible.