Anterior cruciate ligament (ACL) repairs can be done using a piece of tendon from someplace else. Usually the donor graft comes from the patellar tendon below the knee or from the hamstrings muscle behind the knee. The hamstrings muscles has several tendons. The semitentinosus is the tendon of choice.
Many studies have been done to show that the hamstring tendon does regenerate (grow back) in most cases. There have been some studies to suggest that atrophy and shortening of the ST muscle belly occurs in some patients. When this happens, there is loss of strength when the leg is in a position of knee-flexion.
Patients get the best results when the ST grows back and reattaches below the knee joint. This gives it the right position to transmit forces from the ST to the tibia (lower leg one).
Recovery of full strength when the knee is bent is less likely if the tendon doesn't regenerate and/or if the tendon reattaches above the knee. Only athletes who need strength when in a full squat position are affected. This includes judo athletes, gymnasts, and ballet dancers most often.
For the everday, average but active adult, you should expect full recovery of motion and function.