It is generally agreed that the risk of ligament injuries can be reduced by strengthening a specific muscle group by which the dynamic stability of the joint is able to be enhanced and the static stability provided by the ligament can also be compensated. Traditionally, the strengthening exercise of ACL rehabilitation has been putting an emphasis on the hamstring and quadriceps with an attempt to decrease the anterior drawer instability as well as the shearing force of ACL. Nonetheless, from the aforementioned pathomechanism of ACL injuries, the knee rotators should be valued as they are supposed to . The internal rotators of the knee joint (semitendinosus, semimembranosus and gracilis) are highly related to ACL injuries5. However, the knee rotator strength is often neglected and even sacrificed as an autograft for the ACL reconstruction .