A recent study showed that knee strength is actually affected by the muscles around the hip. For some motions, the hip muscles add only a small amount of strength to the knee. Other knee motions, such as turning the knee in or out (rotation) depend much more on hip muscles.
ACL injury affects the muscles around the knee, especially the thigh muscle (quadriceps). Hip muscle strength may be affected by ACL injury, too. One study showed that hip strength is reduced after knee surgery on the affected side. This is especially true for hip extension, or unbending the hip. This information suggests that hip exercises can help you achieve the fullest recovery from your injury.