Repairing the ACL usually means taking a piece of tendon from some other part of the knee and using it as a replacement for the ligament. Studies so far have shown that it takes a long time for this graft to get good blood supply and heal. Starting a vigorous exercise program too soon may result in a knee joint that is too "loose." This can result in poor tennis performance and future injuries.
Patients usually start rehabilitation during the early weeks after surgery. During the first two weeks, the physical therapist will help you reduce the swelling and get full knee extension (straightening the knee). This will involve movement, positioning, and muscle contraction exercises. This is followed by a walking phase combined with the use of a stationary bicycle.
The therapist will prescribe balance and strengthening exercises, starting with the leg that wasn't injured. This quickly progresses to include exercises for balance and control on the injured leg. The therapist will progress the program to include muscle strength, running, and jumping. Any specific movements necessary for tennis will be added as well.