Physiotherapy. Physiotherapist measuring the knee joint flexion of a 36-year-old female patient who is recovering after undergoing surgery to reconstruct the anterior cruciate ligament (ACL) in her knee. The ACL is one of the four major ligaments that hold the knee joint together. It joins the femur (thigh bone) to the tibia (shin bone) and is commonly injured while playing sport. This injury occurred two weeks previously.