After 24-48 hours of acute ACL injury, the patient can apply heat to the knee under medical supervision to help improve blood circulation. However, it is important to note that hot compresses should not be applied within 24 hours of an acute ACL injury, as this may increase the swelling of the joint. The correct way is to apply ice under medical supervision within 24 hours of the injury, usually every 2 hours for 15 minutes at a time.