Injury to the ACL is diagnosed by detecting an increase in the forward movement of the tibia in relation to the femur. Normally such movement is prevented by the ACL. The examining doctor gently grasps the tibia and the femur and tries to detect the increased movement.
Another of the tests is the Pivot Shift test which gently reproduces the giving way sensation as the knee is slowly bent. Done carefully these are painless tests and can accurately diagnose a torn ligament.
Often the diagnosis can be made from the story – a pivoting twisting movement associated with a pop sensation and the feeling of immediately not trusting the knee. Others nearby may have heard a pop and thought the leg was broken. Early swelling is common and it is not usually possible to carry on playing – and any attempt to do so often results in the knee giving way again.