With the patient lying flat and relaxed, the examiner bends the knee slightly (about 20 degrees). The examiner then stabilizes the thigh while pulling the shin forward.
The test places stress on the ACL. Both the amount of movement (shifting) of the shin bone, as well as the feel of the endpoint of movement (how solid the ligament feels), offer information about the ACL. Knees with a damaged ACL may demonstrate more movement and a less firm endpoint during a Lachman test.