The protective relationship of the human anterior cruciate ligament (ACL) by active contraction of the hamstring musculature has been known and understood by sports orthopedists and physical therapists for many years. Rehabilitation programs for patients with torn ACLs as well as those treated with ligament reconstruction have always stressed hamstring strengthening. Research in this area during the past decade has begun to define the proprioceptive mechanism that governs this relationship as well as the actual recording of dynamic muscle firing patterns in pre- and postoperative subjects. Laboratory studies suggest that altered hamstring activity may help these subjects compensate for a knee that is lax because of ACL damage.