Hamstrings--an anterior cruciate ligament protagonist. An in vitro study.

A cadaveric model that incorporated quadriceps and hamstrings muscle loads was developed to simulate the squat exercise. The addition of hamstrings load affected knee kinematics in two ways. First, anterior tibial translation during flexion ("femoral roll-back") was significantly reduced (P = 0.003) and second, internal tibial rotation during flexion was reduced (P = 0.008). However, quadriceps force was unaffected by the addition of hamstrings load. Thus, it seems likely that hamstrings muscle activity that has been observed in vivo during a squat probably functions synergistically with the anterior cruciate ligament to provide anterior knee stability. After the ACL was sectioned, anterior tibial translation was significantly increased during the squat (P = 0.04). The anterior cruciate ligament was then reconstructed using a graft instrumented with a load cell. During passive motion, maximal graft tension was at full extension. During simulated squat exercise, the addition of hamstrings caused a significant decrease in graft load (P = 0.006). During the squat, maximal graft tension was at full extension, and was equal to the graft tension at full passive extension. Thus, the squat exercise may be useful in the early stages of anterior cruciate ligament rehabilitation.