American Journal of Sports Medicine

Robert C. More, M.D., Bryant C. Karras, B.A., Rafael Neiman, B.S., Daniel Fritschy, M.D., Savio L-Y. Woo, Ph.D., Dale M. Daniel, M.D.


A cadaver 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 ACL to provide anterior knee stability.

After the ACL sectioned, anterior tibial translation was significantly increased during the squat (p = 0.04). The ACL was then reconstructed using a graft instrumented with a load cell. During passive motion, minimal graft tension was observed at full extension. During a simulated squat exercise, however, the addition of hamstrings caused a significant decrease in graft load (p = 0.006). During the squat, maximal graft tension was observed 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 ACL rehabilitation.