Injuries to the anterior cruciate ligament (ACL) are at epidemic proportions especially in those involved in athletic endeavors. However much diagnosis, treatment and rehabilitation remain an enigma because of controversy regarding ACL functions. The investigators will use a miniature transducer to measure ACL strain in vivo. The transducer is based on a Hall-effect principle and will be arthroscopically inserted into the ACL in patients undergoing arthroscopic surgery. In all patients the ACL strain will be computed as the knee is passively taken from full extension to 120 degrees of flexion. Strain will also be established under active quadriceps and hamstring contractions. These data will be used to establish the effect of muscle contractions on strain, and will provide a basis for knee rehabilitation regimens. In addition, the electromyographic activity of the hamstrings and quadriceps will be related to ACL strain. ACL strain in vivo will also be established under varus/valgus and external and internal rotation torques and under clinical knee function testing. In some subjects the effect of continuous passive motion on the strain in the ACL will be established. The ACL strain will also be measured during clinical knee testing (Lachman and drawer) and this will be compared to results obtained in a new clinical test machine, the Genucom. The effect of knee rehabilitation machine, Cybex II, in straining the ACL, will be established. The ACL strain will also be determined in various simulated activities of daily living and in sporting activities. Knee braces will be evaluated by establishing the change of ACL strain with and without the braces. These data are expected to be extremely useful to those involved in the evaluation, treatment and rehabilitation of knee injuries.