An in vivo analysis of the effect of transcutaneous electrical stimulation of the quadriceps and hamstrings on anterior cruciate ligament deformation.

Transcutaneous electrical muscle stimulation (TEMS) has been advocated as a method to rehabilitate the postoperative ACL repaired/reconstructed lower extremity. Isolated quadriceps contraction can potentially disrupt the ACL repair/reconstruction; to minimize this risk simultaneous quadriceps and hamstring stimulation has been used. This study measured the in vivo deformation of the ACL during TEMS of the quadriceps and hamstrings. Six legs in four Rhesus monkeys were immobilized in 0 degrees, 45 degrees, and 90 degrees of flexion in neutral rotation using a Hoffman frame and pins placed through the proximal femur and distal tibia. The hamstrings and quadriceps muscles were stimulated with a dual channel electrical stimulator individually and simultaneously at each point of flexion, and ACL deformation was measured using a Hall effect device placed on the anterior medial fibers of the ACL. The following conclusions were made: 1) Isolated quadriceps contraction produces ACL elongation at 0 degrees and 45 degrees of knee flexion and produces ACL shortening at 90 degrees of knee flexion. 2) Isolated hamstrings contraction produces ACL shortening at 45 degrees and 90 degrees of knee flexion and negligible effects at full knee extension. 3) It is not possible to simultaneously contract the quadriceps and hamstrings using separate stimulator pads for each muscle group. 4) At 45 degrees of knee flexion when the quadriceps muscles are stimulated before the hamstring muscles and simultaneous contraction of both is then sustained, ACL lengthening occurs. 5) When the hamstring muscles are fired before the quadriceps muscles and simultaneous contraction of both is sustained, ACL shortening occurs.