Abstract
The objectives of this study were to determine the effects of hamstrings and quadriceps muscle loads on knee kinematics and in situ forces in the posterior cruciate ligament of the knee and to evaluate how the effects of these muscle loads change with knee flexion. Nine human cadaveric knees were studied with a robotic manipulator/universal force‐moment sensor testing system. The knees were subjected to an isolated hamstrings load (40 N to both the biceps and the semimembranosus), a combined hamstrings and quadriceps load (the hamstrings load and a 200‐N quadriceps load), and an isolated quadriceps load of 200 N. Each load was applied with the knee at full extension and at 30, 60, 90, and 120° of flexion. Without muscle loads, in situ forces in the posterior cruciate ligament were small, ranging from 6 ± 5 N at 30° of flexion to 15 ± 3 N at 90°. Under an isolated hamstrings load, the in situ force in the posterior cruciate ligament increased significantly throughout all angles of knee flexion, from 13 ± 6 N at full extension to 86 ± 19 N at 90°. A posterior tibial translation ranging from 1.3 ± 0.6 to 2.5 ± 0.5 mm was also observed from full extension to 30° of flexion under the hamstrings load. With a combined hamstrings and quadriceps load, tibial translation was 2.2 ± 0.7 mm posteriorly at 120° of flexion but was as high as 4.6 ± 1.7 mm anteriorly at 30°. The in situ force in the posterior cruciate ligament decreased significantly under this loading condition compared with under an isolated hamstrings load, ranging from 6 ± 7 to 58 ± 13 N from 30 to 120° of flexion. With an isolated quadriceps load of 200 N, the in situ forces in the posterior cruciate ligament ranged from 4 ± 3 N at 60° of flexion to 34 ± 12 N at 120°. Our findings support the notion that, compared with an isolated hamstrings load, combined hamstrings and quadriceps loads significantly reduce the in situ force in the posterior cruciate ligament. These data are in direct contrast to those for the anterior cruciate ligament. Furthermore, we have demonstrated that the effects of muscle loads depend significantly on the angle of knee flexion.