The influence of muscle load on tibiofemoral knee kinematics.

A comparative kinematics study was conducted on six cadaver limbs, comparing tibiofemoral kinematics in five conditions: unloaded, under a constant 130 N ankle load with a variable quadriceps load, with and without a simultaneous constant 50 N medial and lateral hamstrings load. Kinematics were described as translation of the projected centers of the medial (MFT) and lateral femoral condyles (LFT) in the horizontal plane of the tibia, and tibial axial rotation (TR) as a function of flexion angle. In passive conditions, the tibia rotated internally with increasing flexion to an average of -16 degrees (range: -12/-20 degrees , SD = 3.0 degrees ). Between 0 and 40 degrees flexion, the medial condyle translated forwards 4 mm (range: 0.8/5.5 mm, SD = 2.5 mm), followed by a gradual posterior translation, totaling -9 mm (range: -5.8/-18.5 mm, SD = 4.9 mm) between 40-140 degrees flexion. The lateral femoral condyle translated posteriorly with increasing flexion completing -25 mm (range: -22.6 to -28.2 mm, SD = 2.5 mm). Dynamic, loaded measurements simulating a deep knee bend were carried out in a knee rig. Under a fixed ankle load of 130 N and variable quadriceps loading, tibial rotation was inverted, mean TR = 4.7 degrees (range: -3.3 degrees /11.8 degrees SD = 5.4 degrees ), MFT = -0.5 mm (range: = -4.3/2.4 mm, SD = 2.4 mm), LFT = 3.3 mm (range: = -3.6/10.6 mm, SD = 5.1 mm). Compared to the passive condition, all these excursions were significantly different (p < or = 0.015). Adding medial and lateral hamstrings force of 50 N each reduced TR, MFT, and LFT significantly compared to the passive condition. In general, loading the knee with hamstrings and quadriceps reduces rotation and translation compared to the passive condition. Lateral hamstring action is more influential on knee kinematics than medial hamstrings action.