The Effects of Variable Quadriceps and Hamstring Loading Configurations on Knee Joint Kinematics During In Vitro Testing

Abstract

Previous studies have highlighted the importance of the hamstrings and quadriceps muscles on knee joint mechanics and the effects of their pathologies. It is crucial that the resultant force of theses musculature be accurately represented in in vitro simulation. This study has two objectives to be examined during a deep knee squat: 1) measure the patellofemoral kinematics as a function of different loading configurations of the extensor mechanism and 2) measure the changes in tibiofemoral kinematics after including a direct hamstrings load. Fourteen fresh frozen cadavers were tested using a custom designed muscle loading rig. The rig can statically load the individual heads of the quadriceps and the hamstrings in their anatomical orientation using dead weights or directly drive the rectus femoris quadriceps muscle using a stepper motor. Patellofemoral flexion and shift were the only kinematics that changed significantly between the single line and the physiological based distributed loading configuration of the extensor mechanism, with the largest difference of 2.8° and 0.9 mm at 15° and 45° knee flexion respectively. A weak vastus medialis induced an average lateral shift of 1.5 mm and an external rotation of 0.8° while a 0.9 mm medial shift and 0.6° internal rotation was seen when simulating a weak vastus lateralis relative to the physiological based distributed configuration. The change in patellofemoral kinematics was caused by the non-parallel forces to the axis of the femur generated from the vastus medialis and the vastus lateralis. The flexion moment generated from these forces in the sagittal plane decreased patellar flexion. The vastus lateralis load was larger than that of the vastus medialis causing the resultant force in the frontal plane to be more externally rotated. When the hamstrings were loaded throughout the flexion cycle, the femoral lateral condyle lowest point was more anterior with the largest difference of 1.1 mm at 80° knee flexion. For the iv medial femoral condyle lowest point, loading the hamstrings shifted the lowest point 0.9 mm posterior until 40° flexion. At this flexion angle, the medial lowest point became more anterior for the rest of the flexion cycle (0.9 mm on average). The hamstrings also decreased the medial and lateral lowest point range of motion by 1.7 mm and 0.9 mm respectively. The change in tibia femoral kinematics was larger in deeper knee flexion when the hamstrings were loaded which is due to the increase in the hamstrings moment arm, but it is unclear at this point whether the reduction in tibial internal rotation is due to the isometric loading configuration of the hamstrings. The results from this study demonstrated that different muscle loading configurations of the extensor mechanism and muscle weakness significantly influence patellofemoral shift and tilt while increasing the co-contraction between the quadriceps and hamstrings significantly reduces tibial anterior translation and internal rotation. The study has aided in describing the effects of different muscle loading configurations on knee joint kinematics from simulations and provided important experimental data to investigate changes to improve dynamic simulations using the Kansas Knee Simulator.