Determination of the Knee Joint Internal Loads During Deep Squat

by  CHRISTIAN   E.   SPANU

Abstract

The objective of the present work is to determine the internal loads inside the knee joint during a deep squat while the knee is maximally flexed, up to 150 degrees of knee flexion. A two-dimensional anatomical mathematical model of the human knee joint was developed for this purpose. To collect model input data, X-rays were first obtained to determine the mathematical representation of the tibial and femoral articular surfaces. An X-ray stand was specifically designed and built for this purpose. An integrated human motion analysis system that includes two force plates and a video camera based human motion measurement system was then employed to measure ground reaction forces at different positions during deep squat. The knee joint net equivalent loads - forces and moments – were then determined using a stick diagram representation of the lower limb. This was followed by the determination of the knee joint internal loads including ligamentous forces, tibio-femoral contact force, and quadriceps and hamstrings equilibrating forces. For this purpose, the knee joint has been modeled as two rigid bodies, the tibia and the femur, undergoing general planar motion in the sagittal plane. The tibia was assumed fixed while the femur slides and rolls along the tibial plateau, without losing contact. Point contact was enforced in the analysis. The model included ten ligamentous structures to represent the cruciate and collateral ligaments along with the posterior capsule, and two muscle forces: quadriceps and hamstrings. The quadriceps forces were applied through the patellar tendon. Model calculations were conducted to simulate isometric quad contractions associated with hamstrings co-contractions at different positions during squat.

Results show that in deep flexion, the femoral contact point occurs on the most proximal point of the posterior condyle, and is located posteriorly on the tibia. The most anterior location of the contact point on the tibia occurred when the subject was standing, and the most posterior location occurred when the subject was squatting while the knee was flexed. Increasing quad forces produced an increase in the tibio-femoral contact force. Model calculations have shown that this contact force was much larger at full extension than in flexion. Model calculations have also shown that during the deep squat, hamstrings dominance was required to maintain equilibrium at small flexion angles. As the flexion angle increased, equilibrium was maintained with quadriceps dominance.

The posterior cruciate ligament (PCL) was found to be the most important ligament in the squat. The anterior cruciate ligament was found to carry loads only in a standing position. As the flexion angle increased, the PCL and in particular its anterior fiber bundles, were found to carry very high loads on the order of thousands of Newtons. Numerical calculations have shown that at a given flexion angle, and as the quadriceps force increases, the hamstring forces increase; and this was associated with an increase in the PCL force.

This study represents a first attempt to understand the knee behavior in weight bearing activities with maximum knee flexion. These results clearly show the important role of the posterior cruciate ligament in this position. This may be an important factor when deciding whether to keep or remove this ligament when performing a total knee replacement procedure. Furthermore, the present study shows that when the knee is in deep flexion, contact occurs on the most proximal points of the posterior condyles. This might explain why most commercially available total knee replacements do not allow for maximum flexion angles: the proximal surface of the posterior condyle is not resurfaced.

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