Parameterized analyses of subject-specific femoral stresses in female treadmill runners

Background: Biomechanics play an important role in running, especially regarding prevention of injury. For example, female runners or runners with increased Q angles (between the quadriceps and patellar tendon) are often more prone to knee injury. The purpose of this analysis is to identify areas of stress on the femur in two runners. Determining which areas undergo the highest stress can be used to modify running form and optimize training and recovery.

Methods: This analysis used individualized finite element (FE) stress-strain models based on self-measured palpable dimensions for one leg, angles, body weight, and stride on two different subjects. Measurement for individual model scaling included width across the femoral condyles, anteroposterior dimensions of each condyle, and length video-measured varus (inclination) angle from the greater trochanter to the knee joint. Varus angle and Q angle are positively correlated.

A finite element model was created for each subject based on the material properties of human femoral compact bone, with a medullary cavity but no cancellous bone. Finite element analyses were performed using linear static assumptions in COMSOL Multiphysics. Mid-stance knee-joint reaction forces were derived from multiples of body weight experienced during treadmill running. Quasi-static, instantaneous constraints were applied proximally at the hip to prevent rigid body motion and enable loads to propagate through each femur. We compared effects of constraints at the femoral head alone and at stabilizing muscle attachments.

Results: Proportions: Both subjects had similar measurements, with relatively small differences. One subject had a shorter femur that was more robust, with larger anterior/posterior and transverse measurements. The subject with the more gracile femur had a higher body weight and slightly smaller Q angle, by about 2 degrees.

Stresses: Models for both subjects showed the highest amount of stress on the upper 1/3 of the femur shaft. However, the more gracile femur had higher stress on the medial and lateral sides of the shaft, while the more robust femur had higher strain on the anterior and posterior aspects. The more slender femur also showed higher stress in general.

Interpretations and Conclusion: The greater overall stress in the more slender femur was expected, especially with subject’s higher mass and ground forces. However, the more robust femur’s relatively lower stress medially and laterally was unexpected given the greater angle of inclination and Q angle. These results could all be related to flexibility, muscle strength, and running form. The subjects also have different running forms and exercise routines. We conclude that soft tissue factors may have a greater effect on optimized running form than femoral stresses. This analysis can be expanded upon and refined by including more subjects, different running speeds, and MRI scans of subjects’ legs for more accurate measurements. Incorporating cancellous bone properties will yield better results proximally and distally.