Validation of a lower limb model with in vivo femoral forces telemetered from two subjects.

A mathematical model of the human pelvis-leg system in the sagittal plane, with an anatomical model of the knee, was developed to calculate forces transmitted by the structural elements of the system. The model was used to study the influence of activity of hip flexors and extensors on the forces in the femur during isometric exercises and during level walking. Kinematic and kinetic data together with simultaneous electromyography (EMG) and in vivo axial forces transmitted along the prostheses from two patients implanted with instrumented massive proximal femoral prostheses were obtained. Comparison of the levels of the calculated axial forces in the model femur to the simultaneous telemetered forces showed good agreement for isometric tests. Interaction between the muscles and the bones during isometric tests was examined and bi-articular muscles were shown to play a major role in modulating forces in bones. The study supports the hypothesis that muscles balance the external limb moments, not only at joints but also along the limbs, decreasing the bending moments but increasing the axial compressive forces in bones. It is thus suggested that appropriate simulation of muscle force is necessary in in vitro laboratory experiments and in theoretical studies of load transmission in bones. The sagittal plane model underestimates the value of the maximum axial force in the femur during walking by about 30% but suggests that 70% was due to the action of the extensors or flexors. The results encourage further development of a three-dimensional model with anatomical models of the joints to include coronal and transverse planes for the study of adductors and abductors.