The forces exerted by the soft and hard tissues of the thigh together represent a system in equilibrium. This balance of loads must be considered when the system components are examined independently. However, in many biomechanical analyses of the thigh, the femur is studied without considering soft tissue loading. To improve the understanding of femoral loading a three-dimensional model was developed. Taking into account all thigh muscles, body weight and contact forces at the hip, patello-femoral and knee joints, the internal loads of the bone were calculated.

Internal loads of the femur decreased as a result of muscle activity from proximal to distal at the hip and from distal to proximal at the knee. The load reduction could be up to 50% of the internal forces at the hip, depending on gait phase. Maximal forces were found between 40 and 60% of the stance phase, whereas maximal torsional moments occurred shortly after heel strike.

This model demonstrated that muscles play a substantial role in balancing the loads within the femur. In general, the bone is loaded axially, rather than in bending, with maximum shear forces at the proximal and distal ends. Bending moments are relatively small compared to models which do not consider muscle activity. From one gait phase to another, the femur experiences alternating, rather than one-sided bending load.