Loads on Long Bones
The human skeletal system is a complicated kinematic chain. Loads are transmitted via bones and bone positions and orientations are controlled by muscles. The largest loads in our body can be found in the lower extremity of the skeletal system, namely long leg bones. The total load combines body weight and both external and dynamic loadings.
The investigation and quantification of force distributions in individual muscles and bones during various activities is complicated. The main reason, to the writer's knowledge, is a deficiency of non-operative (non-invasive) and remote data acquisition systems that would enable the accurate measurement of forces inside the human anatomy. Therefore, many investigators have adopted mathematical modelling and simulation to tackle this problem. The results obtained using such models commonly overestimate the magnitudes of forces and bending moments . However, these models can be refined by injecting experimentally-obtained data for muscle and bone properties, and loads. The experimental data available are very limited. Most experimental data come from measurements on animals and in some cases on human anatomy post-mortem. The other source of data is orthopaedic fixators, instrumented with data acquisition systems. Such fixators monitor and measure load patterns and displacements during the patient's daily activities. However, data obtained using this approach are limited and only indicate the partial capacity of the limb. Heller et al  have measured forces in the hip and found the peak force is more than 300 % of body weight for both walking and stair climbing. Their mathematical model was confirmed by in vivo testing and data obtained from hip implants. Duda et al , have modelled load distribution throughout a healthy femur. Their results indicate that the femur transmits loads of more than 230 % of body weight and bending moments of up to 20 % of body-weight-metres during walking activity. Schneider et al  have implanted a telemetrized intramedullary nail into the femur with a midshaft fracture in a 33 year old patient. The maximum measured load in axial bone direction was 120 % of the body-weight and maximum axial moment of 1.3 % body-weight-metres for a single stance. Anterior-Posterior and Medial-Lateral loads were of the order of 8 % of body-weight and moments of up to 5 % of body-weight-metres (data derived from graphs based on body-weight of 750 N). It can be speculated that loads in the tibia are of the same magnitude or higher, based on the geometry and structure of the bone, and direction of gravity.
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4. Schneider E, Michel MC, Genge M, Zuber K, Ganz R, and Perren SM. Loads acting in an intramedullary nail during fracture healing in the human femur. Journal of Biomechanics, 2001. 34(7): p. 849-857.Discuss this article, in our forum.