The human body is a very complex structure and this, combined with the problems of gaining access to specific parts of the structure, can make it very difficult to make reproducible and meaningful measurements. Let us look at some of the problems involved in actually making in vivo measurements of forces and movements.
The vertical forces involved in standing is a good example. Let's assume that a typical adult male has a mass of 70 kg. When standing, there is a vertical force between the feet and ground calculated by 9.81 m/s2 (gravity) * 70 kg = 686.7 N. The force between the feet and the ground will generate a stress. In order to determine the stress, one must first determine the area of contact between each foot and the ground. Let's assume that the typical area of contact for each foot is 0.015m2. Now the average stress can be calculated:
stress = 686.7 N / (0.015m2/foot * 2 feet) = 22,890 Nm-2 = 22.9 kPa
In actuality there are spatial variations of stress due to the changes in the amount of contact between the feet and the ground. Regardless, we can compare the stress between the feet and the ground with the normal arterial pressure of 16 kPa (120 mmHg) during systole and 10.6 kPa (80 mmHg) during diastole and notice that the stress between the feet and the ground is larger than either of these arterial pressures. As a result we can assume that standing causes ischaemia (lack/stoppage of blood flow) in the tissues of the feet. That is why people change their weight distribution from foot to foot while standing.
Similar problems occur within the joints between bones due to the large stresses involved. These large stresses are caused by: the body weight, acceleration of body segments, and the musculature which holds a joint in place. It is very difficult to determine the stresses within joints in vivo because it is difficult to estimate the areas of contact involved. According to Brown and Shaw (1984), the stress determined for the acetabulum of the hip joint was 5.3 MPa due to maximum forces of 2100-2700 N over an area of 5 cm2. Remember, the hip is a ball and socket joint; the ball is the head of the femur and the socket is a region on the edge of the hipbone called the acetabulum. These high stresses clearly explain why articular surfaces and surrounding soft tissues are very important. In cases of osteoarthritis, the tissues of the joint degrade due to the inability to adapt and regenerate to the surrounding conditions. Artificial joints have been designed to correct this condition.
Load cells are transducers which contain a strain gauge (think of a bathroom scale). The load cells can be incorporated into a force plate and used to measure the forces involved in both standing and walking as shown in Figure 1. By using three load cells it is possible to measure the lateral and vertical forces involve in walking across the force plate. Generally, sensors which contain all three load cells are placed at each of the four corners of the rectangular force plate.
|
| Figure 1: The force plate measures vertical, transverse, and forward forces during walking. Load cells are shown at only one corner of the plate, but they are usually placed in all four corners. The graph shows typical values for teh three components of force obtained during walking. The results are expressed as a percentage of body weight. (Brown fig 20.1) |
Brown, B.H. et al. Medical Physics and Biomedical Engineering. Medical Science Series, Institute of Physics Pub. Philadelphia, 1999.
