![]() The gravitational or inertial force acts at this point, the so-called center of mass. To describe the effect of a gravitational or inertial force on a body in a simple fashion, a point in space can be defined where the total mass m of the body is presumed to be concentrated. If high precision is required in the results, the individuals concerned have to be measured beforehand. Uncertainties in the values affect all investigations which use these data as input values, for example gait analysis. This saves having to list values separately for persons of different height or body mass however, individual variations in physique are ignored. For simplicity, the reference values are often given not in absolute units but in relative units, per meter of height or per kilogram of body mass. They are approximately correct, but in the individual case deviations due to sex, age, and physique may occur. For this reason, the numbers cited below for the mass, dimension, location of the center of mass, and moment of inertia of body segments are not representative of both sexes and all age groups. ![]() ![]() The reason is that some values have to be determined in vitro from autopsy specimens, which may not be available in large numbers. Some reference values published in the scientific literature are based on measurements taken from only a small number of subjects. Where exactly, for example, does the trunk end and the thigh begin? In addition, it is often impossible to define the boundary between segments precisely. ![]() In vivo the segments may change shape passively under external forces or actively through activation of the muscles. The body segments are conceptualized as rigid bodies, but this is only an approximation. The arm, for example, can be subdivided into upper arm, lower arm, and hand. Depending on the problem under investigation, further subdivision may be appropriate. To describe the effect of gravity on the human body, and to derive moments and joint forces from observations of the moving body, the body is conceptualized as composed of individual segments connected by joints: for example, head, arm, trunk, thigh, lower leg, and foot. 11 Dimensions, Mass, Location of the Center of Mass, and Moment of Inertia of the Segments of the Human Body ![]()
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