ANTHROPOMETRY - Biomechanics and Motor Control of Human Movement - page 96

4.2 DIRECT EXPERIMENTAL MEASURES

For more exact kinematic and kinetic calculations, it is preferable to have
directly measured anthropometric values. The equipment and techniques that
have been developed have limited capability and sometimes are not much of
an improvement over the values obtained from tables.

4.2.1 Location of the Anatomical Center of Mass of the Body
The center of mass of the total body, called the anatomical center of mass ,is
readily measured using a balance board, as shown in Figure 4.7 a . It consists
of a rigid board mounted on a scale at one end and a pivot point at the other
end, or at some convenient point on the other side of the body's center of
mass. There is an advantage in locating the pivot as close as possible to the
center of mass. A more sensitive scale (0 - 5 kg) rather than a 50- or 100-kg
scale is possible, which will result in greater accuracy. It is presumed that
the weight of the balance board, w 1 , and its location, x 1 , from the pivot are
both known along with the body weight, w 2 . With the body lying prone the
scale reading is S (an upward force acting at a distance x 3 from the pivot).
Taking moments about the pivot:

w 1 x 1 + w 2 x 2 = Sx 3

Sx 3 − w 1 x 1
w 2

x 2 = (4.13)

4.2.2 Calculation of the Mass of a Distal Segment
The mass or weight of a distal segment can be determined by the technique
demonstrated in Figure 4.7 b . The desired segment, here the leg and foot, is
lifted to a vertical position so that its center of mass lies over the joint center.
Prior to lifting, the center of mass was x 4 from the pivot point, with the scale
reading S . After lifting, the leg center of mass is x 5 from the pivot, and the
scale reading has increased to S 1 . The decrease in the clockwise moment
due to the leg movement is equal to the increase in the scale reaction force
moment about the pivot point,

W 4 (x 4 − x 5 ) = (S 1

− S )x 3

S 1

S x 3 −

w 4 = (4.14)

(x 4 − x 5 )

The major error in this calculation is the result of errors in x 4 , usually
obtained from anthropometric tables. To get the mass of the total limb, this
experiment can be repeated with the subject lying on his back and the limb
flexed at an angle of 90 ◦ . From the mass of the total limb, we can now
subtract that of the leg and foot to get the thigh mass.