ANTHROPOMETRY - Biomechanics and Motor Control of Human Movement - page 93

during walking. It may be possible to simulate data for the left side of HAT
and the left limb. If we assume symmetry of gait, we can say that the trajectory
of the left limb is the same as that of the right limb, but out of phase by half
a stride. Thus, if we use data for the right limb one-half stride later in time
and shift them back in space one-half a stride length, we can simulate data
for the left limb and left side of HAT.

Example 4.5. Calculate the total-body center of mass at a given frame
15. The time for one stride was 68 frames. Thus, the data from frame 15
become the data for the right lower limb and the right half of HAT, and
the data one-half stride (34 frames) later become those for the left side of
the body. All coordinates from frame 49 must now be shifted back in the
x direction by a step length. An examination of the x coordinates of the
heel during two successive periods of stance showed the stride length to be
264 . 2 0 . 707 m.

Table 4.2 shows the coordinates of the body segments for both left and right
halves of the body for frame 15. The mass fractions for each segment are as
follows: foot

− 122 . 8 = 141 . 4 cm. Therefore, the step length is 70 . 7cm =

0 . 339. The
mass of HAT dominates the body center of mass, but the energy changes in
the lower limbs will be seen to be dominant as far as walking is concerned
(see Chapter 6).

= 0 . 0145, leg = 0 . 0465, thigh = 0 . 10, 1 / 2HAT =

Center of mass (COM) analyses in three dimensions are not an easy mea-
sure to make because every segment of the body must be identified with
markers and tracked with a three-dimensional (3D) imaging system. In some
studies of standing, the horizontal anterior/posterior displacement of a rod
attached to the pelvis has been taken as an estimate of center of mass move-
ment (Horak et al., 1992). However, in situations when a patient flexes the
total body at the hip (called a “hip strategy”) to defend against a forward
fall, the pelvis moves posteriorly considerably more than the center of mass
(Horak and Nashner, 1986). In 3D assessments of COM displacements, the
only technique is optical tracking of markers on all segments (or as many seg-
ments as possible). MacKinnon and Winter (1993) used a seven-segment total
body estimate of the lower limbs and of the HAT to identify balance mech-
anisms in the frontal plane during level walking. Jian et al. (1993) reported
a 3D analysis of a similar seven-segment estimate of the total body COM
in conjunction with the center of pressure during initiation and termination
of gait and identified the motor mechanisms responsible for that common
movement.
The most complete measure of center of mass to date has been a 21-marker,
14-segment model that has been used to determine the mechanisms of balance
during quiet standing (Winter et al., 1998). Figure 4.6 shows the location
of the markers, and the accompanying table gives the definition of each
of the 14 segments, along with mass fraction of each segment. It is worth