Biomedical Engineering Reference
In-Depth Information
coordinates: ankle (84.9, 11.0), metatarsal (101.1, 1.3), greater trochanter
(72.1, 92.8), and lateral femoral condyle (86.4, 54.9). From Table 4.1, the
foot center of mass is 0.5 of the distance from the lateral malleolus (ankle)
to the metatarsal marker. Thus, the center of mass of the foot is:
x
=
(
84
.
9
+
101
.
1
)
÷
2
=
93
.
0cm
y
=
(
11
.
0
+
1
.
3
)
÷
2
=
6
.
15 cm
The thigh center of mass is 0.433 from the proximal end of the segment.
Thus, the center of mass of the thigh is:
x
=
72
.
1
+
0
.
433
(
86
.
4
−
72
.
1
)
=
78
.
3cm
y
=
92
.
8
−
0
.
433
(
92
.
8
−
54
.
9
)
=
76
.
4cm
4.1.4 Center of Mass of a Multisegment System
With each body segment in motion, the center of mass of the total body is
continuously changing with time. It is, therefore, necessary to recalculate it
after each interval of time, and this requires knowledge of the trajectories of
the center of mass of each body segment. Consider at a particular point in time
a three-segment system with the centers of mass as indicated in Figure 4.4.
The center of mass of the total system is located at (
x
0
,
y
0
), and each of these
coordinates can be calculated separately;
M
=
m
1
+
m
2
+
m
3
, and:
m
1
x
1
+
m
2
x
2
+
m
3
x
3
M
x
0
=
(4.6)
m
1
y
1
+
m
2
y
2
+
m
3
y
3
y
0
=
(4.7)
M
The center of mass of the total body is a frequently calculated variable. Its
usefulness in the assessment of human movement, however, is quite limited.
Some researchers have used the time history center of mass to calculate the
energy changes of the total body. Such a calculation is erroneous, because
the center of mass does not account for energy changes related to reciprocal
movements of the limb segments. Thus, the energy changes associated with
the forward movement of one leg and the backward movement of another
will not be detected in the center of mass, which may remain relatively
unchanged. More about this will be said in Chapter 6. The major use of
the body center of mass is in the analysis of sporting events, especially
jumping events, where the path of the center of mass is critical to the success
of the event because its trajectory is decided immediately at takeoff. Also,
in studies of body posture and balance, the center of mass is an essential
calculation.
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