Biomedical Engineering Reference
In-Depth Information
Figure 5.18
Fifteen major muscles responsible for the sagittal plane moments of force
at the ankle, knee, and hip joints. During weight bearing, all three moments control the
knee angle. Thus, there is considerable indeterminacy when relating knee angle changes
to any single moment pattern or to any unique combination of muscle activity.
where:
N
e
=
number of extensor muscles
N
f
=
number of flexor muscles
F
ei
(t )
=
force in
i
th extensor muscle at time
t
d
ei
(t )
=
moment arm of
i
th extensor muscle at time
t
Thus, a first major step is to make valid estimates of individual muscle
forces and to combine them with a detailed kinematic/anatomical model of
lines of pull of each muscle relative to the joint's center (or our best estimate
of it). Thus, a separate model must be developed for each joint, and a number
of simplifying assumptions are necessary in order to resolve the indeterminacy
problem. An example is now presented of a runner during the rapid pushoff
phase when the plantarflexors are dominant.
5.3.2 Example Problem (Scott and Winter, 1990)
During late stance, a runner's foot and ankle are shown (see Figure 5.19)
along with the direction of pull of each of the plantarflexors. The indeter-
minacy problem is solved assuming that there is no cocontraction and that
each active muscle's stress is equal [i.e., its force is proportional to its phys-
iological cross-sectional area (PCA)]. Thus, Equation (5.11) can be modified
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