Biomedical Engineering Reference
In-Depth Information
humerus
F
B
10-lb. ball
75°
radius
ulna
2 in.
10.5 in.
14 in.
y
x
10 lb.
F
A
F
B
75°
E
F
C
P
B
O
2 in.
mg = 0.022w = 3.5 lb.
0.682L = 7.2 in.
14 in.
FIGURE 4.12
(Top) The forearm held statically fixed in 90
flexion while holding a 10 lb weight at the
hand. (Bottom) Free-body diagram of the forearm system.
Adapted from [5].
Solution
Figure 4.12 (bottom) shows the free-body diagram of this system. Due to the increased number
of unknowns compared to the previous example, both Eqs. (4.35) and (4.36) will be used. From the
anthropometric relationships in Table 4.1, the segment weight (forearm and hand) is approxi-
mated as 2.2 percent of total body weight, with the segment mass located 68.2 percent of the
segment length away from the elbow axis. Note that the segment length for the “forearm and
hand” segment in Table 4.1 is defined as the distance between the elbow axis and the ulnar
styloid.
Summing moments about the elbow at point O, the equilibrium equation
P
M
¼
0 can be
written as
r
OE
F
A
þ
r
OB
ð
10 lb
Þ
j
þ
r
OP
ð
3
:
5lb
Þ
j
¼
0
ð
2in
Þ
i
ð
F
A
Þ
j
þð
12 in
Þ
i
ð
10 lb
Þ
j
þð
5
:
2in
Þ
i
ð
3
:
5lb
Þ
j
¼
0
ð
2in
Þ
F
A
k
ð
120 lb in
Þ
k
ð
18
:
2lbin
Þ
k
¼
0
