Muscle
279
metric tension exerted by the muscle (
X
), and its distance
from the elbow (5 cm); that is, 10
×
35 = 5
×
X
. Thus
X
=
70 kg. The important point is that this system is working at
a mechanical disadvantage because the tension exerted by
the muscle (70 kg) is considerably greater than the load (10
kg) it is supporting.
However, the mechanical disadvantage that most muscle
lever systems operate under is offset by increased maneuver-
ability. As illustrated in
Figure 9–30
, when the biceps short-
ens 1 cm, the hand moves through a distance of 7 cm. Because
the muscle shortens 1 cm in the same amount of time that
the hand moves 7 cm, the velocity at which the hand moves
is seven times greater than the rate of muscle shortening. The
femoris
Gastrocnemius
Gastrocnemius
contracts
femoris
relaxed
femoris
contracts
Flexion of leg
Extension of foot
Figure 9–28
Contraction of the gastrocnemius muscle in the calf can lead either
to ﬂ exion of the leg, if the quadriceps femoris muscle is relaxed, or
to extension of the foot, if the quadriceps is contracting, preventing
the knee joint from bending.
X
= 70 kg
10 kg x 35 cm =
X
x 5 cm
X
= 70 kg
5 cm
30 cm
10 kg
10 kg
Figure 9–29
Mechanical equilibrium of forces acting on the forearm while
Figure 9–29
physiological
inquiry
Describe what would happen if this weight was mounted on a rod
that moved it 10 cm farther away from the elbow and the tension
generated by the muscle was increased to 85 kg.
Answer can be found at end of chapter.
Figure 9–30
The lever system of the arm ampliﬁ es the velocity of the biceps
muscle, producing a greater velocity of the hand. The range of
movement is also ampliﬁ ed (1 cm of shortening by the muscle
produces 7 cm of movement by the hand).
Force
1 cm
7 cm
V
m
= muscle
contraction
velocity
V
h
= hand
velocity
= 7 x
V
m
lever system ampliﬁ es the velocity of muscle shortening so that
short, relatively slow movements of the muscle produce faster
movements of the hand. Thus, a pitcher can throw a baseball
at 90 to 100 mph even though his muscles shorten at only a
small fraction of this velocity.