Chapter 10
The Muscular System
323
10
act as levers. Muscle contraction provides the effort that is ap-
plied at the muscle’s insertion point on a bone. Te load is the
bone itself, along with overlying tissues and anything else you
are trying to move with that lever.
Levers: Power Versus Speed
A lever allows a given effort to move a heavier load, or to move a
load farther or faster, than it otherwise could. If the load is close to
the fulcrum and the effort is applied far from the fulcrum, a small
effort exerted over a relatively large distance can move a large load
over a small distance
(Figure 10.3a)
. Such lever is said to oper-
ate at a
mechanical advantage
and is commonly called a
power
lever
. For example, as shown to the right in Figure 10.3a, a person
can li± a car with a power lever, in this case, a jack. Te car moves
up only a small distance with each downward “push” of the jack
handle, but relatively little muscle effort is needed.
If, on the other hand, the load is far from the fulcrum and
the effort is applied near the fulcrum, the force exerted by the
muscle must be greater than the load to be moved or supported
(Figure 10.3b on p. 324). Tis lever system is a
speed lever
and
operates at a
mechanical disadvantage
. Speed levers are use-
ful because they allow a load to be moved rapidly over a large
distance with a wide range of motion. Wielding a shovel is an
example. As you can see, small differences in the site of a mus-
cle’s insertion (relative to the fulcrum or joint) can translate into
large differences in the amount of force a muscle must generate
to move a given load or resistance.
Regardless of type, all levers follow the same basic principle:
Effort farther than
lever operates at a
load from fulcrum
5
mechanical advantage
Effort nearer than
lever operates at a
load to fulcrum
5
mechanical disadvantage
Bipennate
, in which the fascicles insert into the tendon from
opposite sides so the muscle’s “grain” resembles a feather
(Figure 10.2f). Te rectus femoris of the thigh is bipennate.
Multipennate
, which looks like many feathers side by side,
with all their quills inserted into one large tendon. Te del-
toid muscle, which forms the roundness of the shoulder, is
multipennate (Figure 10.2e).
Te arrangement of a muscle’s fascicles determines its range
of motion (the amount of movement produced when a mus-
cle shortens) and its power. Because skeletal muscle fibers may
shorten to about 70% of their resting length when they contract,
the longer and the more nearly parallel the muscle fibers are to
a muscle’s long axis, the more the muscle can shorten. Muscles
with parallel fascicle arrangements shorten the most, but are not
usually very powerful. Muscle power depends more on the total
number of muscle fibers in the muscle: Te greater the number
of muscle fibers, the greater the power. Te stocky bipennate
and multipennate muscles, which “pack in” the most fibers,
shorten very little but are very powerful.
Check Your Understanding
3.
Of the muscles illustrated in Figure 10.2, which could shorten
most? Which two would likely be most powerful? Why?
For answers, see Appendix H.
Lever Systems: Bone-Muscle Relationships
Te operation of most skeletal muscles involves leverage—using
a lever to move some object. A
lever
is a rigid bar that moves
on a fixed point called the
fulcrum
, when a force is applied to
it. Te applied force, or
effort
, is used to move a resistance, or
load
. In your body, your joints are the fulcrums, and your bones
Load
Effort
Load
Effort
10
kg
1000 kg
Fulcrum
10 x 25 = 1000 x 0.25
250 = 250
Effort
x
length of effort arm
=
load
x
length of load arm
(force
x
distance)
=
(resistance
x
distance)
(a) Mechanical advantage with a power lever
Fulcrum
25 cm
0.25 cm
Figure 10.3
Lever systems operating
at a mechanical advantage and a
mechanical disadvantage.
The equation
at the top expresses the relationships
among the forces and relative distances in
any lever system.
(a)
Mechanical advantage
with a power lever. When using a jack,
the load lifted is greater than the applied
muscular effort. Only 10 kg of force
(the effort) is needed to lift a 1000-kg car
(the load).
(Figure continues on p. 324.)
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