324
UNIT 2
Covering, Support, and Movement of the Body
10
(speed levers), force is lost but speed and range of movement are
gained. Systems that operate at a mechanical advantage (power le-
vers) are slower, more stable, and used where strength is a priority.
Check Your Understanding
4.
Which of the three lever systems involved in muscle
mechanics would be the fastest lever—first-, second-, or
third-class?
5.
What benefit is provided by a lever that operates at a
mechanical advantage?
For answers, see Appendix H.
Major Skeletal Muscles
of the Body
Name and identify the muscles described in Tables 10.1 to
10.17. State the origin, insertion, and action of each.
Te grand plan of the muscular system is all the more impressive
because of the sheer number of skeletal muscles in the body—
more than 600 of them (many more than in
Figures 10.5
and
10.6
combined)! Remembering all the names, locations, and
actions of these muscles is a monumental task. ±ake heart; here
we consider only the principal muscles (approximately 125 pairs
of them). Although this number is far fewer than 600, the job of
learning about all these muscles will still require a concerted effort
on your part.
Memorization will be easier if you apply what you learn in
a practical, or clinical, way; that is, with a
functional anatomy
focus
. Once you are satisfied that you have learned the name of
a muscle and can identify it on a cadaver, model, or diagram,
flesh out your learning by asking yourself, “What does it do?” [It
might be a good idea to review body movements (pp. 256–259)
before you get to this point.]
Classes of Levers
Depending on the relative position of the three elements—
effort, fulcrum, and load—a lever belongs to one of three classes.
In a
first-class lever
, the effort is applied at one end of the
lever and the load is at the other, with the fulcrum somewhere
between. Seesaws and scissors are first-class levers. First-class
leverage also occurs when you li² your head off your chest
(Fig-
ure 10.4a)
. Some first-class levers in the body operate at a me-
chanical advantage (for strength), but others, such as the action
of the triceps muscle in extending the forearm against resistance,
operate at a mechanical disadvantage (for speed and distance).
In a
second-class lever
, the effort is applied at one end of
the lever and the fulcrum is located at the other, with the load
between them. A wheelbarrow demonstrates this type of lever
system. Second-class levers are uncommon in the body, but the
best example is the act of standing on your toes (Figure 10.4b).
All second-class levers in the body work at a mechanical advan-
tage because the muscle insertion is always farther from the ful-
crum than is the load. Second-class levers are levers of strength,
but speed and range of motion are sacrificed for that strength.
In a
third-class lever
, the effort is applied between the load
and the fulcrum. Tese levers are speedy and always operate
at a mechanical disadvantage—think of tweezers and forceps.
Most skeletal muscles of the body act in third-class lever sys-
tems. An example is the activity of the biceps muscle of the
arm, li²ing the distal forearm and anything carried in the hand
(Figure 10.4c). Tird-class lever systems permit a muscle to be
inserted very close to the joint across which movement occurs,
which allows rapid, extensive movements (as in throwing) with
relatively little shortening of the muscle. Muscles involved in
third-class levers tend to be thicker and more powerful.
In conclusion, differences in the positioning of the three ele-
ments modify muscle activity with respect to speed of contraction,
range of movement, and the weight of the load that can be li²ed.
In lever systems that operate at a mechanical disadvantage
Load
Effort
Fulcrum
Load
Effort
100 kg
50 kg
Fulcrum
100 x 25 = 50 x 50
2500 = 2500
(b) Mechanical disadvantage with a speed lever
25 cm
50 cm
Figure 10.3
(continued)
Lever systems
operating at a mechanical advantage
and a mechanical disadvantage.
(b)
Mechanical disadvantage with a speed
lever. When using a shovel to lift dirt, the
muscular force is greater than the load
lifted. A muscular force (effort) of 100 kg
is used to lift 50 kg of dirt (the load). Levers
operating at a mechanical disadvantage are
common in the body.
(Text continues on p. 328.)
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