34 Fitness for Life: Middle School
Lesson 2.2
Biomechanical Principles
Levers
amount of force. For example, first-class levers
allow you to lift your whole body weight with
the relatively small leg muscles (figure 2.2).
A first-class lever also causes a change in the
direction of the force so that the muscles in the
back of your lower leg, which pull upward on
the back of your foot (your heel), make the
front of your foot (your toes) push downward
against the ground.
In a first-class lever, the fulcrum (or pivot
point) is between the resistance (or weight)
and the effort (or force). First-class levers can
be used to either increase the force applied or
increase the speed and distance of movement.
Second-class levers are not common in
the body. However, a good example of a sec-
ond-class lever is seen in a person doing a
push-up (figure 2.3). The body is straight from
the shoulders to the feet. It acts as a lever with
The levers of the body allow you to apply force
to create movements of many different kinds.
A lever is a very basic machine. It is a bar
or a stiff, straight object that can be used to
lift weight, increase force, or create speed. The
bones of your body are levers that allow you
to perform many skills. For example, the bones
of your foot act as a lever when you push with
your foot while you walk and run. The calf
muscles shorten, causing your foot to push
down against the ground. To do its work, a
lever must have a pivot point at its middle or
at its end.
When you use the foot as a lever in walking
and running, the ankle is the pivot point and
the bones of the foot are the levers. When you
use the bones as levers in this way, they are
called first-class levers. First-class levers can
allow you to lift a heavy weight with a small
E6939/Corbin/F02.02/565245/mh-R1
First-class lever:
Toe raise
R
E
F
Muscle shortens
Pivot point (fulcrum [F])
Muscle force (effort [E])
pulls up on lever
Lever pushes down against
resistance (R) of floor
Lever (foot)
Figure 2.2 First-class lever: toe raise.
E6939/Corbin/F02.03/565246/mh-R1
Second-class lever: Push-up Arm muscles
pull on bones
to straighten arms (effort [E])
Arms push
body up
Body weight
is lifted (resistance [R])
Pivot point (fulcrum [F])
R E
F
Lever (body)
Figure 2.3 Second-class lever: push-up.
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