Chapter 10
The Muscular System
325
10
Fulcrum
(a) First-class lever
Arrangement of the elements is
load-fulcrum-effort
(b) Second-class lever
Arrangement of the elements is
fulcrum-load-effort
(c) Third-class lever
Arrangement of the elements is
load-effort-fulcrum
L
L
L
L
L
L
In the body:
A first-class lever system
raises your head off your chest. The
posterior neck muscles provide the effort,
the atlanto-occipital joint is the fulcrum,
and the weight to be lifted is the facial
skeleton.
Example:
scissors
Example:
wheelbarrow
Example:
tweezers or forceps
In the body:
Second-class leverage is
exerted when you stand on tip-toe. The
effort is exerted by the calf muscles
pulling upward on the heel; the joints of
the ball of the foot are the fulcrum; and
the weight of the body is the load.
In the body:
Flexing the forearm by the
biceps brachii muscle exemplifies
third-class leverage. The effort is exerted
on the proximal radius of the forearm, the
fulcrum is the elbow joint, and the load is
the hand and distal end of the forearm.
Load
Load
Load
Load
Load
Load
Load
Load
Effort
Effort
Effort
Effort
Effort
Fulcrum
Fulcrum
Fulcrum
Fulcrum
Fulcrum
Fulcrum
Fulcrum
Fulcrum
Load
Effort
Effort
Effort
Effort
Figure 10.4
Lever systems.
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