1. LEVERS
By Dr.Nijal Parmar

2. Introduction
• Definition:
• Classification of lever:
• MA=W/E
WA
F
EA
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d
e
f
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3. Orders of levers
❑1st Order lever:
✓3 possibilities in 1st order lever
F
l
o
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e
f
f
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4. F
l
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f
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5. F
e
f
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d

6. A first-class lever is a lever system in which the axis lies between the point of application
of the effort force and the point of application of the resistance force, without regard to the
size of EA or RA.
It is order of stability and state of equilibrium can be achieved either with or without
mechanical advantage.
One example of this type of lever is demonstrated during nodding movements of the head,
the skull represents the lever, the atlanto-occipital joints the fulcrum, the weight is situated
anteriorly in the face, and the effort is supplied by the contraction of the posterior neck
muscles.
Second example of a muscle work- ing on a first-class lever is the pull of the supraspinatus
on the humerus.The attachment of the supraspinatus on the greater tubercle of the
humerus is on the opposite side of the composite axis of rotation for the glenohumeral joint
from the CoM of the upper extremity, which is just above the elbow.
Because the muscle and the gravitational force lie on either side of the joint axis,
this remains a first-class lever whether the supraspinatus is contracting
concentrically (as the EF) or eccentrically.

9. • 2nd Order lever:

10. A second-class lever is a lever system in which the resistance
force has a point of application between the axis and the point of
application of the effort force, which always results in EA being
larger than RA.
This is the lever of power as there must always be a mechanical
advantages. An example in the lower limb is demonstrated when
the heels are raised to stand on the toes.The tarsal and
metatarsal bones are stabilised by muscular action to form the
lever, the fulcrum is at the metatarsophalangeal joint, and weight
of the body is transmitted through ankle joint to the talus. The
effort is applied at insertion of the tendon of calf muscle.

12. • 3rd Order lever:

13. A third-class lever is a lever system in which the
effort force has a point of application between the
axis and the point of application of the resistance
force, which always results in RA being larger than
EA .
In the human body, there are more examples of the
3rd order of levers than of any other type.This type
of lever, in the loss of mechanical disadvantage, is
the lever of velocity, gained by speed and range of
movement.
One example of 3rd order lever is to lift heavy weight
holding through wrist. So in that fulcrum is elbow
joint, and resistance is weight being lift and effort is
by brachialis muscle.

15. Mechanical advantage
• Mechanical advantage (M Ad) is a measure of the
mechanical efficiency of the lever (the relative
effectiveness of the effort force in comparison with the
resistance force). Mechanical advantage is related to
the classification of a lever and provides an under-
standing of the relationship between the torque of an
external force (that we can roughly estimate) and the
torque of a muscular force (that we can estimate only in
relation to the external torque). Mechanical advantage
of a lever is the ratio of the effort arm (MA of the effort
force) to the resistance arm (MA of the resistance
force), or
• MA= EA/RA

16. When EA is larger than RA, the M Ad will be
greater than 1. The “advantage” of a lever
with a mechanical advantage greater than 1 is
simply that the effort force can be (but is not
necessarily) smaller than the resistance force
and yet will nonetheless create more torque to
“win.” The torque of the effort force is always
greater than the torque of the resistance
force; that is, (EF)(EA) (RF)(RA). If EA is
greater than RA, then the effort torque can
still be greater than the resistance torque if EF
is smaller in magnitude than RF.

17. In third-class levers, the Mechanical Advantage will
always be less than 1 because EA is always smaller than
RA (the effort force lies closer to the axis than the
resistance force). A third-class lever is “mechanically
inefficient” or is working at a “disadvantage” because the
magnitude of the effort force must always be greater than
the magnitude of the resistance force in order for the
torque of the EF to exceed the torque of the RF (as it must
for the force to “win”). In a first-class lever system, the EA
can be larger than, smaller than, or equal to the RA.
However, because the distal attachment of a muscle tends
to be closer to the joint axis than is the point of application
of an external force even when muscles are working on
first-class levers, muscles working in first-class lever sys-
terms (like those in third-class systems) tend to be at a
mechanical disadvantage.

18. References
• The principles of exercise therapy 4th
edition by DENA GARDINER
• Joint structure and function 4th edition by
CYNTHIA NORKINS

19. Thank you