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By: Dr. M. Abbas Jamil
Lever:
 A lever is a rigid bar that can rotate about a fixed point when a force is
applied to overcome resistance.
 Parts of Lever:
 Levers has 3 parts:
 Force (effort)
 Resistance (load or weight)
 Axis (fulcrum)
 The fixed point about which the lever rotates is the axis (A), sometimes
referred to as the fulcrum.
 The resistance (R) that must be overcome for motion to occur can include
the weight of the part being moved, gravity, or an external weight.
 The force arm (FA) or effort’s arm (EA) is the perpendicular distance, or
length, between the line of force and the axis while the resistance arm
(RA) or weight’s arm (WA) is the perpendicular distance, or length,
between the line of resistance and the axis.
Classes or orders of Lever:
 First-class lever:
 In a first-class lever, the axis is located between the force and the
resistance.
 Most efficient class of lever
 BALANCE
 A good example of this would be a playground seesaw. The seesaw (lever
arm) rotates on a crossbar (axis), which is located somewhere between a
child sitting on one end of the board and pushing down against the ground
(force) and the weight of the child sitting at the other end (resistance).
Examples in human body
 Skull pivoting on the atlas vertebrae of the spine, with weight of the head
held by the trapezius and sternocleidomastoid muscle of the muscle of the
neck.
 A second-class lever has the axis at one end, the resistance in the middle,
and the force at the other end.
 The wheelbarrow is a second-class lever. The wheel at the front end is the
axis, the contents of the wheelbarrow are the resistance, and the person
pushing the wheelbarrow is the force.
 Power
Examples in human body
 Action of the ball of the foot with the gastrocnemius & soleus muscles of
the calf lifting the weight of the body, which acting through foot during
plantar flexion
 Push ups
 A third-class lever has the axis at one end with the force in the middle and
resistance at the opposite end:
Third-class lever
 Most common in the body
 ROM / Speed
 An example of this type of lever would be a screen door that has a spring
attachment . The axis is the door hinges, the force is the spring that closes
the door, and the resistance is the door itself.
 Baseball bat
Examples in human body
 Action of biceps muscles as it lifts a load in the hand
whilst pivoting elbow
 Action of hamstring muscle
Mechanical Advantage
 The efficacy of force in relation to a Lever is dependant upon two factors:
i.
ii.
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Weight or Effort
Weight’s arm or effort’s arm
Moment of Force:
Product of Force exerted (W) or effort (E) and its perpendicular distance
from the Fulcrum (weight’s arm or effort’s arm) is called moment of
force (torque).
 Ratio between weight to the effort.
OR
 Ratio between force arm and resistance arm.
 MA=W/E
 MA = FA/RA
 Force(effort) Arm > Resistance(weight) arm= MA is >1
 Force(effort) Arm < Resistance(weight) Arm = MA is <1
 The Greater the Mechanical Advantage, the less force is needed to cause
motion.
 Mechanical advantage is always obtained in levers of 1st order and 2nd
order when fulcrum is nearer to the weight than to the effort.
 It is not obtained in levers of 3rd order.
 Mechanical disadvantage is obtained in 1st order lever when fulcrum is
nearer to the effort than to the weight and in all levers of 3rd order.
 When the force arm (FA) is greater than the resistance arm (RA), as with a
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second-class lever, the mechanical advantage (MA) is greater than 1. For
example, if a force arm were 2 feet and the resistance arm were 1 foot, the
mechanical advantage would be:
MA= FA/ RA
MA= 2 / 1
MA = 2
This means that the force arm has twice the length as the resistance arm.
Therefore, it has twice the torque (rotary force).
 If, however, the force arm were shorter (1 foot) and the resistance arm were
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longer (2 feet) as with a third class lever, the mechanical advantage would
be:
MA = FA / RA
MA= 1 / 2
MA = 1/2
In other words, the force arm has half the length of the resistance arm.
Therefore, it has half the torque (rotary force).
 An example in therapeutic exercise is the application of force to a patient’s
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lower leg while the patient tries to keep the knee extended. It takes less
force on your part if you place your hand distally versus proximally.
In this case the axis (A) is the knee joint, the resistance (R) is the insertion
of the quadriceps muscle, and the force (F) is your hand on the lower leg.
the resistance arm is 2 inches and the force arm is 4 inches. The mechanical
advantage can be calculated as follows:
MA =FA /RA
MA = 4 / 2
MA = 2
 The RA remains at 2 inches and the FA is 10 inches. Its mechanical
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advantage is calculated as follows:
MA = FA/ RA
MA =10 / 2
MA = 5
Simply stated, less force is needed to cause motion when the mechanical
advantage is greater.
 If the force arm and resistance arm are the same length, and the amount of
force and resistance are equal, then the system is balanced and no motion
will occur. Using the seesaw example, if child A and child B each weigh 40
pounds, and are both sitting 5 feet from the crossbar, the seesaw will be
balanced. If child A moves one foot closer to the crossbar, the system is no
longer balanced. Child A would go up, and child B would go down. Child A
needs to either add weight (more force) or move back (lengthen FA) to
balance the seesaw again. Remember, with a longer force arm and a shorter
resistance arm, less force is needed to cause motion.