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Lever

• It is a simple machine consisting of a relativelyrigid bar-like body that rotates about a fixedpoint called the fulcrum.

•

Basically it consists of two forces. – An effort force

 – A resisting force

• These forces act around a supporting

force which provides for a point ofsupport called the fulcrum

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Levers

 A lever comprises of three components  – 

• Fulcrum or pivot - the point about which thelever rotates

• Load - the force applied by the lever system

•

Effort - the force applied by the user of thelever system

• The way in which a lever will operate is

dependent on the type of lever.

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Classification of Levers

• Class 1 - The fulcrum lies betweenthe effort and the load

•Class 2 - The fulcrum is at one end,the effort at the other end and the load lies

between the effort and the fulcrum

• Class 3 - The fulcrum is at one end,

the load at the other end and the effort lies

between the load and the fulcrum

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Levers

Lever:

Fulcrum:

First class lever

Second class lever

Third class lever

 Anatomical lever

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When muscles develop tension, pulling on

bones to support or move the resistance

created by the weight of the body segment(s)and possibly the weight of an added load, the

muscle and bone are functioning mechanically

as a lever.

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Torque:• T = Fd

 

Moment arm:

• In the body, moment arm of muscle is theperpendicular distance between muscle's line pulland joint center

• Largest moment arm at an angle of pull ~900 

• Vector quantity, magnitude and direction

 – Fd & counterclockwise (+) & clockwise (-)

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Force arm: the distance from theeffort to the fulcrum.

Resistance arm: the distance fromthe resisting force to the fulcrum

Mechanical Advantage: de/dr

 A lever may be used to increaseforce, change the effectivedirection of the effort force, or gaindistance.

In human body, for producingbody movements, bones act aslevers and joints function asfulcrums of these levers.

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Fulcrum

The point of support, or axis, about which a

lever is made to rotate or move.

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First Class Lever

• lever positioned with the

applied force and the

resistance on opposite

sides of the axis ofrotation

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Second Class Lever

• lever positioned with the

resistance between the

applied force and the

fulcrum

• They are levers of

strength, ROM is

sacrificed.

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Third Class Lever

• lever positioned with the

applied force between

the fulcrum and

resistance.

• They allow greater

range of motion.

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 A:

First

Class

B:Second

Class

C:

Third

Class

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Most levers within the body are class three

 A: Biceps at the elbow

B: Patellar tendon at the knee

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Lever Systems

Mechanical advantage = Moment arm (force)

Moment arm (resistance)

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Leverage: It is the mechanical advantage

gained by a lever and is responsible for a

muscles strenght (force of contraction) andrange of motion, ie., the ability to move

bones of joint through an arc.

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• Consider two muscles of same strengthcrossing and acting on the same joint.

•  Assumption:

 – One is attached farther from the jointand one is attached closer.

• The one attached further will producemore powerful movement. Thusstrength depends on the placementof muscles attached.

• The one attached nearer will producegreater range of motion. Thus ROMalso depends on the placement of

muscles attachment.

Hence as strength increases ROM decreases.

Therefore, maximal strength and maximal range are

incompatible with each other.

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 A lever system can serve one of the two purposes:

When the moment arm of the applied force is greater than themoment arm of the resistance, the magnitude of the appliedforce needed to move a given resistance is less than themagnitude of the resistance.

Whenever the resistance arm is longer than the force arm, theresistance may be moved through a relatively large distance.

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 Anatomical Levers

• In the human body, most lever systems are

third class

• Arrangement promotes – Range of motion

 – Angular speed

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Summary

•  A muscle develops tension and producestorque at the joint that it crosses.

• Muscle and bones function as levers.

• The angle of muscle pull on a bone producesrotary and parallel components of force

• When a body is motionless, it is in staticequilibrium.

•

Stability is the resistance to disruption ofequilibrium

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Thank You