Download Levers - POLYTECH High School

A Quick Guide to Levers
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A simple machine used to increase the efficiency
of muscle contractions around a joint
Language of levers:
◦ Muscular effort: how hard a muscle has to work to pull a
load
◦ Fulcrum – the support or point on which a lever moves
 In the case of muscle movement, the joint is the fulcrum
◦ Resistance – the weight of the object you are trying to
move
◦ Distance – the amount of space between the fulcrum and
the load, and the fulcrum and the site of muscular effort
◦ Class – the type of lever; based upon the site of the
fulcrum, the site of the muscular effort, and the site of
the resistance
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The degree of muscular effort required to overcome
resistance to movement at the joint depends upon:
◦ the force of the resistance (weight);
◦ the distance from the fulcrum to the site of muscular effort;
◦ and the distance from the fulcrum to the site of resistance
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*WHAT THIS MEANS*
How hard your muscle has to work to cause
movement at the joint depends upon:
◦ how heavy the object is that you want to move (the
resistance);
◦ how far away the joint (fulcrum) is from the muscle being
used to move the object;
◦ and how far away the joint is from the object that you are
trying to move
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The human body moves all things through force
Bones act as the levers, while joints perform as living
fulcrums
Skeletal muscles create motion by pulling on tough cords of
connective tissue called tendons
◦ These tendons in turn pull on the bone which creates motion
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Muscles move bones through mechanical leverage
◦ As a muscle contracts, it causes the bone to act like a lever with the joint serving as a
fulcrum
Muscle exerts force by converting electrical energy (from the brain)
and chemical energy (created during respiration and digestion) into
tension and contraction
◦ When a muscle contracts, it shortens, pulling a bone like a lever across its hinge
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Muscles move and this causes us to move
We are capable of performing a wide variety of movements, but,
muscle itself moves only by becoming shorter
They shorten and then they rest - a muscle can pull but it cannot
push
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Levers are classified by the following:
◦ The position of the joint (fulcrum) compared to:
 The site of the muscle pull, and;
 The site of the weight (resistance or load) you are
trying to move
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The joint (fulcrum) lies between the pulling muscle
and the weight (load)
Most efficient class of lever
Can provide strength or speed depending upon the
location of the fulcrum
Example – The neck joint during neck extension
◦ Fulcrum = The joints between the cervical vertebrae
◦ Muscle = The muscles on the back of the neck
◦ Load = A weight hanging from the head
L
M
F
M
F
L
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The weight (load) lies between the joint (fulcrum)
and the pulling muscle
Gives the advantage of strength
Example – The metatarsal-phalangeal (toe) joint
during plantar flexion
◦ Fulcrum = The toe joint
◦ Muscle = The muscles of the calf
◦ Load = The body weight
L
M
F
L
M
F
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The pulling muscle lies between the joint
(fulcrum) and the weight (load)
Least efficient class of lever – only advantage
is speed of movement
Example – The elbow joint during flexion
◦ Fulcrum = The ulnar-humeral joint
◦ Muscle = Biceps brachii
◦ Load = Weight in the hand
F
M
L
(M)
(M)