Typical examples of first-class lever are the crowbar, seesaw, and elbow extension. An example of this type of lever in the body is seen with the triceps applying the force to the olecranon (F) in extending the nonsupported forearm (W) at the elbow (A). Other examples of this type of lever may be seen in the body when the agonist and the antagonist muscle groups on either side of a joint axis are contracting simultaneously with the agonist producing force while the antagonist supplies the resistance. A first-class lever is designed basically to produce balanced movements when the axis is midway between the force and the resistance. When the axis is close to the force, the lever produces speed and range of motion (triceps in elbow extension). When the axis is close to the resistance, the lever produces force motion (crowbar).
In applying the principle of levers to the body it is important to remember that the force is applied where the muscle inserts in the bone and not in the belly of the muscle. For example, in elbow extension with the shoulder fully flexed and the arm beside the ear, the triceps applies the force to the olecranon of the ulna behind the axis of the elbow joint. As the applied force exceeds the amount of forearm resistance, the elbow extends.
This type of lever may be changed for a given joint and muscle, depending on whether the body segment is in contact with a surface such as a floor or wall. For example, we have demonstrated the triceps in elbow extension being a first-class lever with the hand free in space where the arm is pushed upward away from the body. By placing the hand in contact with the floor, as in performing a push-up to push the body away from the floor, the same muscle action at this joint now changes the lever to second class because the axis is at the hand and the resistance is the body weight at the elbow joint.
In a first class lever, the weight and force are on opposite sides of the fulcrum:
A small force can be used to advantage over a heavy weight if a long force arm
or lever arm can be used. Examples of this lever include scissors, crowbars, and
An example of a first-class lever is the joint between the skull and the atlas
vertebrae of the spine: the spine is the fulcrum across which muscles lift the
Here the fulcrum lies between the effort and the load. In our bodies, a lever of the first class can be found when the head undergoes nodding movements, i.e. when the occipital condyles articulate with the facets of the atlas. The weight of the face and the head are the resistance. The contraction of the neck muscles is the effort to lift the weight. Another example of a lever of the first class is when the bent arm is straightened . A lever of the first class serves a twofold purpose, i.e. it increases the speed of movement and it overcomes the resistance. In doing so, the resistance (load) is moved in the opposite direction.
Lever of the first class
Second Class Lever
This type of lever is designed to produce force meovements, since a lage rsistance can be moved by a relatively small force. An example of a second-class lever is a wheelbarrow. Besides the example given before of the triceps extending the elbow in a push-up another similar example of a second-class lever in the body is plantar flexion of the foot to raise the body up on the toes. The ball of the foot (A) serves as the axis of rotation as the ankle plantar flexors apply force to the calcaneus (F) to lift the resistance of the body at the tibial articulation (W) with the foot. There are relatively few occurrences of second-class levers in the body.
In the second class lever, the load is between the fulcrum and the force:
A smaller effort can be used to advantage over a larger weight. An example of
this lever is a wheelbarrow.
An example in the human body of a second-class lever is the Achilles
tendon, pushing or pulling across the heel of the foot.
Here the load lies between the fulcrum and the effort. A lever of the second class operates on the same principle as a wheelbarrow. A small upward force applied to the handles can overcome a much larger force (weight) acting downwards in the barrow. Similarly a relatively small muscular effort is required to raise the body weight. In our bodies, a lever of the second class can be found in our feet when we stand on our toes and lift our heels of the ground. The resistance (load) is the weight of our body resting on the arch of the foot. The effort is brought about by the contraction of the calf muscle attached to the heel. This leverage allows us to walk. The main purpose of a lever of the second class is to overcome the resistance.
Lever of the second class
Third Class Lever
With this type of lever the force being applied between the axis and the resistance, are designed
to produce speed and range of motion movements. Most of the levers in the hman body are of this
type, which require a great deal of force to move even a small resistance. Examples include a
screen door operated by a short spring and application of lifting force to a shovel handle with the
lower hand while the upper hand on the shovel handle serves as the axis of rotation. The biceps
brachii is a typical example in the body. Using the elbow joint (A) as the axis, the biceps applies
force at its insertion on the radial tuberosity (F) to rotate the forearm up, with its center of gravity
(W) serving as the point of resistance application.
The brachialis is an example of true third-class leverage. It pulls on the ulna just below the elbow,
and since the ulna cannot rotate, the pull is direct and true. The biceps brachii, on the other hand,
supinates the forearm as it flexes, so that the third-class leverage applies to flexion only.
Other examples include the hamstrings contracting to flex the leg at the knee while in a standing
position and using the iliopsoas to flex the thigh at the hip.
In the third class lever, the force is between the fulcrum and the load:
In this case, there is no force advantage – force is NOT increased. In fact, a
larger force is actually needed to move a smaller weight, so there is a force
disadvantage. The use of this lever is in the gain in speed of movement of the
Examples of this lever class include: The inside door handle of a car, the coiled
spring pulling on a screen door, a pair of finger-nail clippers, and tweezers.
An example of a third-class lever in the human body is the elbow joint: when
lifting a book, the elbow joint is the fulcrum across which the biceps muscle
performs the work.
Here the effort lies between the fulcrum and the load. In our bodies, an example of a lever of the third class is when the biceps contracts, allowing us to lift something in our hand. The elbow is the fulcrum, the hand and its contents are the resistance (or load) and the biceps muscles creates the effort. The load can be moved rapidly over a large distance, while the point of application moves over a relatively short distance. The main purpose of this type of lever is to obtain rapid movement.
Lever of the third class
More Information About Levers & A Brief Review
F A lever is characterized by a fulcrum, a force arm and a weight
F The force arm is the distance from the fulcrum to the point where
force is applied.
F The weight arm is the distance from the fulcrum to the center of
gravity of the weight.
oˆ‚? First Class Lever: The fulcrum is between the force and
oˆ‚? Second Class Lever: The weight is between the fulcrum
and the force.
oˆ‚? Third Class Lever: The force is located between the
fulcrum and the weight.
F Most of the movements of the body are produced by third class
F Third class levers give the advantage of speed of movement rather
F Second class levers give the advantage of strength.
F First Class levers can give the advantage of strength or speed
depending on where the fulcrum is located.
F Since the human body is made up mostly of third-class levers, its
movements are adapted more to speed than to strength. (Short
force arm/long weight arm)
Relationship of the length of lever arms
The ‘resistance arm’ is the distance between the axis and the point of resistance application.
The distance between the axis and the point of force application is known as the ‘force arm’.
There is an inverse relationship between force and the force arm just as there is between
resistance and the resistance arm. The longer the force arm, the less force required to move the
lever if the resistance and resistance arm remain constant. In addition, if the force and force arm
remain constant, a greater resistance may be moved by shortening the resistance arm.
There is also a proportional relationship between the force components and the resistance
components. For movement to occur when either of the resistance components increase, there
must be an increase in one or both of the force components. Even slight variations in the location
of the force and resistance are important in determining the effective force of the muscle.
Decreasing the amount of resistance can decrease the amount of force needed to move the lever.
The system of leverage in the human body is built for speed and range of movement at the
expense of force. Short force arms and long resistance arms require great muscular strength to
produce movement. In the forearm, the attachments of the biceps and triceps muscles clearly
illustrate this point, since the force arm of the biceps is 1 to 2 inches and that of the triceps less
than one inch. Many other similar examples are found all over the body. From a practical point of
view, this means that the muscular system should be strong to supply the necessary force for
body movements, especially in strenuous activity.
Most human activity, and especially strenuous activity, involves several levers working together.
As with throwing a ball, levers in the shoulder, elbow, wrist, hand, and lower extremities
combine to propel the ball. It almost assumes the effect of one long lever from hands to feet. The
longer the lever, the more effective it is in imparting velocity.
Forces in the Body
Athletes display some of the wonderful shows of force that the human body is capable of performing.
Such force is only possible through the arrangement of the muscles, bones and joints that make up the body’s lever systems.
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. 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 chemical energy (created during respiration) into tension and contraction. When a muscle contracts, it shortens, pulling a bone like a lever across its hinge.
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.
There are almost 700 skeletal muscles of the human body, controlled by a few basic principles involving muscle movements or muscular activity.
F Skeletal muscles produce movements by pulling on bones or tendons. The tendon gives a very firm anchorage. The point where a muscle is connected to a bone is called the point of insertion.
F The bones serve as levers and joints act as fulcrums for the levers. Muscles can only contract a short distance, but since they are attached near a joint, the movement at the opposite end of a limb is greatly increased. The biceps muscle of the arm may contract only 89 to 90 mm, but the hand will move about 60 cm.
F The skeletal or voluntary muscles act in pairs rather than singly. One of the muscles produces contraction while the other allows relaxation. Flexion (bending) occurs when contraction causes two bones to bend toward one another, while extension (straightening out) occurs from contraction of muscles, resulting in an increase in angle between two bones. Such pairs of muscles are called antagonistic. Often antagonistic muscles are in groups, for
example, both the brachialis and the biceps muscles flex the arm at the elbow and antagonize the triceps, but only when the palm is facing upwards. In pairs or groups of antagonistic muscle, one is usually much stronger than the other. The biceps, which flex the arm are larger and more powerful than the triceps which extends it.
F When the body is at rest, the some of the antagonistic skeletal muscles remain in a state of contraction, called muscle tone, which holds the body in rigid position. If the person becomes unconscious, or is asleep, muscle tone is lost as the muscles relax completely.
1. A first class lever has the ____________ in the middle.
2. Give an example of a first class lever: ____________________________
3. Draw a diagram of a first class lever:
4. A second class lever has the ______________ in the middle.
5. Give an example of a second class
6. Draw a diagram of a second class lever:
Bones as Levers 8
M. Poarch – 2002
7. A third class lever has the _______________ in the middle.
8. Give an example of a third class lever: ____________________________
9. Draw a diagram of a third class lever:
10. For EACH of the three classes of levers, explain the advantage gained
by using this type of lever.
11. What type of lever do we find most often in the human body?
12. Explain how a muscle exerts force:
Bones as Levers 9
M. Poarch – 2002
13. Examine the following diagrams, write down next to each picture which
class of lever the picture represents and explain why:
Lever in the
Type of lever and why:
Bones as Levers 10
M. Poarch – 2002
14. Describe each of the following:
a. Advantage (mechanical advantage)
g. Force arm
n. Muscle tone
q. Weight arm
Circle and label each
one example of each
class of lever on the
Label the fulcrum,
effort and load for
each class of lever.
CENTER OF GRAVITY: The point in any solid where a single applied force could support it; the point where
the mass of the object is equally balanced. The center of gravity is also called the
center of mass. (When a man on a ladder leans sideways so far that his center of
gravity is no longer over his feet, he begins to fall.)
GRAVITATION (GRAVITY): The force, first described mathematically by Isaac Newton, whereby any two
objects in the Universe are attracted toward each other. (Gravitation holds the
moon in orbit around the earth, the planets in orbit around the sun, and the sun in
the Milky Way. It also accounts for the fall of objects released near the surface of
the earth. Objects near the surface of the earth fall at a rate of 32 feet per second.)
FREE FALL: In physics, the motion of a body being acted on only by gravity.
FRICTION: The force of one surface sliding, rubbing, or rolling against another. Friction slows down the motion
of objects, and can create heat. Friction can also stabilize motion.
FULCRUM: The fixed point about which the lever moves. The point at which energy is transferred.
INERTIA: The tendency for objects at rest to remain at rest, and objects in uniform motion to continue in
motion in a straight line, unless acted on by an outside force.
LEVER: A rigid rod or bar to which a force may be applied to overcome a resistance. A lever (or a
combination of levers) is a simple machine used to gain force, gain speed, or change directions.
LEVERAGE: To wield power with levers. Understanding where the fulcrum is located allows us to position
ourselves to gain our greatest leverage.
MACHINE: A device (or system of devices) made of moving parts that transmits, send or changes a force.
Machines are often modeled on how the human body works.
SCIENCE: An organized body of information or HOW THINGS WORK!
SIMPLE MACHINE: Machines powered by human force (as opposed to batteries, electricity or burning fuel)
In bio-mechanics, the body mass is referred to as load.
If an object is picked up, the load will
be that of the body plus the object
been picked up. The body weight
place a load on the bone and muscle
structures. If no load is applied, the
body will stand still (inertia).
To move the body load, force needs to
be applied. A lighter body load
requires less applied force to be
moved and a stronger body will be
able to move the body load faster.
The secret of success is for the body to become stronger without the body gaining weight.
2.2. FORCE (MOTIVE FORCE)
Force = Mass x Acceleration. Force is the strength of the muscle push or pull required to move
the body mass (load). As long as the force applied on the muscle is equal to the load of the body,
the body will not move and will be in equilibrium (state of rest).
The force applied by the muscles must be bigger than the body mass (load) for the body to move.
Strengthening the muscles will enable the body to apply a larger force on the bones.
The more force muscles apply on the bones, the faster the movement of the limbs will be.
The long jump run-up clearly
demonstrates how the body
angle change in accordance
with the force applied. The
body angle will change to
accommodate the force
The key factor is how much strength training can be applied on the muscle
in an attempt to develop force before an injury will occur in the form of a torn
muscle or a broken bone.
2.2.1. Static force is a force that does not produce motion (The ‘set’
position in the 100m start).
2.2.2. Centrifugal force is the force pulling outwards during rotation (The
discus pulling in the hand during rotation).
2.2.3. Centripetal force is the force pulling inwards during rotation. (The
force pulling in the shoulder while delivering a discus).
2.2.4. Eccentric force is an off-centre force. The centre of
gravity in the human body is more or less situated at
the navel. Delivering a shot needs an eccentric force to
deliver the shot, as the shot is held next to the shoulder
while delivering the shot. Eccentric force requires more
muscle strength than force executed in line with the
centre of gravity. In the sketch it can be seen that the
shot is not in line (above) with the centre of gravity. The
key-factor is to reduce movement away from the centre
of gravity by either bringing the source that requires the
applied force e.g. the shot, closer to the body to avoid
2.2.5. Internal forces will be the force that is applied by the muscles on the bones in the limbs.
2.2.6. External forces will be the force acting outside the body such as the gravity of the earth and
friction between bodies such as the feet and the ground.
Inertia is the body’s resistance to change position (Newton’s 1st law – Law of inertia). If no force is
applied on the body, the body will not move.
2.3.1. Moment of inertia = mass x radius squared. Moment of inertia, normally a very short
period of time, is the moment the body is standing still or in a state of rest e.g. in pole vault,
the trajectory of the body will follow an upwards and downwards motion. At the point where
upwards motion change to downwards motion, a moment of inertia will exist.
Work is force x distance in the direction of force e.g. the amount of time the push or pull of the
muscles is required to move the body over a 1500m x the 1500m = work required.
The key factor is to develop the capacity of the body to operate at a work rate of e.g. 110% during
training. The athlete will then be able to operate at 91% (100% ? 110%) during competition to
achieve success, with less injury risk to the body.
If an 800m athlete wants to run 60 seconds per 400m lap in competition, the training repetitions
should be at 54.6 seconds. Training at repletion times of 54.6 seconds will enable the athlete run at
91% capacity and run a time of 60 seconds in per 400m lap.
Mechanical work = product of weight lifted x distance lifted
Gravity is a force that is always present. It is the magnetic force of the earth which pulls objects
vertically downwards to the centre of the earth.
2.5.1. Centre of gravity is the point
in a body where force acts
through. A solid body like
the shot or discus will have
a fixed centre of gravity but
in the human body the
centre of gravity will be
determined by the position
of the body.
Torque is the force causing an object to rotate x length of lever arm e.g. a longer arm requires more
force to deliver a javelin than a shorter arm.
Key factor – If sufficient force can be exerted on a longer arm, the longer arm is likely to generate
more torque e.g. a longer arm will throw a javelin further than a short arm because more torque can
be applied on the javelin during the process of delivery.
2.6.1. External unbalanced torque must be applied to create
Newton’s 1st law – A body will remain at rest, or
motion will be in a uniform straight line, until an
external force is applied to change its direction is
To deliver a javelin, an upward and forward
movement of the arm is required. The arm holding the
javelin will have to exceed the force required to move
the javelin forward as well as to overcome the
downward force of gravity, before a javelin will be
able to travel in a temporary upwards trajectory after
An axis is a straight line about which a body rotates.
2.7.1. Vertical axis of the body passes through
body from top to bottom when standing in
the upright position.
2.7.2. Sagittal (also called anteroposterior) axis of
the body is an axis parallel to the ground
which passes through the body from front
to back. Key factor – The sprinter will move
from start to finish as fast as possible
without changing the distance of the
sagittal axis from the ground (Moving up
2.7.3. Frontal axis of the body is the axis parallel
to the ground passing through the body
from side to side e.g. the shortest distance
between 2 points is a straight line. Key
factor – The sprinter will move from start to
finish as fast as possible without changing
the distance of the frontal axis from the
sagittal axis (Moving side to side).
2.7.4. Horizontal (also called transverse) axis is an axis which is parallel to the ground and can be
sagittal or frontal.
The sketches below show how the 3 axis’s is applied in bio-mechanics:
When the body is moving, the speed that it is moving, and the time it
takes to move from one point to the next point defines acceleration.
Acceleration is the rate of change of velocity.
Acceleration of the body is in proportion with the force applied by
the muscles in the body. More force will ensure greater acceleration.
2.8.1. Angular acceleration is the rate of change of angular velocity e.g the angular acceleration of
a high jumper crossing the cross bar.
2.8.2. Positive acceleration means the velocity increases faster and faster e.g. a sprinter running
the 1st 100m of a 400m sprint.
2.8.3. Negative acceleration is velocity decelerating (slowing down) e.g. a sprinter running the last
100m of a 400m sprint and exhaustion is resulting in a reduced muscle force.
2.8.4. An object free falling downwards accelerates at 9.8m/sec. e.g. to deliver a javelin, the force
applied must be more than the body mass, the mass of the javelin and gravity force.
After delivery of the javelin in an upwards direction, gravity will continuously pull the javelin
back to earth at a rate of 9.8m/sec. The point of return will be when the combined force of
the body the javelin and gravity are reduced to a force less than the force of gravity
The trajectory of the javelin will consist of positive acceleration (going up), a moment of
inertia (changing direction) and negative acceleration (going down).
Key factor – The bigger the eccentric force applied during the delivery of the javelin, the
longer negative acceleration will be delayed. (The javelin will travel further before returning
to the ground).
Speed is the rate of change of a position. For a sprinter speed will mean the stride length x stride
frequency. For a jumper speed will mean the speed during take-off. For a thrower the speed will
mean the speed during delivery of the implement.
Once the force applied on the body (muscle contraction), is bigger than the load (body mass), the
body will start moving (positive acceleration). The speed per second that the body change position
in a given direction = velocity. If a sprinter covers 100m in 10 seconds the velocity of the athlete will
be 100 ? 10 = 10m/s.
2.10.1. Optimal velocity is sometimes called maximum velocity
2.10.2. Angular velocity is the angle through which the body turns per second e.g. during the period
of time that the jumper travels through air after take off.
Motion is the continuous change of position. As long as force is applied, motion will take place e.g.
as long as the athlete is running motion takes place.
2.11.1. Linear motion is movement in a straight line from one point to another e.g. a sprinter from
start to finish.
2.11.2. Rotational motion is movement around an axis of rotation e.g. the arms and legs of a
sprinter is moving in circular movements while moving forward.
2.11.3. General motion is a combination of linear motion and rotational motion e.g. In the 100m, the
body of the sprinter is moving forward in a straight line but the arms and legs is moving in a
circular motion. In discus the thrower moves from the back of the circle to the front of the
circle while the body is turning around in circles in an attempt to gain maximum speed of the
discus prior to delivery.
2.11.4. Uniform motion is steady, constant motion with unchanged speed e.g a 10000m athlete will
try to run economically in an attempt to maintain the pace of running (uniform motion) as
long as possible.
Momentum is the quantity of motion of a moving body. Momentum = mass x velocity
2.12.1. Angular momentum is the moment of inertia x angular velocity
The level of smoothness of two surfaces making contact will determine the level of friction. The
smoother the surfaces, the more likely a gliding (slip) motion will appear when force is applied at an
A sprinter has to accelerate as fast as possible. To do this force has to be applied through the feet
onto the ground in a running action to ensure forward movement. Fast acceleration may cause the
feet to slip on the ground. To avoid slipping the friction between the feet and ground is increased.
This is done by wearing spikes in the running shoes to create as much friction as possible between
the surfaces of the track and the running shoes.
Equilibrium is another word for balance. When the resultant of all forces acting on a body are zero
(neutralizing each other), the body is in equilibrium.
A body at rest is in equilibrium. The sprinter in the set position is in
equilibrium. When you lie still on a bed, the body is in equilibrium. The
force of the body pressing against the bed and the force of the bed
pushing back are equal, r