Biomechanics of Resistance
• Biomechanics is the study of
body movement
• The body's mechanics are
muscles, bones, tendons and
• Makes use of the laws of physics
and engineering concepts
• Defined as any interaction, a push or pull,
between two objects that can cause an
object to accelerate or decelerate
• Forces are characterized by magnitude
(how much) and direction (which way are
they moving)
• For example a push on the ground
generated by knee extension and hip
extension may cause the body to
accelerate upward-jump
Characteristics of a Force
• Point of Application:
–Specific point at which the force is
applied to an object
–Insertion of the muscle
• Line of Action:
–Direction of the force
–A straight line of infinite length
Types of Forces
• Shear
• Gravity
• Torsion
• Inertia
• Friction
• Muscle Force
• Tension
• Ground
• Compression Reaction Force
• Downward force on an object
• Equal to the object’s mass multiplied
by motion
• Mass is the amount of matter that
constitutes an object and expressed
in kilograms
• Mass is constant, regardless of where
it is measured (earth or on the moon)
• The pound is a unit of force not mass
–The mass of a BB stays constant while
its weight varies according to motion
• The kilogram on a weight refers to its
• It is incorrect to say that an object weighs
a certain number of kilograms (weight
relates to force not mass)
–The mass of the BB is 45kg
Center of Gravity
The CG (center of mass) of a body refers to its
balance point
The CG is that point where all of the weight of the
body is concentrated (CG over BOS)
Anterior to S2 when standing
Center of Gravity
• Changes when body is in motion
– Crouching, kneeling, or a sitting position will lower
the center of gravity and increase stability.
– A wrestler and defensive lineman will increase
stability by lowering their CG
Center of Gravity
• Narrow base of support is unstable
• When lifting or carrying, keep objects COG
close to yours
• Lifting objects far from COG may result in
fatigue, sprain, strain, joint damage…
Center of Gravity and Exercise
• Move limbs and weight away from COG to increase
• Keep limbs and weight close to COG to decrease
• Resistance to movement
• Related to mass of the object
• The greater the mass of an object, the
greater its inertia and harder to move it
• “Cheat lifting movements”
• A baseball player can train with a heavy
bat that provides greater inertial
resistance, which slows down the swing
• A force acting parallel (sandwiched)
between two objects during motion
–When two surfaces pressed
together rub against each other.
–Belt or brake pad, slide board,
football sled
• Fluid Resistance:
–Pushing an object through a liquid
or gas.
–Swimming, cycling, skydiving,
baseball, rowing, kayaking, golf
• A stretching force pulling at both ends
–Pull of a contracting muscle tendon
• A force that tends to flatten or squeeze an object
– Lifting a BB presses the ends of the bones together
and is produced by muscles and gravity along the
length of the bones
• Necessary for the development of bone growth
• If a large compressive force is applied, and the load
surpasses the stress limits, a fracture or break will
Shear Force
• Applied parallel to the surface of the object
– Across the surface of a bone
• A good example of shear force is seen with a
simple scissors.
• The two handles put force in different directions
on the pin that holds the two parts together.
• Twisting force
– Foot is planted and the body changes
direction (ankle sprain)
• The force which causes rotation
• Rotation occurs about a pivot point
• Line of action of force must act at a
distance from the pivot point
Muscle Force
• A muscle can only generate a pulling or
tensile force
–Elbow extension: the triceps brachii
pulls on the olecranon process of the
–Elbow flexion: biceps brachii pulls on the
radial tuberosity of the radius
–Movement at any joint occurs by
opposing pairs of muscles and gravity
assists in the motion
Ground Reaction Force (GRF)
• Force provided by the surface an individual is moving
on (i.e., sandy beach, gym floor, grass lawn, concrete
sidewalk) and the reaction to the force the body exerts
on the ground
• If the body is pushing down and forwards, the ground
reaction force is up and backward; if the body pushes
down and backward, the ground reaction force is up
and forwards
Levers of the Musculoskeletal System
• Involve bones, joints, and skeletal
• Bring about most movements of the limbs
and the whole body
• Body movements characteristic of sport
and exercise act primarily through levers
of the skeleton in order to exert forces on
the ground, object, and other people
• Muscles not acting through bony levers
include those of the face, tongue, heart,
arteries, and sphincters
Parts of a Lever
• Lever: a rigid or
partially rigid
structure that can
rotate about a pivot
point or fulcrum.
• Fulcrum (pivot
point): axis of
rotation or the point
about which the
lever pivots
• The axis is the point
of rotation about
which the lever
• Bones of the body act
as levers which
create a mechanical
advantage of speed
or strength
• The fulcrum is formed
by the joint
• Effort is any force
applied to the lever
• Load or resistance is
a force that resists
the motion of the
Muscle Force
• Effort Arm/Force Arm
• The force exerted by a muscle at either of its
ends when its electrochemically stimulated to
Moment Arm (Lever Arm or Force Arm)
• Moment arm of
the muscle force
• Moment arm of
the resistive
• Distance
between the
point where the
force acts and
the point of
• Levers =
humerus, radius,
and ulna
• Effort=
contraction of
• Load= weight of
the arm, gravity…
• Fulcrum or axis
of rotation =
humeroulnar joint
Mechanical Advantage (MA)
• Mechanical
Advantage: created
by a machine that
enables people to do
work while using less
• Used to allow a small
effort to move a large
• It is calculated by
dividing the load by
the effort
Mechanical Advantage (MA)
• A normal human can't lift a 1,000-pound vehicle
off the ground, so you need the mechanical
advantage of a jack.
• If the weight of the vehicle (the load) is 1,000
pounds and the weight of the jack handle (the
effort) is seven pounds, then the mechanical
advantage provided by the jack is 1,000/7, or
"one thousand to seven."
Mechanical Advantage (MA)
• A MA greater than
1.0 means that
muscle force exerted
is less than resistive
• A MA less than 1.0
means that muscle
force exerted is
greater than the
resistive force
• A MA less than 1.0
is a disadvantage
Musculoskeletal System
• Most of the skeletal muscles operate at a
considerable mechanical disadvantage.
• Variations in Tendon Insertion
– Tendon insertion farther from the joint
center results in the ability to lift heavier
• This arrangement results in a loss of
maximum speed.
• This arrangement reduces the muscle’s
force capability during faster movements
Three Classes of Levers
• Each providing
different levels of
• The levers are
referred to as class
1, class 2 and class
• Differs by the force
you apply (effort),
load (opposing
force), and fulcrum
(pivot point)
First Class Lever
• Fulcrum is placed between the load and the
• The force you apply is on the opposite side of
the fulcrum to the force you produce
First Class Lever
• If the two arms of the lever are of equal
length, the effort must be equal to the load
• If the effort arm is longer than the load
arm, as in a crowbar, the effort travels
farther than the load and is less than the
• The mechanical advantage is calculated
by measuring the length of the lever on
either side of the fulcrum.
First Class Lever
• Triceps
–Muscle force
and resistive
force act on
sides of the
First Class Lever
• The head is raised off
the chest
• As the head is raised,
the facial portion of
the skull is the
resistance, the
fulcrum is between
the atlas and occipital
bone, and the effort is
the contraction of the
muscles of the back
First Class Levers
• Balanced Movements
– Fulcrum near the middle
– Seesaw
– Erector spinae extending the head
• Speed and Range of Motion
– Fulcrum is near the effort
– Scissors
– Triceps extending the elbow
• Force motion
– Fulcrum is near load
– Crow bar
Second Class Levers
• Load between the effort and the fulcrum
• A wheelbarrow is a second-class lever. The wheel’s
axle is the fulcrum, the handles take the effort, and the
load is placed between them.
• The effort always travels a greater distance and is less
than the load.
Second Class Lever
• Gastrocnemius and
soleus in
plantarflexing the foot
to raise the body on
the toes
• The ball of the foot is
the fulcrum,
gastrocnemius and
soleus are the muscle
force, and the body is
the resistance
Second Class Levers
Third Class Levers
• Effort placed between the load and the fulcrum
• Increase distance and speed not force
• A hammer acts as a third-class lever when it is
used to drive in a nail: the fulcrum is the wrist,
the effort is applied through the hand, and the
load is the resistance of the wood.
Third Class Lever
• DB Biceps Curl
• The fulcrum is the elbow, the effort is applied by
the biceps muscle, and the load is the weight in
the hand.
• Most movements of the body are produced by
third class levers
Third Class Lever
• Pitchers use their arms
as third-class levers.
• During a pitch, the
pitcher’s forearm pivots
at the elbow, which is
the fulcrum.
• The muscles of his
forearm supply the
force, and the ball
provides the resistance.
• The lever action of a
pitch magnifies the
speed a pitcher can
give to the ball.
Summary of Levers
Most body movements produced are
third class levers
We are more adapted to speed than
strength (short force arm/long weight
First class levers give the advantage
of strength or speed depending on
where the fulcrum is located
Second class levers give the
advantage of strength
• First Class Lever : E F L or L F E
• Second Class Lever: F L E or E L F
• Third Class Lever: F E L or L E F
(F E L)
1. Identify the 3 parts of a lever.
2. Draw and label a diagram of
each of the 3 types of levers
3. In lifting an object, the biceps
represents the _____________
force; the elbow represents the
_____________; the object lifted
represents the ______________
force. This is an example of a
______________ class lever.
Identify the Classes of Levers
5. In a first class lever, the
_________________ is between the
effort and the load.
6. In an off-center first class lever (like
a pliers), the load is larger than the
effort, but is moved through a
7. Identify two examples of first class
8. In a second class lever, the
________________ is between
the fulcrum and the effort.
9.Identify two examples of second
class levers.
10. In a third class lever, the
____________________ is
between the fulcrum and the load.
11. Identify three examples of third
class levers.
12. A man lifts a rock weighing 500
pounds by standing on the end of a
lever. When the man later stood on a
scale and weighed himself, he
weighed in at 100 pounds.
How much mechanical advantage did
the lever provide to the man in lifting
the rock?
13. A person wants to use a lever to lift a
large piece of machinery. The person
uses a lever that was previously used to
lift another piece of similar machinery and
will be set up identically for this lift. The
mechanical advantage in the last set up
was 8. The machinery weighs 800
pounds. What force or weight must the
person place on the lever to move the
• MA = 500 pounds/100 pounds = 5
• Conclusion: The man's body weight was
magnified 5 times by using the lever as a
simple machine to lift the rock.
• Force with simple machine = (Force
without simple machine)/(MA)
Force with simple machine = 800/8 = 100
• Conclusion: The person would have to
weigh or apply a force of 100 pounds to
move the machinery.
• Strength:
– The capacity to exert force at any given
• Acceleration:
– Increase in velocity or speed
• Deceleration:
– Decrease in velocity or speed
• The force that a muscle exerts decreases as
movement speed increases
• Power:
– The time rate of doing work
– force x distance/time = work/time
• The effect of a force applied
over a distance
• Work = Force x Distance
• If a work crew uses a pulley to hoist an
800-pound piano off the ground 20 feet
into a second-story window, the amount of
work done is 20 feet times 800 pounds,
equaling 16,000 foot-pounds of work
• If it took the crew 40 seconds to hoist the
piano, the average power would be
16,000 foot-pounds divided by 40
seconds, equaling 400 foot-pounds per
Work and Power in SI Units
• The worldwide standard of measures
is called System International (pg.35)
• Unit of Force is the newton (N)
• Power is Watts (W)
• Unit of Distance is the meter (m)
• Unit of Mass is the kilogram (kg)
Calculating Resistance Training Work
• To accurately assess the work in a lifting
session measure the vertical distance
the weight moves per repetition
• Work = Weight x Vertical Distance x
• Consider the low and high points during
the exercise movement
• Weight of the body should be considered
• Compare a squat to a leg press
Biomechanical Factors in Human Strength
Neural Control
Muscle Cross-Sectional Area
Arrangement of Muscle Fibers
Muscle Length
Joint Angle
Muscle Contraction Velocity
Strength-to-Mass Ratio
Neural Control
• How many and which motor units are involved
in a muscular contraction (recruitment)
• During the first few weeks of resistance
training, much of the strength improvement
can be attributed to neural adaptations
• The brain learns how to produce more force
from a given amount of muscle tissue
• Muscle force is greater when:
– More motor units are involved in a
contraction, the motor units are greater in
size, or the rate of firing is faster.
Muscle Cross-Sectional Area
• Cross sectional area is proportional to tension
• Area is measured perpendicular to fiber length
Muscle Cross-Sectional Area
• Two people who have different heights
but the same body fat % and weight will
not have the same cross sectional area.
• The taller person will have less cross
sectional area and less strength in
proportion to body mass.
• Taller person will find it more difficult to
perform pull-up’s, push-up’s and sprints.
• Strength of a muscle is not related to it’s
Arrangement of Muscle Fibers
Pennate Muscles
• Pennate muscles (i.e., deltoid, rectus femoris,
tibialis posterior) have fibers that align obliquely
with the tendon, creating a featherlike
• Pennate muscles have greater cross-sectional
area (and greater tension development
capability) than parallel muscles (extensor
Muscle Length
• At resting length: actin and myosin filaments lie
next to each other; maximal number of potential
cross-bridge sites are available; the muscle can
generate the greatest force.
• When stretched: a smaller proportion of the
actin and myosin filaments lie next to each other;
fewer potential cross-bridge sites are available;
the muscle cannot generate as much force.
• When contracted: the actin filaments overlap;
the number of cross-bridge sites is reduced;
there is decreased force generation capability
Length-Tension Relationships
• The length at which a muscle can produce
the greatest force
Joint Angle and Muscle Contraction
• Joint Angle:
– Higher muscle forces results in higher torque,
which means a greater ability to rotate the limb
about a joint against resistive torque
• Muscle Contraction Velocity:
– Muscle force (tension) and speed of a
contraction (rate) are inverse
– As the speed of a concentric muscle contraction
increases, its ability to produce force decreases
(overlapping of myosin and actin)
Muscle Actions
• Concentric: the muscle
shortens because the
contractile force is greater than
the resistive force. The forces
generated within the muscle
and acting to shorten it are
greater than the external forces
acting at its tendons to stretch it
Muscle Actions
• Eccentric: the muscle lengthens because the
contractile force is less than the resistive force.
The forces generated within the muscle and
acting to shorten it are less than the external
forces acting at its tendons to stretch it.
• Isometric: the muscle length does not change
because the contractile force is equal to the
resistive force. The forces generated within the
muscle and acting to shorten it are equal to the
external forces acting at its tendons to stretch it.
Force-Velocity Relationship
Strength-to-Mass Ratio
• Equals the force the person can exert
during movement divided by the mass
of the body
• An individual weighing 150 pounds
can leg press 200 pounds, the leg
press strength to mass ratio is
200/150 = 1.33
• However, if the same strength gain is
accompanied by a body weight
increase of 170 pounds, the strengthto-mass ratio is decreased to 1.29
Strength-to-Mass Ratio
• As body size increases, body mass increases
more rapidly than does muscle strength
• Given constant body proportions, the smaller
athlete has a higher strength-to-mass ratio than
does the larger athlete
• Bodybuilders vs. Gymnasts vs. Rock Climbers
• Individuals must consider whether the exercises
they select will likely add body mass to a
greater extent than increase in strength
• Greater strength-to-mass ratio means improved
physical performance
Sources of Resistance to Muscle
Fluid Resistance
Electronically Controlled Devices
• A chain, belt or rope wrapped around a wheel
• The mechanical advantage of a pulley system
is approximately equal to the amount of
supporting ropes or strands.
• If you had a weight of 60 kg and you wanted to
lift it using two pulleys that used two ropes
looped between them, you would have a
mechanical advantage (MA) of two
• Having two ropes that give you a mechanical
advantage (MA) of two means your weight feels
like one half of what it really is.
• When lifted with the help of the pulleys 60kg
would only feel like 30 kg.
• Thus, the effort weight equals 30 kg
• If a pulley setup has three supporting strands,
what would be the MA of the setup?
• If the weight of an object being lifted is 100 kg
and the number of supporting ropes the pulley
system has is four; what would be the systems
• How much effort weight would you actually be
• The weight of an object is 30 kg, the
mechanical advantage is three, how much effort
weight would you need to raise the object?
• (Answers: 3, 4, 25 kg, 10 kg)
Elasticity and Electronically Controlled
• Elasticity:
–Springs or bands for resistance
–Tension increases with stretch
–Tension begins low and ends high
unlike human muscle groups in which
there is a decline in force toward the
end of the movement
• Electronically Controlled Devices:
– Regulate degree of resistance (Life Circuit)
• When the weight is horizontally farther
than the joint, it exerts more torque
• When the weight is horizontally closer
to the joint, it exerts less torque
• During a BB curl the horizontal
distance from the BB to the elbow is
greatest when the forearm is
• The lifter must exert the greatest
muscle torque to lift the weight
• During a squat the combined center of
mass of the body and BB must be
positioned above the foot so that the
lifter does not fall
• Placing the bar in different positions
causes the lifter to adjust body
posture to keep center of mass over
base of support to avoid falling
• BB positioned low on the upper back,
the trunk leans forward to keep the
center of mass over the base of
support which brings the bar farther
from the hip and closer to the knees
(hip extensors work harder)
• BB positioned high on the back or in
front of the shoulders brings less trunk
lean so the load is placed on the
• The Smith machine has the
advantage of allowing the feet to be
positioned behind or in front of the
center of mass of the body without
causing a fall
• With feet farther forward, the trunk
stays more upright which reduces
the torque the back muscles
generate making the quadriceps do
more work
• Any exercise in which the whole
body is lifted involves work against
• Doing an exercise faster increases
acceleration forces; more work
against gravity and inertia
–Power Clean
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