Biomechanics refers to the study of how anatomical components create movement through muscles generating tension that is transmitted through tendons to bones. There are three types of muscles based on their involvement in movement: agonists, which directly cause movement; antagonists, which slow or stop movement; and synergists, which assist movement indirectly. Levers, fulcrums, moment arms, and mechanical advantage are also discussed in relation to human movement. Power is then defined as the rate at which work is done, where work is the product of force and distance moved.
2. What is Biomechanics
• Biomechanics refers to the study of the
mechanisms by which anatomical components
create movements
– Muscles create tension
– This tension is transmitted to tendons
– Tendons attach to bones
– The transmission of tension through the tendons
pulls on bones
– Bones move through space
4. • The human body is capable of moving in
almost every imaginable direction at velocities
ranging from joint actions that are faster than
a snake’s bite to quasi-isometric slow control
5. Skeletal Muscle
• Skeletal muscle is excitable tissue that
contracts (shortens) via the sliding filament
theory following neural stimulation
• There are 2 sides to every muscle, and in
almost all circumstances, both sides of a
skeletal muscle attaches to bone via tendons
6. Muscle tissue merges into tendon tissue, and the
tendon tissue hooks onto a knobby prominence jutting
off of a bone
7. Origins and Insertions
• Muscles have two attachment sites to bones
• The origin of a muscle is always the part that is
either
– More superior (Trunk muscles/Axial skeleton)
– More proximal (Limb muscles/Appendicular skeleton)
• The insertion of a muscle is always the part that is
either
– More inferior (Trunk muscles/Axial skeleton)
– More distal (Limb muscles/Appendicular skeleton)
8.
9. Types of Muscles – Based on
Involvement in Movement
• We can classify muscles based on the way that
they contribute to a given human movement
• 1. Agonists
• 2. Antagonists
• 3. Synergists
10. Agonists and Antagonists
• 1. Agonists
– The muscle most directly involved in causing a
movement
– The “Prime Mover”
• 2. Antagonist
– A muscle that can slow down or stop a movement
– The antagonist assists in joint stabilization and
decreasing the velocity of a limb towards the end
of a fast movement
11. Synergists
• A synergist assists a movement indirectly
– Often times synergists are considered to be
“stabilizers”
– Without synergists, prime movers would be
ineffective in causing a movement
• Synergists are required to control body
motion when the agonist is a 2 joint muscle
13. Multi-Joint Movements and Synergists,
Part 1
• You cannot perform an effective multi-joint
movement without effective synergist activity
because of the dual actions of 2 joint muscles
• Let’s look at the Olympic style squat
– The Olympic style squat is a knee dominant
exercise (Knee extension is the primary
movement)
– The rectus femoris is a quadriceps muscle that is
considered to be a 2 joint muscle
14. Notice that Rectus Femoris Attaches to
the Pelvis, The Other Quads Don’t
15. Multi-Joint Movements and
Synergists, Part 2: Rectus Femoris
Example
• RF attachments and insertions are
• The actions of the rectus femoris are the
following
– Knee extension (primary movement of squatting)
– Hip flexion (the opposite movement that takes
place during the concentric portion of the squat)
17. Multi-Joint Movements and
Synergists, Part 3: Rectus Femoris
Example
• When the rectus femoris contracts, it causes both
knee extension and hip flexion
• Rising from a deep squat involves both knee
extension and hip extension
• If the rectus femoris is to act to extend the knee
as a person rises without inclining the trunk
forward, then hip extensor muscles (glute max)
must act as synergists to counteract the hip
flexion activity of rectus femoris
19. Levers
• A lever is a rigid object that is used with an
appropriate fulcrum or pivot point to multiply the
mechanical force (effort) that can be applied to
another object (load)
• A lever has the potential to produce or resist
forces
– The lever acts on forces via rotation around a
fulcrum/pivot point
– The lever can create force or resist forces acting on it
20. Lever Systems
• A lever system consists of 7 components
– 1. The lever
– 2. The fulcrum
– 3. The moment arm
– 4. Torque
– 5. Force (muscular force)
– 6. Resistive force
– 7. Mechanical advantage
21.
22. The Fulcrum
• The fulcrum is the pivot point of the lever
• The pivot point creates the central axis of
rotation for a lever
• All gross human movements are lever
movements
– The bone is the rigid body of the lever
– The joint is the fulcrum
23. Types of Levers
• There are 3 classes of levers
– First Class Lever
– Second Class Lever
– Third Class Lever
24. First Class Levers
• First class levers are those where the muscular
force and the resistive force act on opposite
sides of the fulcrum
Resistive
arm
Muscular arm
25. Second Class Levers
• A lever where the muscle force
and the resistive force are on
the same side of the fulcrum
– With second class levers, the
muscular moment arm is longer
than the resistive moment arm
26. Third Class Levers
• A lever for which the
muscle moment arm
and the resistive
moment arm are on
the same side of the
fulcrum
– With third class
levers, the resistive
moment arm is longer
than the muscular
moment arm
27. Torque
• Torque is the degree to which a force tends to
rotate an object about a specified fulcrum
• Torque is the magnitude of a force times the
length of its moment arm
– The longer the muscular moment arm, the less
force required
– The shorter the muscular moment arm, the more
force is required
28. Longer moment arm, less force
needed
Shorter moment
arm, more force
needed
29. Muscle Force
• Force generated by the sliding filament theory
of muscular contraction
• Muscles contract via myosin cross bridges
pulling on actin
– Pulls the ends of the muscle toward the center
– Contraction/shortening
30. Most Muscles Operate at a Severe
Mechanical Disadvantage
• Most human muscles that rotate limbs
operate at a mechanical disadvantage
31. Moment Arm
• The moment arm describes a DISTANCE
• The moment arm distance is the perpedicular
distance between the object creating force
and the fulcrum around which a lever moves
32. Moment Arm of Muscular Force
• The muscular moment arm of the
triceps is a very short distance that
goes perpendicular from the elbow
joint to where triceps force comes
from
33. The Moment Arm of Muscular Force
Length of the muscular
moment arm.
Direction of
Muscular Force
Production
(Contraction)
***Notice how
the muscular
moment arm runs
perpendicular to
the direction of
muscular force
production
34. Moment Arm of Muscular Force &
Cross-Sectional Area
• The larger the cross-sectional area of a
muscle, the larger the moment arm becomes
– Arnold’s triceps are bigger than yours (or mine)
• The mechanical advantage will increase for
the muscular moment arm when the cross-
sectional area increases
35. Resistive Force
• The force generated by a source external to
the body that acts contrary to muscle force
– The 3 major types of resistive forces that muscles
work against are
• 1. Gravity
• 2. Inertia
• 3. Friction
36. Moment Arm of Resistive Force
• The perpendicular distance
between the resistance and
the fulcrum
• The resistance moment is
the perpendicular distance
from the resistance to the
elbow joint
• How does the resistance
arm change as the rep
progresses?
37. Moment Arm of Resistive Force
This is the moment arm of
resistance. Notice how it runs
perpendicular to the direction of
resistance
Note the differences in resistance
to muscle moment arms
This is the direction of resistance
(straight down to the center of the
Earth)
38. Mechanical Advantage
• Mechanical Disadvantage is…
• Moment arm of resistance > moment arm of
muscle force
• Arnold’s triceps are at a tremendous
mechanical disadvantage for the exercise
pictured
39. Calculating the Mechanical Advantage
• To calculate the mechanical advantage, you
must measure the distance of the muscular
moment arm (MM) and the distance of the
resistive moment arm (RM)
• Then divide the MM by the RM
– MM/ RM a
– > 1 mechanical advantage
– < 1 mechanical disadvantage
40. Is this an advantage or disadvantage?
Distance of Rm = 40 cm
Distance of Mm = 5 cm
-Mm = 5 cm
- Rm = 40 cm
- 5/40 = 0.125
-The muscle would have to
generate more than 8 times the
force of the resistance to rotate
the elbow towards the “up” phase
of the exercise
41. What about this?
Resistive arm = 10 cm
Muscular arm = 20 cm
Mm = 20 cm
Rm = 10 cm
20/10 = 2
The muscle only has to generate
half the force of the resistance to
rotate the joint into the “up” phase
of this exercise
44. Tendon Insertions and Mechanical
Advantage
• What are the advantages when the tendon
inserts further from the joint?
• Why?
• If the tendons of a muscle insert farther away
from the joint fulcrum, the muscular moment
arm will be longer, and the mechanical
advantage will be increased
45. Tendon Insertions and Mechanical
Advantage
• What are the disadvantages when the tendon
inserts further from the joint?
• Why?
• If the tendon is inserted farther from the joint
center the muscle has to contract more (in
distance) to make the joint move through a given
range of motion
– A given amount of muscle shortening results in less
rotation of a body segment about a joint – this
translates into less speed
46. Speed of Rotation and Tendon
Insertion
• Get up!!!!!
• We’re going to do a drill
53. Human Strength & Power
• We will now shift the focus to discussing factors related
to how humans express strength and power
• There are a number of variables within the strength
continuum that need to be examined
– Work
– Power
– Displacement
– Velocity
– Neural control
– Types of muscular contractions
– Types of resistances
55. Force
• In physics, the concept of force is used to
describe an influence which causes a free
body to undergo an acceleration
• Force can also be described by intuitive
concepts such as a push or pull that can cause
an object with mass to change its velocity i.e.,
to accelerate, or which can cause a flexible
object to deform
56. Factors Related to Force
• Related concepts to accelerating forces include
• 1. Thrust
– Any force which increases the velocity of the object
• 2. Drag
– Any force which decreases the velocity of any object, and
• 3. Torque
– The tendency of a force to cause changes in rotational speed about an
axis.
• Forces which do not act uniformly on all parts of a body will also
cause mechanical stress, a technical term for influences which
cause deformation of matter
60. Trying to Define Strength
• Strength is intimately related to maximal force
production
• Force = mass x acceleration
– Often times people forget about the acceleration
component of force
– Acceleration = over coming gravity
• Acceleration is a change in velocity per unit time
– Velocity is the rate of change in position
• The best definition of strength is
– The maximal force that a muscle can generate at a
specified velocity
61. Force
Mass = 50 kg
Acceleration:
Gravity = 9.8 m/s2
Force = Mass x Acceleration
The force of this barbell = 50 kg x 9.8 m/s2
62. Force = 50 x 9.8
Force = 490
Velocity = constant
Strength = Maximal Force Produced
At a specific velocity
Because the acceleration of gravity does not change on
earth, then only way to change the force is by increasing
the mass
64. Defining Power
• Power is intimately tied to Work
– Power is work per unit time
• Work refers to the force exerted on an object and the
distance the object moves in the direction in which the
force is exerted
– Work = Force x Distance
• Power is how much work was performed in a given
time
– Power = Work/Time
65. Force = 50 x 9.8
Force = 490
Distance = .75 m
Power = Work / Time
Work = Force x Distance = 490 x .75 = 367.5
In a deadlift, work does not change for a given weight.
Thus, the time taken to do the work will determine power.
Calculate the difference in power when the movement takes 3 sec
vs. 2 sec
Time taken
66. Lets Review…
• Force is an influence which causes an object to
undergo a change acceleration
– Force = Mass x Acceleration
• Strength is the maximal force a muscle can
generate at a specified velocity
– Strength = force x velocity
• Power is the work done in a given amount of time
– Work = force x distance
– Power = work/time
67. Calculating Work – Knowing the Units
of Measure
• Without including units of measure, these
calculations are meaningless
• Work = Force x Distance
• The unit of measure for Work that we will be
using is Joules
• The unit of measure for Force that we will be
using is Newtons
• The unit of measure for Distance that we will be
using is Meters
68. Calculating Work – Gravitational
Forces
• Most resistance training involves gravitational
forces – since gravity is a force, there are 2
variables of interest
– Mass and acceleration
– The mass of the resistance and the acceleration of
Earth’s gravity
• Mass can be represented by kilograms
• The gravity of Earth pulls all objects down
towards the center of the earth at the same
acceleration
– 9.8 meters/second2
69. Calculating Work – Determining
Weight
• To calculate work performed on Earth, we
must determine the weight of the resistance
• To calculate the weight of the resistance, the
scientific community uses the unit of measure
called the Newton
• The Newton equals the mass of the resistance
(kg) multiplied by the acceleration of gravity
– Weight (Newtons) = mass x acceleration of gravity
70. Determining Work – An Example Using
the Deadlift
• We will say that the approximate distance
traveled in the deadlift is 1 meter
• The mass of the barbell being deadlifted will
be 100 kg
• Step 1: determine the weight/force of the bar
– Mass x The Acceleration of Gravity
71. Determining Weight of the Bar – An
Example Using the Deadlift
• Weight/Force (Newtons) = 100 kg x 9.8 m/s2
• Weight/Force (N) = 980 N
• This value in Newtons gives us our FORCE
variable in the equation
• Remember, Work = Force x Distance
• Now we must calculate the distance traveled
72. Determining the Distance Traveled –
An Example Using the Deadlift
• As was previously stated, the distance covered
by the barbell during the deadlift was 1 meter
per repetition
• You must take into account number of
repetitions
• If 10 repetitions are performed, then the
distance traveled = 10 meters
– 10 reps x 1 meter = 10 meters
73. Force = 100 x 9.8
Force = 980
Distance = 1 meter
Calculating Work
10 repetitions were performed for the set, thus:
10 x 1 m = 10 m of distance
74. Determining the Work of a Set – An
Example Using the Deadlift
• Work (Joules) = Force (Newtons) x Distance
(Meters)
• For 1 repetition
– Work (Joules) = 980 N x 1 meter
– Work = 980 Joules
• For 10 repetitions
– Work (Joules) = 980 n x 10 meters
– Work = 9,800 Joules
75. Calculating Power – An Example Using
the Deadlift
• Power = Work/Time
• The unit of measure for power is the Watt
• The unit of measure for time is the second
• To calculate Power, divide the work by the # of
seconds it took to perform the work
76. Calculating Power – An Example Using
the Deadlift
• Let’s say that the set of 10 deadlifts took 100
seconds to perform
– Remember, the total work for the 10 deadlifts was
9,800 Joules
• Power (Watts) = Work/Time
– 9,800 Joules/100 seconds = 98 Watts
77. Power, a case of tall vs. short
• Both athletes are bench pressing 100 kg for 5
repetitions
• Athlete 1 has a ROM of .65 m and takes 7
seconds
• Athlete 2 has a ROM of .5 m and takes 6
seconds
• Who was more powerful?
78. Calculate Work
• Force = 980 newtons ( 100 kg x 9.8 m/s2)
• Work = force x distance
• Athlete 1 work
– 5 x .65 m = 3.25 m (distance)
– Work (joules) = 3.25 m x 980 n = 3185 j
• Athlete 2 work
– 5 x .5 m = 2.5 m
– Work = 2.5 m x 980 n = 2450 j
79. Calculate Power
• Power (watts) = work/time
• Athlete 1
– 3185 j / 7 s = 455 watts
• Athlete 2
– 2450 j / 6 s = 408 watts
• Who expressed more power?
• What did we learn?
80. How Powerful are YOU?!
• Based on force (subject mass x gravity)
– Divide Pounds by 2.2 to = KG, multiply by 9.8
• Calculate work = Force X Distance
– Distance = VJ height in Meters
• Calculate power for all 5 subjects
– Find the time for your VJ height
– Power (watts) = Work / Time
• Who was most powerful
• Was this what you expected?
82. • A more practical formula was devised by (Fox
& Mathews, 1974). The Lewis formula is:
• Power (watts) =
• (square root of 4.9) x body mass(kg) x (square root of jump distance(m)) x 9.8
• More practical because it is difficult to
accurately measure flight time by hand.
83. How can we use VJ to assess power
training?
• If weight stays the same but VJ height
increases?
– If weight decreases but VJ height stays the same?
• If weight increases but VJ height stays the
same?
– If weight increases and so does VJ height?
• If both increase?
• If both decrease?
84. The Interconnectedness of Strength &
Power
• Strength and power share these variables
– Force
– Acceleration
– Velocity
• Strength and power are what separate lesser
qualified athletes from elite athletes
– Genetic
– Training
85. Further Discussion of the Relationship
Between Strength & Power
• You will often hear people describe certain
exercises as strength exercises or power
exercises
– I.e.: Deadlift is strength because it is low speed
– I.e.: Power clean is power because it is fast speed
• It is not correct to associate strength with low
speed and power with high speed
86. Further Discussion of the Relationship
Between Strength & Power
• Strength is the capacity to exert force at any
given speed
– IOW: most force that can be exerted in a single
contraction
• Power is the mathematical product of force
and velocity (distance per unit time) at
whatever speed is happening for the exercise
87. Muscular Power
• Power = Velocity of muscular force
development x amplitude of muscular force
development
– Velocity is how fast the fiber contracts
– Amplitude is how much tension it produces
88. The Practical/Critical Component to
Understand
• Critical: ability to exert force at speeds
characteristic with a given sporting movement
– A powerlifer squatting maximal weight is
producing the highest possible muscular force at
low velocities
• The resistance is high
• A shot-put thrower is producing the highest
possible muscular force at high velocities
• The resistance is low
92. Velocity Assessment
• Training athletes who compete in different sports
requires tremendous velocity assessment and
training specificity
– If you train an offensive lineman exclusively with high
velocity training, you will not be improving the low-
speed strength qualities that are so important to the
sport very effectively
– If you train the shot-put thrower exclusively with low
velocity training, you will not be improving the high-
speed strength qualities that are so important to the
sport very effectively
93. So what?
• A focus of work done should match the force x
velocity continuum in which the athlete
competes
• Work outside this continuum should be
geared to improve force production at
competition speed
94. If we increase this
curve
It will push
the power
curve to
the right
95. Strength will benefit the shot-put
thrower
• Higher load, moderate speed training will
improve absolute tension production
– More force generation per single contraction
• With appropriate realization training, this means
more force can be produced at higher velocities.
– Which means increased power at competition speed!!
• Compare the deadlift to a power clean
96. • The stronger muscle can contract faster at a given load
• Relate this to loads encountered during competition
– For a given tension (say… BODY WEIGHT) the stronger muscle has a greater velocity of contraction
– Recall, power = tension x velocity
99. The application of strength to power
• Improvements in deadlift (high external
load, moderate speed) must be matched by
equal or greater improvements in power clean
(moderate load, fast speed).
• IOW: Focus should be on transforming deadlift
strength to vertical jump or clean power
100. Neural Factors Related to Muscular
Strength & Power
• The 2 neural control factors that are intimately
involved with displaying strength and power
are
– Recruitment
– Rate Coding
101. Muscle fibers
• Type I – slow twitch, fatigue resistant, low
force production
• Type IIA – fast twitch, high glycolytic capacity,
fatigue resistant, higher force production
• Type IIX – fast twitch, largest, highest force
production
102. Motor unit
• A motor neuron and all the muscle fibers it
recruits
• Muscle fibers within the MU are all the same
type (I, IIA, IX, etc.) and size
• The larger the muscle fiber, the more impulse
needed to excite it, and the larger the motor
neuron must be
103. Neural Factors Related to Muscular
Strength & Power
• In general, muscular force is greater when
– 1. More motor units are involved with the contraction
– 2. The motor units are greater in size (fast twitch
motor units, which are the most difficult to recruit)
– 3. The rate of neural firing increases in speed and
efficiency
• Rate coding – how well a message goes from your brain and
down your spinal cord to eventually reach and signal the
individual muscle motor units to fire
104. 1. More motor units are involved with the contraction
105. 2. The motor units are greater in size (fast twitch motor units, which are the
most difficult to recruit)
106. Rate coding - speed
• The repetitive firing of all available motor
units occurs so quickly that there is a
summation of force
• The ability to produce tension is magnified
beyond what you get from regular recruitment
• In essence, your muscles get supercharged
and are able to generate tension above and
beyond what they normally would.
111. Application - Maximal Effort Lifting
• For the shot-put thrower
– Maximal Number of motor units activated
– Maximum discharge (neural stimulation)
– Improve intra- and inter-muscular coordination
112. Application – The Transfer
• Rate coding improves with
– Practice
– Stimulation
• I.e.: Post activation potentiation
– Perform one high tension exercise to increase CNS
recruitment
– After 5 min rest, perform a low tension, high speed movement
to take advantage of increased CNS drive
• Over time the body becomes more sensitive to the
neural discharges and “learns” to accept a new level of
force as being normal for a particular movement
114. Muscular Cross-Sectional Area
• All else being equal, the force a muscle can
exert is directly related to its cross-sectional
area
• The large the fiber size, the more contractile
proteins within in, the greater the tension
development
115.
116. Muscle CSA
• Muscle “Volume”
– CSA X Muscle Length
• Muscle volume does not seem to be a
contributing factor to the muscular force
production
117. • If the CSA of the muscles of their limbs are the same
• They will have the same force production capabilities
40 cm2 arm CSA
26 cm
humerus
length
23 cm
humerus
length
118. Comparing the Volume to the Cross-
Sectional Area
– Both individuals have the same absolute force
production capabilities
– Taller athlete has more muscle volume
– Thus, his/her bodyweight is greater
– Athlete 1:
• Arm CSA 40 cm2 , arm length 26 cm
• 1200 N / 1040 cm3 = 1.15 N/cm3
– Athlete 2:
• Arm CSA 40 cm2, arm length 23 cm
• 1200 N / 920 cm3 = 1.30 N/cm3
119. Strength-to-Mass Ratio
• Strength-to-Mass ratio is intimately related to
cross-sectional area and muscular volume
• Most athletics involved moving the body
through space
• The strength-to-mass ratio directly reflects the
athlete’s ability to accelerate his or her body
120. Strength-to-Mass Ratio
• Strength-to-mass ratio of larger athletes is often
less than smaller athletes
• Greater muscle volume relative to CSA
– CSA is a 2 dimensional phenomenon
– Volume is a 3 dimensional phenomenon
• Hypertrophy
– Volume increases in greater proportion compared to
CSA
• Thus, when body size increases, body mass
increases more rapidly than does muscular
strength
121. Body Size
• The reason that smaller athletes are stronger than
larger athletes pound for pound is because of the fact
that smaller athletes can have equal cross-sectional
area of muscles as larger athletes, but the smaller
athletes have less muscular volume
• So what?
• If an athlete increases muscle mass by 15% and force
production by 10%
• Reduction in ability to accelerate his/her body
• More strength focus in taller athletes
123. Human Strength and Power
• Biomechanical Factors in Human Strength
– 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.
125. Length-tension ratio
• A muscle contracts best when it is at its
optimal length, which is either at resting
length or slightly stretched at 1.2 times its
resting length, depending on the muscle
• Pennated muscles contract best when they
are stretched 1.25 to 1.33 times their resting
length
126. Length-tension ratio applied
• Recall our vertical jump last week…
• Which muscles were involved
• How are they stretched when activating the
jump
• How would they be stretched if starting too
deep or straight up?
127. Arrangement of Muscle Fibers
• There are 2 major arrangement techniques
when it comes to the direction of muscle
fibers inside of a muscle
– Pennate muscles
– Fusiform muscles
129. Pennate Muscle
• The word pennate
traces its routes back
to the Greek word
for “fan”
• These muscles
feature fibers that
“fan out” from the
central aspect of the
muscle belly
130. Fusiform Muscle
• Fusiform muscles feature
fibers that run in straight
lines
• There is no central region
of the belly where the
fibers fan out
• Instead, the fibers run
parallel to one another in
series
131. Pennate Muscles
• Pennate muscles are excellent at generating
low-speed strength
• Demonstrate tremendous “fiber packing”
– more muscle fibers fit into the pennate
arrangement vs. fusiform arrangement
• Pennate muscles have a lower maximal
shortening velocity compared to fusiform
muscles
132. Thoughts on Hamstring Injuries Based
on Fiber Arrangement
• The Hamstrings are a fusiform muscle group
• Fusiform muscles are great for fast contractions;
• their absolute force of contraction is small compared
to pennate muscle groups
• The quadriceps are a pennate muscle group
– Quadriceps become dominant during strength training
– Quads can already be dominant in many athletes
– As a result of this imbalance Q>H , the hamstrings
are more likely to tear
133.
134. Thoughts on Hamstring Injuries Based
on Fiber Arrangement
• Its not just Q>H that matters!
• The glutei muscles are also pennate muscles
– Most people have poorly developed glutei
muscles!!!!!!
– The glutei should be the primary hip extensors
during powerful movements like sprinting
– Most hamstring injuries are due to Q > G
135. Why are Hamstring Injuries Caused by
Weak Glutei?
• When performing powerful hip extension activities, the
body will recruit whatever muscles necessary to
perform action
• Weak glutes = extra hamstring recruitment
• Essentially, the body starts asking the hamstrings to
perform a function that they are not intended to do
– As a result of this, the hamstrings start working in force
ranges that are above and beyond their capabilities
– The connection point between the hamstring and the
tendon or the tendon and the bone typically suffers some
sort of injury as a result
136.
137. Train the Glutes
• With anteroposterior loading
– Increases activation
– Mimics hip extension during sprinting
– Greater activation at full hip extension
– Glutes contract best at nearly full-hip extension
140. Keeping the Bar Close to the Body:
Examining the Resistive Moment Arm
• When someone is lifting barbells or
dumbells, the Moment Arm of Resistive Force
is always oriented in the same direction
– Horizontal
Why?
– The acceleration of gravity is always applying force
in the same direction
• Straight down to the center of the Earth
141. Keeping the Bar Close to the Body:
Examining the Resistive Moment Arm
Direction of gravity’s
resistance
Direction of the
moment arm of
resistive force
If the Rm ends here, the
mechanical advantage is
increased because the Rm is
shorter
If the Rm ends here, the mechanical
advantage is decreased because the Rm
is longer. The longer the Rm, the greater
the muscular force production required
to overcome the resistive force
142. Moment Arms of the Deadlift:
Mechanical Disadvantage
Moment Arms of Muscular Force.
Notice that with a movement like
the deadlift, there are a number of
muscle groups working. Despite
having so many muscle groups
working, all of these Moment Arms
of Muscular Force are much
shorter than the Moment Arm of
Resistive Force
Very long moment Arm of Resistive
Force. The dead lift puts the
muscles at the hip and back at
tremendous mechanical
disadvantage
144. Keeping the Bar Close to the Body:
Examining the Resistive Moment Arm
• Benedikt Magnusson’s 1100 pound deadlift
• From a practical standpoint, the closer to your
body the bar is held, the more you reduce the
moment arm of resistive force
– Closer to body = decreased mechanical
disadvantage
– This is why good deadlifters always have scraped
up shins
– This is critically important for preventing back
injuries
145. The Major Advantage of Standing Free
Weight Exercises Machines
• Standing free weight exercises are the best form
of anti-gravitational resistance training available
– Better than machines, better than cables
• Every muscle in body must contract to stabilize
body
– Machines do not recruit stabilizer muscles
– Machines do not mimic real world scenarios
• Such weight bearing exercises promote bone
mineralization, which fights against osteoporosis
146. Most Common Back Injuries
• Between 85 to 90% of all intervertebral disk
herniations occur at either
– The junction of L4 & L5 or
– The junction of L5 & S1
• Factors that lead to injury
– 1. The tremendous mechanical disadvantage imposed
upon the spinal musculature during 2 foot barbell lifts
– 2. When people allow their lumbar and thoracic spine
to move into kyphosis
147. Moment Arms of the Deadlift:
Mechanical Disadvantage
Moment Arms of Muscular Force.
Notice that with a movement like
the deadlift, there are a number of
muscle groups working. Despite
having so many muscle groups
working, all of these Moment Arms
of Muscular Force are much
shorter than the Moment Arm of
Resistive Force
Very long moment Arm of Resistive
Force. The dead lift puts the
muscles at the hip and back at
tremendous mechanical
disadvantage
148. Kyphotic Back: Absolute Disaster
• The primary way that
people hurt themselves
during 2 foot barbell
lifts is by moving the
lumbar and thoracic
spine into a kyphotic
position
• The cardinal rule to
follow in any weight
room is to never let the
back roll over
149. Disk Injuries
• Kyphosis sets the
stage for possible
vertebral disk
injuries
• Bulges or herniations
of the disk always
occur on the
posterior aspect of
the vertebrae
150. Disk Injuries
• The anterior portion
of the spinal discs
DECREASE in space
upon flexion.
• Both the anterior
and posterior
aspects of the discs
INCREASE in space
upon extension.
151. Protecting the Disk
• The lower back should be moved into lordosis
during 2 foot barbell lifting
– An arched back position
• It has been shown that the muscles of the low
back are capable of exerting considerably
higher forces when the back is arched rather
than rounded
152. Proper Back Positioning
• You’ll see how the lifter in
this picture has her lumbar
vertebrae moved into
lordosis
• Anytime you see an
experienced individual
training with barbells, you
will see them making sure
the low back is arched with
all major lifts
153. Abdominal Tension and Pressure – The
Fluid Ball
• The contents of the abdomen include
– Abdominal muscles
– Parts of the digestive tract
– Parts of the diaphragm
• The most abundant substance in the abdomen is fluid
– The majority of mass in the abdomen is water
• Contract the abdominal muscles forcefully during 2
foot barbell lifts pressurizes the fluids in the abdomen
• Increased abdominal pressure significantly reduces the
forces imposed on spinal erector muscles, and
significantly reduces compressive forces on the disks
155. The Valsalva Maneuver
• Probably every competitive strength athlete
utilizes the Valsalva Maneuver during the
execution of 2 foot barbell lifts
– The Valsalva significantly increases intrabdominal
pressure
• The Valsalva Maneuver involves attempting to
forcibly exhale against a closed glottis
• The Valsalva Maneuver significantly increases
blood pressure in the chest and reduces venous
return to the heart
• Blackout is associated with prolonged Valsalva
156. Friction Resistance
• ALL GRAVITATIONAL FORCES ARE ENTIRELY
VERTICAL IN NATURE
• FRICTIONAL RESISTANCE TRAINING IS
ENTIRELY HORIZONTAL IN NATURE
• Athletes encounter horizontal/frictional forces
during competition, therefore, not training
these forces is a mistake
157. Friction Resistance/Horizontal Force
Production
• The best way to get frictional
resistance/horizontal force production in
during the training process is through the use
of sled work
– Dragging sleds and pushing sleds is a
tremendously challenging form of exercise
• The limitation to sled work is that it is always
harder to get the sled moving, and it is always
easy to keep the sled moving once it starts
159. Coefficients of Friction
• The coefficient of static friction is always
greater than the coefficient of sliding friction
– This is why it is hard to start a sled, but easy to
keep it moving
• Once the sled is moving, the resistance stays
the same, so the resistance does not change
as the speed increases
– The faster the sled goes, the greater the power
output
160. Sled Work is a Killer!
• http://www.youtube.com/watch?v=1ieOBgyq
88E
• http://www.youtube.com/watch?v=lWMEz8Ift
sg&feature=related
• Don’t believe me, check this out
161. So, Have We Learned Anything?
• Tell me anything that you think you have
learned thus far from our biomechanics
discussions
– Now tell me if you can think of any way that you
can apply this new found knowledge