Biomechanics

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Biomechanics
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Biomechanics -- Defined

• Bio - life living organism
• Mechanics - the branch of physics concerned with
  the analysis of the action of forces on matter or
  material systems
• Biomechanics the study of forces and their
  effects on living systems
• Exercise Sport Biomechanics the study of
  forces and their effects on humans in exercise
  and sport
• Applied or Functional Biomechanics (the focus
  of this class) the examination of the
  application of biomechanics in the exercise and
  sports field

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Human Biomechanics

• Applications of biomechanics (human biomechanics)
• Purpose of the science understand, protect and
  enhance human function
• Role in sport ultimately, to improve
  performance
• Role in therapy rehabilitate, re-educate
• Role in product design to design products that
  
• optimally support human
  function
• Role in injury prevention to minimize adverse
  stress and strain on the body through movement
  analysis, technique design and product
  development
• Role in the workplace Ergonomics - to maximize
  productivity by minimizing worker fatigue and
  discomfort
• Who uses biomechanics?

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Mechanics - analysis of the action of forces on
matter or material systems
Mechanics
Deformable Body Mechanics
Fluid Mechanics
Rigid Body Mechanics
Relativistic Mechanics
Quantum Mechanics
Rigid Body objects are assumed to be perfectly
rigid Deformable Body objects can be deformed
by a force Fluid Gas or fluid
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Humans Rigid or Deformable?

• Biological tissue, including the human body, is
  by nature, deformable. It can absorb forces, it
  can stretch, bend, compress.
• With regards to gross human movement, these
  deformations are relatively small, and for the
  sake of simplicity, Applied or Functional
  Biomechanics largely ignores these properties.
• Each segment of the body is considered a rigid
  body linked together by joints.
• In reality, repeated plastic deformation of
  biological tissue will result in injury.

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Stress Strain Curve

• Import curve here

Repetitive or prolonged stress at this strain
will eventually result in microdamage
(i.e. stress fracture)
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Bone Stress-Strain Curve
Bone is relatively rigid note the rapid strain
Boney body segments determine human rigidity in
biomechanical terms
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Branches of Rigid Body Mechanics
Rigid Body Mechanics
Statics
Dynamics
Statics mechanics of objects
Kinematics
Kinetics
at rest, or at constant velocity
Dynamics mechanics of objects in accelerated
motion
Kinematics describes the motion of a body
without regard to the forces or torques that may
produce the motion
Kinetics describes the effect of forces on the
body i.e.. muscular force, gravitational force,
external resistance force, ground reaction force,
etc.
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Basic Dimensions and Units of Measurement Used in
Mechanics Biomechanics

• Biomechanics is a quantifiable science,
  measurable, and can be expressed in numbers
• Systeme-Internationale dUnites (SI Units)
• Length measured in meters (m)
• Time measured in seconds (s)
• Mass measured in kilograms (kg), the measure of
  inertia, or resistance to a change in motion of
  an object

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Mass vs. Weight

• Mass is the measure of inertia, whereas Weight is
  the measure of the force of gravity acting on an
  object.

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Additional Dimensions Units of Measure

• Length millimeter (mm), centimeter (cm),
  kilometer (km), etc. are all based on the meter
  (m)
• Time Minutes, hours, days, weeks, months,
  years, etc. can all be derived from the second
  (s)
• Mass milligram (mg), gram (g), etc. are all
  based on the kilogram (kg)

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Forces Torques

• Force a push or pull exerted by one object on
  another come in pairs (Newtons 3rd Law)
  creates acceleration or deformation (Newtons 2nd
  Law) causes an object to start, stop, change
  direction, speed up or slow down (Newtons 1st
  Law)
• SI Unit of Force is the Newton (N) force
  required to accelerate a 1 kg mass 1/m/s/s
• Force is described by its size (magnitude) and
  direction
• The angular equivalent of F is Torque (T) a
  Torque rotates an object about an A of R
• T F x moment arm
• Resultant Force the summation of all forces
  acting on a body determines the direction of the
  body

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Forces (cont.)

• Internal Forces and Torques forces or torques
  that act within the studied object i.e. the
  human body, or the object being manipulated by
  the human pole vault, soccer ball, etc. Internal
  forces can cause movement of body segments at a
  joint but cannot produce a change in the motion
  of a bodys C of M. Muscular force is the primary
  internal force examined in biomechanics. As the
  overwhelming majority of motion in the human body
  is angular, torque forces are more applicable in
  biomechanics.
• (The terms Force and Torque will be used
  interchangeably throughout this course.
  Essentially, if the term Force is used to
  describe angular motion, "Torque is implied.)

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Forces (cont.)

• External Forces forces that act on an object as
  a result of its interaction with the environment
  surrounding it
• Most External Forces are contact forces,
  requiring interaction w/ another object, body or
  fluid
• Some External Forces are non-contact forces
  including gravitational, magnetic and electrical
  forces
• The science of biomechanics largely deals with
  contact forces and gravity (weight), which
  accelerates objects at 9.8 m/s
• Contact forces can be sub-divided into normal
  reaction force and friction

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Contact Forces

• Normal Reaction Force
• line of action of the force is
• perpendicular to the surfaces in
• contact
• Friction Force line
  of action
• of the force is
  parallel to the
• surfaces in contact

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Reaction Friction Forces
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Newtons Laws of Motion

• Newtons Laws help to explain the relationship
  between forces and their impact on individual
  joints, as well as on total body motion.
• Knowledge of these concepts can help one
  understand athletic movement, improve athletic
  function, understand mechanisms of injury, treat
  and prevent injury

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Newtons Laws (cont.)

• Newtons 1st Law Law of Inertia
• A body remains at rest or in motion except when
  compelled by an external force to change its
  state. A force is required to start, stop, or
  alter motion
• Inertia the tendency of a body to remain at
  rest or resist a change in velocity
• Inertia is directly proportional to its mass
• The angular equivalent is Mass Moment of Inertia

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Mass Moment of Inertia

• Mass Moment of Inertia (I) The resistance to
  change in a bodys angular velocity
• Dependent on both the objects mass and on the
  distribution of mass about its axis of rotation
• Radius of Gyration the average distance between
  the A of R and the C of M of a body (p)
• I mass of the object multiplied by the square
  of the R of G
• I m x p2

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Law of Inertia Biomechanical Application

• How can an athlete control their Mass Moment of
  Inertia? In other words how can they manipulate
  the resistance to change in angular velocity to
  attain a goal?

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Newtons Laws (cont.)

• Newtons 2nd Law Law of Acceleration
• The acceleration of a body is directly
  proportional to the F causing it, takes place in
  the same direction in which the F acts, and is
  inversely proportional to the mass of the body
• A velocity / time
• F ma (Force mass x acceleration) (linear)
• Angular equivalent of F is Torque (T)
• T F x moment arm (rotational force applied to
  the A of R, through a moment arm)
• T has the same relationship with direction and
  mass moment of inertia as F has with direction
  and mass
• As I (moment of Inertia) increases (due to
  increased R of G or increased mass), Acceleration
  decreases

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Newtons Laws (cont.)

• Newtons 2nd (cont.)
• Impulse-Momentum Relationship from Fma, we can
  derive Momentum (p) and Impulse
• Impulse Force x time (Ft)
• Momentum mass x velocity (mv)
• Ft mv (impulse momentum)
• If Ft increases, mv increases
• Mass is considered constant
• within biomechanics, therefore,
• an increase in impulse implies an
• increase in velocity
• How are the principles of
• Impulse and Momentum
• used in the design of sports
• equipment?

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Newtons 2nd (cont.) Impulse-Momentum

• Because Mass is constant, and because external
  forces are largely non-modifiable, in the world
  of sports and exercise, the duration of force
  application is the most modifiable
• If the Force is not constant, impulse is the avg.
  force times the duration of that average force
• Essentially, calculating force as average force
  holds that force as a constant, however it is the
  peak force that we need to minimize
• If the application of Force is prolonged
  (increased time), in order to maintain the same
  magnitude of impulse (Ft), the Force magnitude
  (average and peak) must be lowered
• Conversely, if the application
• of Force happens more rapidly
• (decreased time), there will be a
• higher Force (avg. peak) in
• order to maintain impulse

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Newtons 2nd (cont.) Impulse-Momentum
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Newtons 2nd Law (cont.) Impulse-Momentum
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Newtons 2nd Law (cont.) Impulse-Momentum
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Newtons Laws (cont.)

• Newtons 2nd Law (cont.)
• Work-Energy Relationship -- from Fma,
• we can also derive Work (W)
• Work Force x Distance
• (W FD) (linear)
• Angular equivalent Torque x Angular
  displacement (T x degrees)
• Measured in Newton meters (Nm)
• Work is a measure of strength,
• measured by the extent to
• which a force moves a body over
• a distance without regard to time

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Newtons Laws (cont.)

• Newtons 2nd (cont.)
• Power (P) the rate of work W/time
• W/t F x D/t or F x Velocity (WFV)
• Training power in an athlete requires doing work
  quickly, or explosively
• How is Power measured and trained in sport and
  exercise?

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Measuring and Training Power in the Athlete
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Power in Sport
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Newtons Laws (cont.)

• Newtons 3rd Law Law of Action-Reaction
• For every action, there is an equal and opposite
  reaction
• The two bodies react
• simultaneously, according
• to Fma each body
• experiences a different
• acceleration effect which
• is dependent on its mass

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References

• Neumann, D.A. (2002). Kinesiology of the
  Musculoskeletal System. St. Louis, Missouri.
  Mosby.
• McGinnis, P.M. (2005). Biomechanics of Sport and
  Exercise 2nd ed. Champaign, IL. Human Kinetics.

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