College of Nursing and Health Sciences at umass by sanmelody


          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

   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
              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?
    Mechanics - analysis of the action of
     forces on matter or material systems


              Deformable       Fluid
Rigid Body                                 Relativistic   Quantum
              Body            Mechanics
Mechanics                                  Mechanics      Mechanics

       Rigid Body – objects are assumed to be perfectly rigid
       Deformable Body – objects can be deformed by a force
       Fluid – Gas or fluid
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.
             Stress – Strain Curve
   Import curve here or prolonged stress at
              this strain % will eventually
              result in microdamage
              (i.e. stress fracture)
         Bone Stress-Strain Curve

            Bone is relatively rigid – note the rapid strain

Boney body segments determine human rigidity in biomechanical terms
  Branches of Rigid Body Mechanics
                                     Rigid Body

                   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.
Basic Dimensions and Units of Measurement
    Used in Mechanics & Biomechanics
   Biomechanics is a quantifiable science,
    measurable, and can be expressed in numbers
   Systeme-Internationale d’Unites (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
             Mass vs. Weight
Mass is the measure of inertia, whereas Weight is
 the measure of the force of gravity acting on an
     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)
               Forces & Torques
   Force – a push or pull; exerted by one object on
    another; come in pairs (Newton’s 3rd Law); creates
    acceleration or deformation (Newton’s 2nd Law); causes
    an object to start, stop, change direction, speed up or
    slow down (Newton’s 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
                        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 body’s 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
     (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.)
                       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
               Contact Forces
Normal Reaction Force –
line of action of the force is
perpendicular to the surfaces in

Friction Force – line of action
of the force is parallel to the
surfaces in contact
Reaction & Friction Forces
       Newton’s Laws of Motion
   Newton’s 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
              Newton’s Laws (cont.)
   Newton’s 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
             Mass Moment of Inertia
   Mass Moment of Inertia (I)– The resistance to
    change in a body’s angular velocity
   Dependent on both the objects mass and on the
    distribution of mass about it’s 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
    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?
               Newton’s Laws (cont.)
   Newton’s 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
                  Newton’s Laws (cont.)
   Newton’s 2nd (cont.)
    • Impulse-Momentum Relationship – from F=ma, 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
    Newton’s 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
Newton’s 2nd (cont.) Impulse-Momentum
Newton’s 2nd Law (cont.)
Newton’s 2nd Law (cont.)
               Newton’s Laws (cont.)
   Newton’s 2nd Law (cont.)
    • Work-Energy Relationship -- from F=ma,
      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
            Newton’s Laws (cont.)
   Newton’s 2nd (cont.)
    • Power (P) – the rate of work; W/time
       • W/t = F x D/t or F x Velocity (W=FV)

    • Training power in an athlete requires doing
      work quickly, or explosively

    • How is Power measured and trained in
      sport and exercise?
Measuring and Training Power in the Athlete
Power in Sport
           Newton’s Laws (cont.)
   Newton’s 3rd Law – Law of Action-Reaction
   For every action, there is an equal and opposite
   The two bodies react
    simultaneously, according
    to F=ma ; each body
    experiences a different
    acceleration effect which
    is dependent on its mass
   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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