Kinetic Analysis of Human Motion

Previous             To "Methodology in Biomechanics"             Next

¡@

Basic Kinetics
Loads Acting on the Human Body
Instruments for Measuring Kinetic Variables
Derived Kinetic Variables

  1. Hall, 2003¡GChapter 3, 12, and 14
  2. Chaffin & Andersson, 1999¡G pp. 101-124, 146-158, 167-170 ¡@

¡@

Basic Kinetics

Force

  • an action that changes the state of rest or motion to which it is applied
  • The action of a force results in acceleration of a body
    F = ma
  • SI unit¡G Newton (N)
    1 N = (1 kg)(1 m/s2)
  • external force vs. internal force
  • strength¡G maximum force that a body can generate or be loaded

    • ¡@

    Body

  • an object that may be real or imaginary but represents a definite quantity of matter (mass), with certain dimensions, occupying a definite position in space
  • rigid body vs. deformable body

    • ¡@

    Effect of forces on a body

  • in dynamic sense
  • linear motion (translation) in the direction of net force
  • rotary motion (rotation) in the direction of net moment
  • in static sense
  • static equilibrium if the body is rigid or if the stress is low or if the duration is short
  • deformation (shape changes) if the body is deformable
  • long-term effect on human body¡G biological changes
  • growth
  • injuries
  • degeneration

    • ¡@

    Stress and Strain

  • stress¡G the intensity of force per unit area
  • normal stress¡G the intensity of internal force acting perpendicular to a plane
    s = F / A
  • shear stress¡G the intensity of internal force acting tangent to a plane
    t = F / A
  • SI unit = N / m2 = Pa (Pascal)
  • strain¡G the degree of deformation per unit area
  • normal strain¡G the ratio of the change in length to the original length
    e = DL / L
  • tensile strain is positive while compressive strain is negative
  • shear strain¡G the intensity of internal force acting tangent to a plane
    g = d / h
  • SI unit  noraml strain = %
                shear strain = rad
  • stress-strain curve
  • elasticity¡G the ability of a body to resume its original size and shape on removal of the applied loads
  • elastic (Young's) modulus¡G E = s / e
  • modulus of rigidity (shear modulus)¡G G = t / g
  • plasticity
  • yield point
  • failure point
  • strength¡G maximum force that a body can generate or be loaded
  • e.g. muscle strength or strength of a material

    •   back to top

    Loads Acting on Human Body

    ¡@

    Types of External Loads
  • tensile stress
  • the force applied perpendicular to the body and take it apart
  • the body tends to be elongated in the direction of the applied forces
  • one kind of normal force
  • compressive stress
  • the force applied perpendicular to the body and put it together
  • the body tends to be shrink in the direction of the applied forces
  • one kind of normal force
    •
  • shear stress
  • the force acting in directions tangent to the area resisting the force
  • also named as tangential force
  • bending stress
  • failure under bending stress
  • three point bending: failure at the point of the middle force
  • four point bending: failure at the weakest point
  • torsion stress
  • combined stress

    • Factors Affecting the Extent of Deformation

  • mechanical properties
  • size of the body
  • shape of the body
  • temperature
  • humidity
  • magnitude, direction, and duration of applied forces

    •   back to top

    Instruments for Measuring Kinetic Variable

    ¡@

    Instruments for Measuring Muscle Forces

  • electromyography (EMG)¡G the technique of recording electric activity produced by the muscle
  • muscle activity¡G the change in electric current or voltage as tension is developed by a muscle
  • EMG signals¡G changes in electrical potential across the muscle finer membrane
  • resting potential of a muscle fiber = -90mV
  • action potential of a muscle fiber = 30-40 mV<
  • motor unit action potential (MUAP): EMG signal from the depolarization of a motor unit<
  • to use electrodes recording the level of muscle activity at a given time interval
  • types of electrode
  • surface electrode
  • wire electrode (indwelling electrode)
  • needle electrode
  • parameters
  • activity pattern
  • integrated EMG
  • pecentage of maximum voluntary contraction (MVC)
  • relationship between EMG and force
  • not a linear relationship
  • EMG records the recruitment of motor unit
  • dynamometer
  • localized static strength measurement systems
  • hand-held dynamometer¡G electronic strain gauge
    disadvantages¡G only measuring peak force
  • seated strength tester
  • localized dynamic strength measurement systems
  • Cybex isokinetic system¡G dynamometer
  • Kin-Com isokinetic system¡G load cells
                         
  • whole body static strength measurement system
  • position of load cell can be adjusted to different heights
  • position of load cell can be adjusted to different directions
  • load cell can be attached with different handles
  • whole body dynamic strength measurement system
  • isokinetic lift strength tester
  • using simple electromechanical measuring system for performing a lifting task
  • components of the system
    i. electronic load cell and velocity transducer connected to a readout device ii. constant-velocity motor with adjustable speed control
  • isoinertial strength test (Liftest test)
  • lifting loads with different weights until one¡¦s psychophysiological limit is reached
  • used for personnel selection in US military department
  • Factors Affecting Muscle strength
  • gender
  • static strength: female = 65-85% of male
  • knee isokinetic strength: 70-75% of male
  • age
  • greatest around late 20¡¦s
  • at age of 40, 5% loss of young
  • at ahe of 60, 20% loss of young
  • anthropometric variables
  • body height
  • lean body weight
  • cross-sectional area of muscle
  • pain
  • physical training

    • ¡@

    Instrument for Measuring External loads

  • force transducer¡G a force measuring device that gives an electric signal proportional to the applied force
  • types of transducer
  • capacitive sensor
  • conductor sensor
  • strain gauze
  • piezoelectic sensor
  • capacitive sensor
  • consisting of e electrically conducting plates that lie parallel to each other, separated by a distance that is small compared to the linear dimensions of the plates
  • the space between the plates is filled with dielectric (non-conducting electrical material)
  • A change in force produces a change in the thickness of the dielectric material which is inversely proportional to a current which can be measured
  • F µ 1/Q
    where F= force, Q= total charge of on each plate
  • conductor sensor
  • consisting of 2 layers of conductive material and a conductive material in between
  • the space between the plates is filled with conducting material
  • An increase in force produces a decrease in electric resistance between 2 plates
  • strain gauze
  • made in electric types
  • electrical resistant transducer: wire
  • piezoresistive transducer: silicon
  • piezoelectic sensor
  • non-conducting crystal that exhibits the property of generating an electrical charge when subjected to mechanical strain, e.g. quartz
  • compressive forces produce a change in the electric charges on the surfaces where the force has been applied.
  • shear forces produce a change in the electric charges on the surfaces perpendicular to the applied forces
  • advantage: wide range in measurement of force
  • selection of force transducer
  • capacitive or conductor sensors
  • for measuring forces on soft or uneven surfaces or pressure distribution
  • less accurate (20% of error)
  • strain gauze or piezoelectic sensor
  • for measuring forces on rigid body
  • more accurate (5% of error)

    • ¡@

    Instrument for Measuring Ground Reaction Forces

  • force platform system¡G a ground reaction force measuring system that records forces in vertical, lateral, and anteroposterior directions with respect to the plate itself
  • types of force plate
  • four-corner type¡G a rectangular flat plate with 4-triaxial force transducers mounted at each corner
  • central support type¡G one centrally instrumented pillar which supports an upper flat plate
  • pressure plate system¡G a pressure map system that provides graphical or digital map of pressure across the plantar surface of the feet
  • types of pressure plate system
  • mattress type
  • shoe-insert type

    •   back to top

    Derived Kinetic Variables

    ¡@

    Resultant Force

  •  the net force resulting from the summation of several acting forces on a body
    FR = åFi
  • SI unit¡G Newton (N)
    1 N = (1 kg)(1 m/s2)
    •

    ¡@

    Pressure

  • the force over a given area
    P = F / A
  • SI unit¡G Pascal (Pa)
    1 Pa = (1 N) / (1 m2)
    •

    ¡@

    Moment of Force (Torque)

  • the effect of a rotary force acting on a body
  • the product of force and the perpendicular distance from the point of force action to the axis of rotation
    M = Fd      or      T = Fd
  • SI unit¡G Newton-Meter (N-m)
    1 N-m = (1 N) (1 m)
    •

    ¡@

    Momentum

  • quantity of motion
  • the product of the mass and its velocity of a rigid body in motion
    L = mv
  • SI unit¡G kilogram-second (kg-s)
    1 kg-s = (1 kg) (1 s)
  • principle of conservation of momentum¡G in the absence of external forces, the total momentum of a given system remains constant
    m1v1 = m2v2
  • When a collision occurs between two objects, there is a tendency for both objects to continue moving in the direction of motion originally possessed by the object with the greater momentum. The magnitude of the final velocity is
    v = (m1v1 + m2v2) / (m1 + m2)
    •

    ¡@

    Impulse

  • a large force applied to a rigid body through a small period of time
  • the product of impulse force and the time over which the forces acts
    impulse = F t
  • SI unit¡G Newton-second (N-s)
    1 N-s = (1 N) (1 s)
  • relationship between impulse and momentum
    impulse = F t = m a t = m (vi+1 - vi) = m vi+1 - m vi = DL
    •

    ¡@

    Work

  • product of the force along the direction of displacement and the displacement of a rigid body in motion
    W = F d
  • SI unit¡G joule (J)
    1 J = (1 N) (1 m)
    •

    ¡@

    Power

  • the work done per unit of time

  • the product of the mass and its velocity of a rigid body in motion
    P = W / Dt = F d / Dt = F v
  • SI unit¡G watts (W)
    1 W = (1 N)(1 m) / (1 s) = (1 J)/ (1 s)
    •

      back to top

    Best Viewed with 800 ´ 600                      Use Microsoft Internet Explorer 5.0 or above
    Established on 04/06/2003 and Last Updated 04/07/2003 by Huei-Ming Chai, PT, PhD.
    © 2003 ®ã´f±Ó at School of Physical Therapy, National Taiwan University, Taipei
    All Right Reserved