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Biomechanics: Outline. Definition Types of Motion Measuring Motion Describing the Geometry of Motion: Kinematics Linear Angular Describing the Forces of Motion: Kinetics Linear Angular Fluid Mechanics. Biomechanics.

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biomechanics outline
Biomechanics: Outline
  • Definition
  • Types of Motion
  • Measuring Motion
  • Describing the Geometry of Motion: Kinematics
    • Linear
    • Angular
  • Describing the Forces of Motion: Kinetics
    • Linear
    • Angular
  • Fluid Mechanics
  • The study of the structure and functions of biological systems by means of the methods of mechanics

Hatze, 1974

We might think of biomechanics as the “physics of human movement”

  • Kinematics
    • describing movements with respect to time and space
  • Kinetics
    • examines the forces that produce the movement and result from the movement
why study biomechanics
Why study biomechanics?
  • skill analysis
    • correction
    • pinpointing errors
  • developing a new technique
  • adapting to new equipment
  • understanding complex movement behavior
types of motion
Types of motion
  • Linear (translation)
    • all parts travel the same distance in the same time along the same path
type of motion
Type of Motion
  • Angular motion
    • parts rotate around an axis of rotation
general motion
General Motion

Most movements are

likely a combination of

both linear and

angular motion

measuring motion

High speed cinematography

High speed Videography






Pressure and Force transducers

Force Platform

Isokinetic dynamometer



Measuring Motion
kinematics film analysis
Kinematics: Film Analysis




what might we measure
What might we measure?

KINEMATICS: Spatial component

  • Position
    • location in space relative to some spatial coordinate system reference (e.g., center of joint, COG, COM, point of contact)
  • Displacement
    • is the straight line distance and direction
  • Distance
    • the length of the path traversed
what might we measure1
What might we measure?
  • Center of gravity

the point about which a body’s weight is equally balanced in all directions (Hall, 1995)

kinematics film analysis1
Kinematics: Film Analysis






(10, 570)

what might we measure2
What might we measure?

Kinematics: Spatial and temporal components

  • Speed
    • distance / time (m/s)
  • Velocity
    • displacement / time (m/s)
  • Acceleration
    • velocity / time (m/s2)
what might we measure3
What might we measure?


  • Inertia
    • a body’s resistance to being moved
  • Force
    • a push or pulling action on the body (lbs, N)

(nb: 1 lb = 4.45N)

motion force and sir issac
Motion, Force, and Sir Issac
  • First Law (Inertia)
    • a body continues in a state of rest or uniform motion until acted upon by an external force of sufficient magnitude to disturb its current state
motion force and sir issac1
Motion, Force, and Sir Issac
  • Second Law (Acceleration or F=ma)
    • the acceleration of the body is proportional to the force exerted on it and inversely proportional to its mass

e.g.1, a soccer ball (of fixed mass) will experience greater acceleration when kicked with more force

e.g.2, for kick (of given force) a lighter soccer ball will experience greater acceleration

motion force and sir issac2
Motion, Force, and Sir Issac
  • Third Law (action-reaction)
    • every action has an equal and opposite reaction (important for conservation of momentum)
angular motion
Angular Motion
  • When a force is not exerted along a line that passes through a body’s center of gravity (eccentric force), the body will experience angular (rotary) motion
what might we measure4
What might we measure?
  • Angular displacement
    • change in location of rotating body
  • Angular distance
    • angle between initial and final positions when measured by following the path of the body

angular motion consider in degrees, revolutions, or radians

1 radian = 57.3 degrees

1 revolution = 360 degrees

1 revolution = 6.28 radians

what might we measure5
What might we measure?
  • Angular Velocity
    • angular displacement / time (degree/s)
  • Angular Acceleration
    • angular velocity / time (degrees/s2)
what might we measure6
What might we measure?

Angular Kinetics

  • Torque
    • turning effect on a body measured as the product of force and moment arm length (e.g., changing tires)
  • Moment of inertia
    • resistance to rotary motion that results from combination of mass and distribution of the mass of an object
      • minimize resistance to angular rotation must move

mass closer to axis of rotation (e.g., choking-up

in baseball, spinning in skating or gymnastics)

moment of interia relative
Moment of Interia: Relative
  • Tuck
  • Pike
  • Full body rotating around center of mass
  • Full body rotating around a bar

Extended swing

  • around bar
  • Extended swing
  • around central axis
  • Pike
  • Tuck




M = mass

d = distance from

axis of rotation

fluid mechanics
Fluid Mechanics
  • Drag
    • Fluid force that opposes the forward motion of the body and reduced the body’s velocity.
  • Lift
    • Component of air resistance that is directed at right angles to the drag force

Fluid force that opposes the forward motion of the body and reduced the body’s velocity.

Will depend on:

  • fluid density
  • frontal area of body (e.g., rowing shells)
  • drag coefficient (dependent on shape)
  • movement velocity
forms of drag
Forms of Drag
  • Surface (hydrodynamic drag)
    • referring to interaction between body surface and the water
      • water temperature, water viscosity, body surface area, movement velocity
  • Profile (Form)
    • refers to resistive forces resulting from poor body position
  • Wave
surface drag
Surface Drag

Water particles attract other water particles and

will increase with “roughness of skin”

profile drag
Profile Drag
  • Low pressure pocket forms
  • and “holds back” the
  • cyclist. As velocity doubles this
  • resistive force quadruples!!!!
  • Important factors:
  • Shape
  • smoothness
  • orientation (crouch can lower
  • resistance ~30%
reducing drag
Reducing Drag
  • Frame designs
    • on bikes are often
    • “tear-shaped” to
    • reduce drag
  • Drafting within 1 m
    • can reduce drag
    • accounting for 6% of
    • energy cost (e.g., ducks flying)

Component of air resistance that is directed at right angles to the drag force




Air Flow

lift common example

High velocity/Low Pressure

Low velocity/High Pressure

Lift - common example

According to Bernoulli's Law, faster air has lower

air pressure, and thus the high pressure beneath the

wing pushes up to cause lift.

lift and formula i
Lift and Formula I
  • The desire to further increase the tire adhesion led the major revolution in racing car design, the introduction of inverted wings,which produce negative lift or 'downforce'.
magnus effect
Magnus Effect

Intended Direction



  • Force first discovered by Magnus. It explains the curving of a spinning ball. As the spinning object pushes the air from one side to the other, it will create a lower pressure zone, making the object move faster on one side.

Air Flow




linear motion

displacement, velocity..

Angular Motion

angular displacement…


linear motion

mass, inertia

Angular Motion

torque, moment of inertia

  • Fluid dynamics
    • drag
    • lift