L6 - Reference Frames & Coordinate Systems

• What is a Reference Frame?

  • A coordinate system that allows us to interpret measurements
  • Consists of 1 axis per measurement dimension
  • eg video image has 2 axes (x, y)
  • Quadric 1 = pos x & pox y, then goes clockwise
  • x = horizontal, y = vertical
  • Axes are perpendicular & fixed relative to each other & intersect at an origin
• Absolute Reference Frames:

  • Used to describe landmark (eg CoM) or segment positions
  • Can describe segment angles
  • Used to describe things relative to the world
  • Eg floor, gravity
• Absolute Reference Frames:

  • 1 direction only care about progression forwards - horizontal (x)
  • Their position relative to earth, start line 0
  • When looking at orientation of body parts/segments
  • Origin at proximal end of segment we are interested in
  • Need to describe it to know what angle (180°) is relative too
  • Absolute angle for segment relative to space
  • Are missing adjacent segment, other knee angle is totally different
• Absolute Reference Frames:

  • Either know where in space or relative to each other → depends what you need to know
  • eg lifting would be important to do with absolute
  • Relative tells us what's going on with muscles because they cross joints
• Relative Reference Frames:

  • Used for determining relative segment position (eg stance width) & joint angles
  • Describes limb orientation/segment relative to another
  • Angles normalise body size & absolute location
• Relative Reference Frames:

  • Relative goes with joint angle
  • 1 segment relative to another segment
  • Usually distal segment relative to proximal segment
  • Inside angle & supplementary angle have to add to 180 °, have to say which is which
  • As flex knee, supplementary angle gets bigger, inside angle decreases
  • Inside more common for coaching (tools used); supplementary more used in biomechanics
• Positive Axis Direction:

  • Cross Product
  • x * y = xy sin(𝜃)
  • Right-Handed Coordinate System
  • Thumb → right → x
  • 1st finger → anterior → y
  • 2nd finger → up → z
  • z adds depth
• Coordinate System Orientation:

  • For consistency, use conventions to define coordinate system orientation
  • x-axis→ mediolateral
  • (side to side)
  • y-axis → anteroposterior
  • (forward & backwards)
  • z-axis → longitudinal
  • (movement across many planes)
  • Positive directions - right, anterior, up
• Coordinate System Orientation:

  • x-axis
  • Mediolateral
  • side to side
  • In colonel plane
  • Adduction
  • pos for left, neg for right
  • Abduction
  • neg for left, pos for right
  • Positive direction = right
  • Negative direction = left
• Coordinate System Orientation:

  • y-axis
  • Anteroposterior
  • forward & backwards
  • In sagittal plane
  • Flexion
  • pos, besides for knee
  • Extension
  • neg, besides for knee
  • Positive directions = forward/flexion
  • Negative directions = backwards/extension
  • Opposite for knee
• Coordinate System Orientation:

  • z-axis
  • Longitudinal
  • movement across many planes
  • Transverse plane
  • Rotation
  • Positive direction = up
  • Negative direction = down
• Positive Rotation Direction:

  • Right Hand Rule
  • Thumb pointed in positive axis direction
  • Fingers curl in positive rotation direction
  • Put this hand on top of the right hand axes?
• Exceptions to Right Hand Rule:

  • By convention we change some angles to help with interpretation:
  • Knee flexion
  • Knee normally neg; change so all pos
  • Left side ab/adduction
  • Abduction neg on left, pos on right; opposite for adduction; change?
  • Left side int/external rotation
  • Need to double check