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Psyc lecture Psyc Ergonomics Biomechanics

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Psyc lecture Psyc Ergonomics Biomechanics Powered By Docstoc
					Psyc 552 Ergonomics &
Biomechanics
Lecture 12
Psychophysics
   A study of the relationship between the
    physical qualities of a stimulus and the
    perception of those qualities.
   Weber’s Law (circa 1800) – showed the
    relationship between the stimulus and the
    sensation can be defined by a constant.
Weber’s Law



   I=…
   ∆I = …
   K = Constant
Fechner’s Law & Steven’s Power Law
   Related the strength of a sensation to k and
    the intensity of the physical stimulus.
       S = k log I


   Steven’s modified Fechner’s Law to read:
       S = Kin
                                   Stevens' Power Law
                           8
Psychophysical Magnitude




                           6


                           4


                           2


                           0
                               0      100     200       300   400   500
                                             Physical Value
Psychophysics and work
   Psychophysical criteria
       Are correlated with physiological methods
       Are correlated with some injury measures


   The goal of work psychophysics…
Limits
   Traditional psychophysical methods required
    people to make judgments based upon a
    standard.

   Different people picked different standards.

   The response were…
The Borg scale
   Used categories rather than ratios.

   He used a variety of verbally anchored scales
    that had good inter-rater reliability.
Borg Scale
   The first Borg scales used the following anchors.
       3 – extremely light
       5 – very light
       7 – light
       9 – rather light
       11 – neither light nor laborious
       13 – rather laborious
       15 – laborious
       17 – very laborious
       19 – extremely laborious
Ratings of Perceived Exertion (RPE)
   6                          13 – somewhat hard
   7 – very, very, light      14
   8                          15 – hard
   9 – very light             16
   10                         17 – very hard
   11 – fairly light          18
   12                         19 – very, very hard
                               20
Category Ratio Scale
   0 – nothing at all        6
   .5 – very, very weak      7 – very strong
   1 – very weak             8
   2 – weak                  9
   3 – moderate              10 – very, very strong
   4 – somewhat strong       Maximal
   5 – strong
 Visual Analog Scale (VAS)
              10 cm line




 Easiest                      Hardest
Imaginable                   Imaginable
  Work                         Work
 Visual Analog Scale (VAS)
              10 cm line




 Best Class                   Best Class
Imaginable                   Imaginable
 Visual Analog Scale (VAS)
              10 cm line




    Best                         Best
 Instructor                   Instructor
Imaginable                   Imaginable
Preferred Maximums
   A manual material handling task can vary in
       Weight
       Size
       Frequency
       Distance
   We can instruct people to work keeping all
    variables but one constant
   Other instructions include…
Correlations
   Physiological measures (e.g., heart rate, O2
    consumption) are well correlated with
    activities of the larger muscle groups in the
    lower body.
   They are not well correlated…
Psychometric Qualities
   Scales…



   Manual material handling…
Psychometric Qualities (cont)
   Upper body work…



   Preferred maximums…
Ergonomic Applications
   Snook – Use of psychophysical guidelines can
    reduce work related back injuries by 1/3rd

   Matching work to individual perceptions of exertion
    can reduce over exertion injuries.

   In the absence of specific biomechanical or
    physiological models or guidelines, psychophysical
    methods might be the best, or only, method of
    evaluating work.
Work variables
   Weight – As weight increases, so to does
    perceived exertion.

   With repetitive lifting, the preferred
    maximum might be only 1/5th of an
    individuals maximum lifting ability.
Work variables (cont)
   Frequency – as the frequency of lifts increase,
    so to does the perception of exertion.

   Increasing frequency increase the energy
    expenditure to perform the work.
Work variables (cont)
   Distance
       Vertical distance -- lift distance
       Horizontal distance – knuckle height from floor
       Push / pull distance

				
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posted:4/5/2011
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