psyc 552 ergonomics & biomechanics lecture 12. psychophysics a study of the relationship...
Post on 21-Dec-2015
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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.
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
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 7 – very, very, light 8 9 – very light 10 11 – fairly light 12
13 – somewhat hard 14 15 – hard 16 17 – very hard 18 19 – very, very hard 20
Category Ratio Scale 0 – nothing at all .5 – very, very weak 1 – very weak 2 – weak 3 – moderate 4 – somewhat strong 5 – strong
6 7 – very strong 8 9 10 – very, very strong Maximal
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…
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.