Psyc 552 Ergonomics & BiomechanicsLecture 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

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8Stevens' Power Law

Physical Value

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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

Visual Analog Scale (VAS)

Easiest Imaginable

Work

HardestImaginable

Work

10 cm line

Visual Analog Scale (VAS)

Best ClassImaginable

Best ClassImaginable

10 cm line

Visual Analog Scale (VAS)

Best InstructorImaginable

10 cm line

Best InstructorImaginable

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