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 = Ki n

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 Hardest Imaginable Work 10 cm line

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

Visual Analog Scale (VAS) Best Instructor Imaginable 10 cm line Best Instructor 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 1/3 rd  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 1/5 th 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