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Abstract
Changes in psychophysical lifting capacity with reductions in shelf opening clearances were experimentally determined. Three levels of shelf opening clearances were studied: unrestricted shelf opening clearance, loose shelf opening clearance (a clearance of 15 mm between the box width and the shelf opening), and tight shelf opening clearance (a clearance of 3 mm between the box width and the shelf opening). Two levels of lifting frequency (1 and 4 lifts/min.) and two lifting heights (floor to 0·81 m and 0·81 m to 1·52 m) were included. The metabolic energy expenditure rates and heart rates of the subjects (8 males and 8 females) at psychophysically acceptable weights of lift were also recorded to assess the physiological burden. The results indicated a decline in the psychophysical lifting capacity of the subjects as the shelf opening clearance became narrower. The decline in psychophysical capacity, compared to unrestricted shelf opening clearance for males was 9% when the shelf opening clearance was 15 mm. A further decline of almost 4% was recorded when the shelf opening clearance narrowed down to 3 mm. For females, the corresponding declines were approximately 10% and 4%. The effects of lifting frequency and height of lift were found to be consistent with previous findings.