The effects of speed, frequency, and load on measured hand forces for a floor to knuckle lifting task.
Ergonomics 1992 Jul; 35(7-8):833-843
The effects of speed, frequency, and load on hand forces during floor to knuckle lifting were investigated. The study group consisted of five male volunteers, 18 to 30 years old. They lifted a box filled with weights from floor to knuckle height at the most comfortable speed (normal speed) at lift rates of 1, 4, or 8 lifts per minute (min). The maximum acceptable weights of lift (MAWLs) were determined for each lift. Subjects then pulled on a specially designed strain gauge apparatus which simulated a loaded box at forces equal to lifting at 35, 60, or 85% of their MAWLs at their normal speed or as fast as possible (fast speed) at the rates 1, 4, or 8 lift/min. Horizontal and vertical hand forces were measured during the lifts. The maximum vertical and horizontal hand force components for each lift occurred about the time of lift off. The peak hand force was manifested as a distinct spike in plots of hand force versus lift time for all fast lifts. The spikes had rise and fall times of 0.04 to 0.12 second. For the normal speed lifts, the spikes were present in hand force versus time plots of four subjects. The presence of the spikes indicated that the subjects were not lifting smoothly. The subjects always pulled on the load with resultant forces that were greater than the load. For the fast speed, the peak vertical force averaged 3 to 3.5 times the load weight and the horizontal hand force was approximately equal to the load weight. For normal speed lifting, the peak vertical hand force was approximately twice the load weight and the horizontal force about half the load weight. The hand forces increased with increasing load lifted expressed as a percentage of the MAWL. The hand forces decreased with increasing lift frequency. The authors conclude that individuals, in general, do not lift objects smoothly. Jerks may be required especially when attempting to lift loads rapidly. The hand force pattern for floor to knuckle lifting is reproducible and can be used in advanced biomechanical models and simulations.
NIOSH-Publication; NIOSH-Grant; Musculoskeletal-system-disorders; Biomechanics; Task-performance; Laboratory-testing; Humans; Musculoskeletal-system; Manual-lifting; Manual-materials-handling
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