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A single metric for quantifying biomechanical stress in repetitive motions and exertions.

Lin-ML; Radwin-RG; Snook-SH
Ergonomics 1997 May; 40(5):543-558
The relative effects of repetitive motion, force, and posture on subjective discomfort during repetitive physical work were examined. The purpose of the study was to determine if repetition, force, and posture could be combined into a single biomechanical metric that could be used to predict subjective discomfort during a repetitive physically demanding task. Seven volunteers, six males, 21 to 25 years (yr) old, performed a task involving repetitive wrist flexions at a pace of 2 and 4 motions per minute (min), at angles of 15 and 45 degrees (deg), and force levels of 15 and 45 newtons (N) for 1 hour (hr). The subjects rated their perceived level of discomfort on a 10 centimeter visual analogue scale ranging from 'no discomfort' to 'very high discomfort'. Linear regression analysis treated pace, force, and wrist flexion angle as independent variables and perceived discomfort as a dependent variable to generate a frequency weighted digital filter that could be used to convert continuous recordings of repetitive motions and exertions into an output variable that was proportional to discomfort. Perceived discomfort increased significantly as the pace increased from 4 to 20 motions/min, force increased from 15 to 45N, and wrist flexion angle increased from 15 to 45N. No interactive effects between these variables were seen. Linear regression analysis produced a model having a high/pass digital filter with 22 decibels per decade of attenuated slope. The model was tested by application to a group of six male volunteers, 19 to 22yr old, who performed a simulated industrial task involving transferring pegs across a horizontal bar and inserting them into holes against a controlled resistance for 1hr. The task was performed at rates of 6, 12, and 15 motions/min, wrist flexion angles of 15 and 45deg, and resistance forces of 15 and 45N. Perceived discomfort increased with increasing pace, wrist flexion angle, and resistance force. The fit of the model with the data obtained with each individual subject could be described by Pearson correlation coefficients of 0.90 to 1.00. The correlation coefficient for the entire data set was 0.67. The authors conclude that this type of approach may be useful for assessing exposure to physical stressors and designing repetitive forceful tasks that are less stressful.
NIOSH-Publication; NIOSH-Grant; Biomechanics; Repetitive-work; Simulation-methods; Hand-injuries; Injury-prevention; Musculoskeletal-system-disorders; Laboratory-testing; Ergonomics; Psychophysiology; Physical-stress; Mathematical-models; Statistical-analysis
University of Wisconsin-Madison, Department of Industrial Engineering, 1513 University Avenue, Madison, WI 53706
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University of Wisconsin-Madison, Department of Industrial Engineering, Madison, Wisconsin 53706