Modelling of shoulder and torso perception of effort in manual transfer tasks

Abstract

The aim of the present study was to develop statistical models of perceived effort at the shoulder and torso levels associated with manual load transfer tasks. The motions were directed from a home location toward one of twenty-two target shelves distributed in the right hemisphere. A total of 2149 ratings were obtained from 31 subjects for effort perception at the selected joints, using a ten-point modified Borg scale. Regression models, developed for the perception associated with each body part, included target locations (azimuth, height and distance), posture constraints (standing or sitting), task types (one or two handed transfer conditions), and demographic and anthropometric measures (stature, body weight, gender, and age) as parameters. The models provide a prediction of effort perception with adjusted r-square coefficients of 0.41 and 0.50 for the shoulder and torso, respectively. The results indicate that space and posture interact in a complex way to affect the rating of perceived effort, and are in agreement with the hypothesis that the ‘sense of effort’ is primarily associated with the efference copy of the descending motor command. Since a level of effort is not associated with a unique pattern of motor command, it is proposed that effort perception is likely to result from a summation of the components of the motor command. The models can be applied to optimize the spatial organization of the work environment in an attempt to reduce the risk of musculoskeletal injury.

Keywords:

• Sense of effort
• Manual material handling
• Workspace
• Psychophysics

Acknowledgements

The present study was sponsored by the partners of the HUMOSIM project (DaimlerChrysler Company, EDS - PLM Solutions, Ford Motor Company, General Motors Corporation, International Truck and Engine Corporation, Lockheed Martin Corporation/Sandia National Laboratories, University of Michigan Automotive Research Center, US Army – Tank-Automotive and Armaments Command, and United States Postal Service), and partially by the National Institute on Disability and Rehabilitation Research of the United States Department of Education, Grant no. H133E980007, and Rehabilitation Engineering Research Center at the University of Michigan. The opinions contained in this publication are those of the grantee and do not necessarily reflect those of the United States Department of Education.