Computers ind. Engng Vol. 21, Nos 1--4, pp. 601--605,1991 0360-8352/91 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1991PergamonPress plc A 2-DIMENSIONAL COMPUTERIZED BIOMECHANICAL MODEL S.S. Asfour °, S.M. Waly° and M.W. Fahmy** * Department of Industrial Engineering and ** Department of Civil Engineering University of Miami Coral Gables, FL 33124, U.S.A The main objective of this study is to present a user-friendly computerized biomechanical static model which can be used to estimate the forces and the moments at the major joints of the human body. The model estimates the compression and the shearing forces at the L5/S1 using three different approaches. Both symmetric and asymmetric postures can be analyzed. The symmetric model consists of seven links that represent the hands, lower arms, upper arms, trunk, upper legs, lower legs, and the head. In the asymmetric model, two more links were incorporated to represent the left and the right legs. The user has the choice to run the analysis for a given individual or for a percentile of the population. The program can be used for industrial applications and work place design. It can be also utilized in comparing different methods of performing a specific task. INTRODUCTION Despite the high technology currently available, many activities are still performed manually. Manual materials handling is one of the leading causes of injuries in industrial settings. NIOSH (1981) indicated that over exertion in manual activities accounts for a significant proportion of work related injuries. Several approaches have been utilized for the design of manual tasks. The biomechanical approach has been used to control musculoskeletal injuries. The main objective of this approach is to maintain job requirements within the capabilities of the worlfforce. Industrial tasks such as manual materials handling activities have been analyzed using the concept of job stress index (JSI) which is defined as the ratio of job demands and employee capability (Ayoub, 1991). From a biomechanical point of view, if a task results in biomechanical stresses (i.e. compressive force at I_5/$1) which exceed the tolerable limits of the musculoskeletal system, the individual is exposed to a high risk of injury. Biomechanically, job demands are quantified in terms of the stresses (e.g., forces and torques) imposed on the different joints, muscles and ligaments. These stresses are estimated using biomechanical models. Both static and dynamic biomechanical models have been utilized. It should be noted that the application of biomeehanical models depends on the assumptions made. Static biomechanical models primarily estimate forces and torques at various articulations of the body during voluntary actions. Since the work of Pearson et al. (1961), many researchers have developed different static models (Chaffin, 1969, Chaffin and Baker, 1970, Martin and Chaffin, 1972, Park and Chaffin, 1974, Garg and Chaffin, 1975, Schultz and Andersson, 1981, Granhed et al., 1987 and Bean et al., 1988). Several dynamic models have been reported in the literature (Fisher, 1967, El-Bassoussi, 1974, Muth et al., 1978, Smith et al., 1982, Freivalds et al., 1984, Kromodihardjo and Mital, 1986, Mital and Kromodihardjo, 1986, Chen and Ayoub, 1988 and Jager and Luttmann, 1989). Garg et al. (1982) reported that static biomechanical simulation of lifting loads grossly underestimates the stresses on the museuloskeletal system. Compressive forces at the lumbosacral joint, estimated from the static biomechanical simulation, were within the action limit defined by NIOSH, while those forces estimated from the dynamic biomechanical simulation exceeded the maximum permissible limit. However, according to Garg et al, (1982), the predictions based on static biomechanical simulation were in general agreement with the psychophysical weight limits in lifting tasks. In the present study only static analyses were considered in the development of the presented model. Chaffin (1975) discussed the limitations of the static biomeehanical models. Static models, which neglect the effects of acceleration and momentum, could underestimate the biomeehanical stresses on the musculoskeletal structures. The main advantages of static biomechanical models are that they require relatively simple calculations as compared to dynamic models. They can also serve the purpose of providing quick estimates of the biomechanieal stresses imposed on individuals engaged in manual tasks. In addition, they can be widely used in the analysis of weight holding and slow dynamic tasks. 601