Within and Between-Subject Reliability Using Classic Jack for Ergonomic Assessments

As the use of computer-aided ergonomic tools become more prominent in performing ergonomic evaluations early in the design phase, the drive to improve upon the validity, reliability, and accuracy of the technology will increase. Posturing a digital human (DH) in a virtual environment proves to be a challenging task. There are a very large number of possible positions in which the DH can be positioned for any given task, and the position that the DH is postured into may differ depending on the experiences of the user. This may lead to different conclusions regarding the acceptability of an operation. Digital human model manipulation requires that an Ergonomist use computer software to manipulate a digital human model within a virtual environment, and requires that assumptions be made regarding how the workers would position themselves in such an environment. To use this method effectively, it is necessary to accurately posture the digital human [3] and some have shown that large errors can result when these postures are compared to those adopted by real workers [5, 6]. It has also been suggested in the literature that static posturing may underestimate risk [1]. Lamkull et al [4] studied four automotive assembly simulation cases with computer software (Ramsis) that included posture prediction algorithms with which the subjects could apply manual adjustments. Each case assessment was repeated six times. They note the challenges facing the assessor, such as the determination of the most appropriate “frozen” moment to analyze whether two hands can be used and whether it is possible to use a support arm for one handed tasks. They results indicate that significant variability existed within and between subjects for a number of joint angles. While some studies do exist, much more data is needed to evaluate the reliability and validity of using digital human model manipulation for performing proactive ergonomic analyses of musculoskeletal injury risk. The purpose of this study was to evaluate the between and within subject reliability when using the Classic Jack Human Simulation Program (UGS, Plano, Texas, USA) for the ergonomic analysis of workstations. It is anticipated that this study will result in the development of guidelines that will determine when the use of static postures should or should not be employed.