Bracing during Kinematically Constrained One-hand Isometric Force Exertions: Predicting Force and Posture for Ergonomic Analysis.

Abstract

Workers performing industrial tasks routinely encounter obstructions in their environment that limit the postures that they can achieve. However, many obstacles also provide an opportunity for bracing with a hand or thigh. Observations of automotive workers have shown that hand or thigh bracing is common during assembly tasks, suggesting that bracing may increase worker capability or decrease stress. Biomechanical analyses of tasks with bracing are difficult to conduct because the addition of the unknown bracing forces produces a mechanically indeterminate system. To address this issue, force and posture data were gathered in a laboratory study of 22 men and women with a wide range of body size. Subjects exerted one-handed isometric backward, forward, and upward exertions at four task-handle positions in the presence of a kinematic constraint that afforded thigh and hand bracing opportunities. Bracing with the contralateral hand or thigh was hypothesized to improve force-exertion capability, and both postures and bracing forces in the presence of kinematic constraints were hypothesized to depend on task conditions and bracing availability. Bracing with the contralateral hand and/or thighs significantly increased one-hand force exertion capability by 40% on average. A method was developed to categorize bracing forces with respect to their contribution to task hand force generation. Decomposition of the bracing force vectors into opposing and non-opposing components enabled patterns of bracing forces to be classified into five distinct Force-Generation Strategies (FGS). Each FGS was associated with a particular posture pattern. Statistical models were developed to predict specific FGS and to predict posture variables and the magnitudes and directions of the task hand and bracing forces within each FGS and nominal task hand force direction. A conceptual model based on biomechanics principles was developed that accounts for the observed behaviors and forms a template for development of posture- and force-prediction models for use in industrial ergonomics. Guidelines for practitioners are presented to account for the effects of bracing forces on task-exertion capability.