Tool-handle design based on a digital human hand model

A significant part of manual work is still done using hand-tools. Therefore, a correct design is crucial for preventing upper-extremity musculoskeletal disorders, such as carpal tunnel syndrome, hand-arm vibration syndrome, tendonitis, etc. When considering the ergonomics of a hand-tool, in addition to its main functionality, the most important part is the tool's handle. Most of the authors have considered cylindrical handles and provided guidelines and mathematical models for determining optimal diameters in order to maximise finger-force exertion, comfort, contact area, thus minimising the chances of cumulative trauma disorders (CTD). However, they have not taken into account the shape of the hand during optimal power-grasp posture when determining the tool-handles' shapes, which could additionally improve the handles' ergonomics. In order to overcome this limitation, we have developed an anatomically accurate static digital human-hand model (DHHM). The developed DHHM allows direct tool-handle modelling and does not require an iterative design process when designing a tool-handle with improved ergonomics. In order to develop DHHM, anthropometric measurements on ten subjects were performed for the manufacturing of corresponding optimal cylindrical pre-handles with variable diameters for each finger. Outer hand moulds were manufactured based on the pre-handles for obtaining the shape of the hand with skin and subcutaneous tissue undeformed. Magnetic resonance imaging was conducted with the outer hand moulds attached, and segmentation and 3D reconstruction were performed on the images to obtain the DHHMs for each subject. Tool-handles based on DHHM were then obtained within common Computer-Aided Design software. Measurements on the handles based on the DHHM have shown that they provide; on average; an over 25% higher contact area compared to the corresponding cylindrical handle. With higher contact area and anatomical shape of the handle, extensive deformation of the soft tissue can be avoided, thus preventing excessive load on the hand. Subjects also compared these DHHM handles with cylindrical handles regarding perceived subjective comfort-rating. It was shown that those tool handles based on the DHHM provided a higher overall comfort-rating compared to cylindrical handles. It has also been demonstrated that anatomically shaped tool-handles based on the developed DHHM can improve user performance and lower the risk of CTD. Relevance to industry: This paper introduces methods for developing a static DHHM for an optimal power-grasp posture by directly modelling a tool-handle with improved ergonomics. It also demonstrates that anatomically-shaped tool-handles based on the developed DHHM with optimal power-grasp posture increases the contact area and the subjective comfort-rating, thus increasing user performance and lowering the risk of CTD. (C) 2013 Elsevier B.V. All rights reserved.