Design and Evaluation of Grasp Assistive Devices in an Industrial Environment

Abstract

Carpal tunnel syndrome and tendonitis are two common upper extremity cumulative trauma disorders related to repetitive and forceful activities in the workplace. The objective of this research was to reduce the hand force during an activity, as reducing task repetition would negatively affect productivity. Two devices were developed to achieve this objective: a soft pneumatic grasp assist device to augment grasp strength, and a novel grip training device to visually alert the user when more force than necessary is used. Device effectiveness was quantified by measuring muscle activity and grip force during an in vivo study of a common industrial activity. Nine associates experienced with power tools employed by an automobile manufacturer installed 18 fasteners using a pistol grip DC tool under three conditions: a typical manner (no device or prompting), with the grasp assist, and with the grip training device. Surface electromyography (sEMG) was used to measure the activity of four muscles commonly associated with grasping – flexor digitorum superficialis (FDS), flexor carpi ulnaris (FCU), extensor digitorum communis (EDC), and flexor carpi radialis (FCR). Results showed that both the grasp assist and grip trainer significantly reduced the mean, combined, normalized muscle activity compared to the typical condition by 18% and 23% respectively (p<0.05). Muscle activation results were contextualized using the revised strain index (RSI), a clinical tool to evaluate the safety of an activity by considering activity specific ergonomic factors. The grasp assist and grip trainer both yielded a significantly lower mean RSI value than the typical condition by 13% and 17% respectively (p<0.05). Grip force was measured using a flexible pressure transducer affixed to the pistol-grip handle of the DC tool. Again, the grasp assist and grip trainer yielded significantly lower values than the typical trial by 47% and 36% respectively (p<0.001). Between devices, the grasp assist yielded a significantly lower grip force the grip trainer (p<0.001); however, the mean muscle activation was not significantly different, which suggests that the four muscles measured in this study do not completely capture grip force. A large variation in grip force was measured for all three conditions with a weak, positive correlation between power tool experience and force applied. Knowledge of the voluntary variation in grip force can be used to educate workers on minimizing the force exerted during an activity.