The influence of robotic grip augmentation on reducing muscular effort and fatigue during spacesuit glove use
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Hand, finger, and forearm fatigue are amongst the top three most common types of injuries endured by astronauts during EVA missions. The three-layered extravehicular activity (EVA) spacesuit gloves, a 4.3psi spacesuit pressure differential, and the heavy reliance upon using the hands in zero gravity contribute to this high statistic. The Spacesuit RoboGlove (SSRG), a Phase VI spacesuit glove modified with robotic grasp assist capabilities, has been developed to improve astronaut performance and reduce the risk of injury during EVA missions. A preliminary study has shown that the SSRG can consistently augment the user’s grip strength, however, further analysis is needed to evaluate its potential to reduce muscular effort and forearm fatigue. Thus, the purpose of this study was to quantify spacesuit glove-induced muscular effort and forearm fatigue to: i) identify the muscles that are in need of robotic assistance while wearing a spacesuit glove, and ii) evaluate the influence of robotic grip assistance on diminishing spacesuit glove-induced forearm muscle effort and fatigue. Six subjects performed a fatiguing task consisting of cyclic dynamic gripping interspersed with constant force contractions. Each subject performed the task under three conditions: barehand, Phase VI glove pressurized to 4.3 psi (SSG), and SSRG pressurized to 4.3 psi. Surface electromyography (sEMG) from seven muscles of the forearm (flexor digitorum superficialis (FDS), flexor carpi radialis (FCR), flexor carpi ulnaris (FCU), extensor digitorum (ED), extensor carpi radialis longus (ECRL), extensor carpi ulnaris (ECU), and extensor indices (EI)), force data from a hand dynamometer, and subjective fatigue ratings were collected concurrently throughout each condition. Trends in integrated EMG (iEMG), amplitude (RMS), and median frequency (MF) of the sEMG signals were used to quantify expended effort and fatigue-induced changes within each muscle. These metrics were compared across the three experimental conditions. Subjective fatigue ratings revealed that SSRG aided the subjects in feeling less fatigued over the first half of the experiment. iEMG showed that the FDS, FCR, and ED muscles exerted the most effort and were most prone to fatigue during the SSG condition. The SSRG helped to reduce muscular effort in the flexor muscles (FDS, FCR, and FCU) compared to the SSG condition. However, the SSRG increased muscular effort of the extensors, most notably ED, compared to the SSG condition. Results from four subjects showed that the SSRG was able to reduce muscular effort to near barehanded levels for the FDS, FCR, and ECU muscles. These results indicate that the SSRG shows promise as a grip assist device that reduces expended effort of the flexor muscles, however, further design improvements are still needed. For most conditions, the expected trends in fatigue metrics (i.e. decrease in MF and increase in RMS) were not seen. Modifications to the protocol should be made for future experiments to improve the outcome of these metrics and allow for a more conclusive argument to be made concerning the effectiveness of SSRG in reducing forearm muscle fatigue.