Which subject-related variables contribute to movement variability during a simulated repetitive and standardised occupational task? Recurrence quantification analysis of surface electromyographic signals

Effects of arm-support exoskeletons on pointing accuracy and movement

Exoskeletons are wearable devices that support or augment users' physical abilities. Previous studies indicate that they reduce the physical demands of repetitive tasks such as those involving heavy material handling, work performed with arms elevated, and the use of heavy tools. However, there have been concerns about exoskeletons hindering movement and reducing its precision. To this end, the current study investigated how proprioception enables people to point to targets in a blindfolded, repetitive pointing task, and their ability to recalibrate their pointing movement based on visual feedback during an intervening calibration phase, both with and without an arm-support exoskeleton. On each trial, participants were instructed to follow a 40 BPM metronome to point six times alternating between two target points placed either on a vertical or horizontal line. Within a trial, each pointing movement alternated between flexion and extension. Results indicate that participants' average pointing error increased by 4% when they wore an exoskeleton, compared to when they did not. The average pointing error was 12% lower when the target points were aligned vertically as compared to horizontally. It was also observed that the average pointing error was 14% lower during flexion as compared to extension movement. Surprisingly, accuracy did not improve in the post-test as compared to the pre-test phase, likely due to accuracy being high from the beginning. Participants' movement dynamics were analyzed using Recurrence Quantification Analysis. It was found that movements were less deterministic (1% reduction in percentage of determinism) and less stable (13.6% reduction in average diagonal line length on the recurrence plot) when they wore the exoskeleton as compared to when they did not. These results have implications on the design of arm-support exoskeletons and for facilitating their integration into the natural motor synergies in humans.