This study examines muscle tension and subjective muscle tension awareness while using a computer mouse positioned to the right of a standard computer keyboard and a centrally positioned trackball. Seventeen volunteer subjects experienced in mouse and trackball use were seated at an ergonomically adjusted workstation. Surface electromyography (sEMG) and subjective muscle tension levels were recorded from four muscle groups (left sternocleidomastoid/scalene, right upper trapezius, right posterior deltoid, and right lower trapezius/rhomboids) during 1-min trials with subjects resting with hands in their lap, while using a trackball below the centre of the keyboard, and while using a mouse immediately to the right of a 101-key keyboard. All subjects showed significantly higher mean sEMG activity recorded from the right upper trapezius, right posterior deltoid, and right lower trapezius/rhomboids during mouse use to the right of a standard keyboard (arm abducted) compared to using a trackball positioned centrally, (p < 0.001). sEMG levels remained elevated during the entire trial period of right side mouse use without evidence of microbreaks (< 1 s epochs of low sEMG activity). sEMG activity from the left sternocleidomastoid/scalene muscles showed no significant change from baseline in any condition. Subjective reports of muscle tension did not correlate with sEMG activity. The authors predict that there will be an overall increase in reports of upper extremity musculoskeletal disorders (UEMSD) and computer related disorders (CRD) when people abduct their arms in order to reach a mouse positioned to the side of standard width, or wider keyboards. Discussed are the applications of sEMG for evaluation of computer keyboard and pointing device use, appropriate ergonomic equipment design, and a methodology for improving muscle awareness, strengthening, relaxation, and workstyle practices to promote healthier computing.
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