title: Trapezius muscle activity increases during near work activity regardless of accommodation/vergence demand level.
abstract: OBJECTIVE
  To investigate if trapezius muscle activity increases over time during visually demanding near work.
  
  
  METHODS
  The vision task consisted of sustained focusing on a contrast-varying black and white Gabor grating. Sixty-six participants with a median age of 38 (range 19-47) fixated the grating from a distance of 65 cm (1.5 D) during four counterbalanced 7-min periods: binocularly through -3.5 D lenses, and monocularly through -3.5 D, 0 D and +3.5 D. Accommodation, heart rate variability and trapezius muscle activity were recorded in parallel.
  
  
  RESULTS
  General estimating equation analyses showed that trapezius muscle activity increased significantly over time in all four lens conditions. A concurrent effect of accommodation response on trapezius muscle activity was observed with the minus lenses irrespective of whether incongruence between accommodation and convergence was present or not.
  
  
  CONCLUSIONS
  Trapezius muscle activity increased significantly over time during the near work task. The increase in muscle activity over time may be caused by an increased need of mental effort and visual attention to maintain performance during the visual tasks to counteract mental fatigue.
authors.affiliation: Department of Occupational and Public Health Sciences, Faculty of Health and Occupational Studies, Centre for Musculoskeletal Research, University of Gävle, 801 76, Gävle, Sweden, hrr@hig.se.
authors.forename: H O
authors.initials: HO
authors.lastname: Richter
authors.forename: C
authors.initials: C
authors.lastname: Zetterberg
authors.forename: M
authors.initials: M
authors.lastname: Forsman
mesh_terms: D000065:Accommodation, Ocular; D000328:Adult; D003290:Convergence, Ocular / Q000502:physiology*; D004576:Electromyography; D005260:Female; D006801:Humans; D007908:Lens, Crystalline / Q000502:physiology; D008297:Male; D008875:Middle Aged; D064171:Superficial Back Muscles / Q000502:physiology*; D014937:Work; D055815:Young Adult