title: Body sway at sea for two visual tasks and three stance widths.
abstract: BACKGROUND
  On land, body sway is influenced by stance width (the distance between the feet) and by visual tasks engaged in during stance. While wider stance can be used to stabilize the body against ship motion and crewmembers are obliged to carry out many visual tasks while standing, the influence of these factors on the kinematics of body sway has not been studied at sea.
  
  
  METHODS
  Crewmembers of the RN Atlantis stood on a force plate from which we obtained data on the positional variability of the center of pressure (COP). The sea state was 2 on the Beaufort scale. We varied stance width (5 cm, 17 cm, and 30 cm) and the nature of the visual tasks. In the Inspection task, participants viewed a plain piece of white paper, while in the Search task they counted the number of target letters that appeared in a block of text.
  
  
  RESULTS
  Search task performance was similar to reports from terrestrial studies. Variability of the COP position was reduced during the Search task relative to the Inspection task. Variability was also reduced during wide stance relative to narrow stance. The influence of stance width was greater than has been observed in terrestrial studies.
  
  
  CONCLUSIONS
  These results suggest that two factors that influence postural sway on land (variations in stance width and in the nature of visual tasks) also influence sway at sea. We conclude that--in mild sea states--the influence of these factors is not suppressed by ship motion.
authors.affiliation: School of Kinesiology, University of Minnesota, 1900 University Ave. SE, Minneapolis, MN 55455, USA. tas@umn.edu
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Stoffregen
authors.forename: Sebastien
authors.initials: S
authors.lastname: Villard
authors.forename: Yawen
authors.initials: Y
authors.lastname: Yu
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008889:Military Personnel; D009039:Motion Perception; D004856:Postural Balance; D013647:Task Performance and Analysis; D014796:Visual Perception