title: Reliability of muscle blood flow measurements in orbicularis oculi.
abstract: OBJECTIVE
  Orbicularis oculi muscle tension and muscle blood flow have been shown to be objective measures of eyestrain during visually demanding activities, such as computer work. In line with this, positive associations between eye-related pain and muscle blood flow in orbicularis oculi have been observed. A hypothesis regarding work situations with cognitive tasks and low-level muscle activity, such as computer work, proposes that muscle pain originates from the blood vessel-nociceptor interactions of the connective tissue of the muscle. Noninvasive muscle blood flow measurements in the orbicularis oculi muscle are preferable to using an invasive technique. The aim of this study was to test reproducibility and stability of muscle blood recordings in orbicularis oculi using photoplethysmography.
  
  
  METHODS
  In the reproducibility tests, 12 subjects were tested twice within 1 to 5 weeks. To study the stability of the method, six of the subjects were randomly selected and tested four more times within 2 to 6 weeks. Test subjects were doing identical visually demanding computer work for 10 minutes in each test.
  
  
  RESULTS
  The short-term repeatability of muscle blood flow measurements was considered good, but the stability of blood flow recordings over time in orbicularis oculi was low because of a greater within-subject maximum variability compared with between-subject average variability.
  
  
  CONCLUSIONS
  Investigators should be aware of the effect of time, possibly attributed to confounding factors such as environmental changes and mental stress, when comparing photoplethysmography muscle blood flow recordings.
authors.affiliation: *PhD †MSc ‡PhD, MD Department of Optometry and Visual Science, Buskerud and Vestfold University College, Kongsberg, Norway (H-MST, MH, AA, TMK, GH); and Department of Biomedical Engineering, Linköping University, Linköping, Sweden (LGL).
authors.forename: Hanne-Mari Schiøtz
authors.initials: HM
authors.lastname: Thorud
authors.forename: Magne
authors.initials: M
authors.lastname: Helland
authors.forename: Arne
authors.initials: A
authors.lastname: Aarås
authors.forename: Tor Martin
authors.initials: TM
authors.lastname: Kvikstad
authors.forename: Lars Göran
authors.initials: LG
authors.lastname: Lindberg
authors.forename: Gunnar
authors.initials: G
authors.lastname: Horgen
mesh_terms: D000328:Adult; D001248:Asthenopia / Q000503:physiopathology; D001783:Blood Flow Velocity / Q000502:physiology; D016288:Computer Terminals; D005152:Facial Muscles / Q000098:blood supply*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009801:Oculomotor Muscles / Q000098:blood supply*; D017156:Photoplethysmography / Q000379:methods*; D012039:Regional Blood Flow / Q000502:physiology; D015203:Reproducibility of Results