title: Neck mobility measurement by means of the 'Flock of Birds' electromagnetic tracking system.
abstract: OBJECTIVE
  To establish the accuracy and reliability of a six-degrees-of-freedom electromagnetic tracking device, the "Flock of Birds", for measuring neck rotations and to identify the main sources of error.
  
  
  METHODS
  Ten human subjects made the same types of maximal neck rotation, both actively and passively: axial rotation in neutral position, from a flexed position and from an extended position, flexion/extension and lateral flexion. The same movements were mimicked in a 'dummy head' set-up.
  
  
  METHODS
  One Flock of Birds receiver was mounted on the thorax, one on the head. By means of a third receiver, mounted on a stylus, bony landmarks on head and thorax were palpated. These served to define two anatomically based local coordinate systems, to which the rotations were referred.
  
  
  RESULTS
  Measurements were accurate with a maximal measurement error of 2.5 degrees. No significant difference between active and passive rotation was seen. The intra-subject variation was low within the same session, SD between 2 degrees and 4 degrees. Between sessions the variability was considerable, SD between 5 degrees and 16 degrees.
  
  
  CONCLUSIONS
  The Flock of Birds method is reliable and sufficiently precise. The variability in measured range of motion between sessions is a point of concern in interpreting follow-up studies in patients.
  
  
  CONCLUSIONS
  A reduced range of neck motion is a major complaint in pathologies of the cervical spine or the shoulder. A method is described in which neck rotations are related to well-defined bony landmarks. In combined rotations, e.g. flexion combined with axial rotation, the measured range of motion can sometimes fluctuate strongly (up to 30 degrees ) between measurements, without apparent pathology.
authors.affiliation: Laboratory of Human Movement Analysis, University Hospital, P.O. Box 30001, NL-9700 RB, Groningen, The Netherlands.
authors.forename: C L
authors.initials: CL
authors.lastname: Koerhuis
authors.forename: J C
authors.initials: JC
authors.lastname: Winters
authors.forename: F C T
authors.initials: FC
authors.lastname: van der Helm
authors.forename: A L
authors.initials: AL
authors.lastname: Hof
mesh_terms: D000328:Adult; D055590:Electromagnetic Phenomena / Q000295:instrumentation* / Q000379:methods*; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation* / Q000379:methods*; D008297:Male; D009068:Movement / Q000502:physiology; D009333:Neck / Q000502:physiology*; D016059:Range of Motion, Articular / Q000502:physiology; D012016:Reference Values; D015203:Reproducibility of Results; D012399:Rotation; D012680:Sensitivity and Specificity