Neck mobility measurement by means of the 'Flock of Birds' electromagnetic tracking system

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.

Design. 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 2.5degrees. No significant difference between active and passive rotation was seen. The intra-subject variation was low within the same session, SD between 2degrees and 4degrees. Between sessions the variability was considerable, SD between 5degrees and 16degrees.

Conclusion. 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.