Journal of Biomechanics 32 (1999) 629 } 633 Technical Note Calibration of the &&Flock of Birds'' electromagnetic tracking device and its application in shoulder motion studies C.G.M. Meskers , *, H. Fraterman, F.C.T. van der Helm, H.M. Vermeulen, P.M. Rozing  Orthopaedic Laboratory, Department of Orthopaedic Surgery, Leiden University Medical Center B0-57, P.O. Box 9600, 2300 RC Leiden, Netherlands  Man Machine Systems and Control Group, Department of Mechanical Engineering and Marine Technology, Delft University of Technology, Delft, Netherlands  Department of Physical Therapy, Leiden University Medical Center, Leiden, Leiden University Medical Center B0-57, P.O. Box 9600, 2300 RC Leiden, Netherlands Received in "nal form 10 December 1998 Abstract In this paper the applicability in terms of measurement accuracy of the &&Flock of Birds'' six D.O.F. electromagnetic tracking device in shoulder research is investigated. Position measurements in a workspace of approximately 1 m were performed using a stylus. The andom error at the stylustip appeared to be 1.86, 1.98 and 2.54 mm for x-, y- and z-coordinate, respectively. The error caused by distortion of the magnetic "eld by metal in the concrete of especially the #oor was 20.8, 22.2 and 20.4 mm for the x-, y- and z- coordinate, respectively. Calibration and leaving out the measurements closests to the #oor lowered this error to 2.07, 2.38 and 2.35 mm. Orientation errors of the shoulder bones evolving from the measurement inaccuracy were estimated from repeated measurements of shoulder bony landmarks of ten subjects by means of the stylus. These errors were generally below 23. This is lower than found for the same measurements using a spatial linkage digitizer. It is concluded that the &&Flock of Birds'' is a useful tool for shoulder kinematic studies.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Electromagnetic tracking device; Calibration; Shoulder; Kinematics 1. Introduction Van der Helm and Pronk (1995) described a palpation technique to obtain 3D positions of bony landmarks of the shoulder. The main advantage of this technique is that scapula orientations can be measured quite easily. Furthermore, bone- referenced local coordinate systems (LCS) can be constructed (Van der Helm and Pronk, 1995; Van der Helm, 1997). So far, the bony landmark position recordings were performed using a spatial link- age digitizer (Pronk and Van der Helm, 1989), having the disadvantage that every bony landmark had to be meas- ured separately. Using a six degree-of-freedom (D.O.F.) tracking device would make measurements faster by measuring both the position and the orientation of mul- tiple receivers simultaneously. Construction of referenced * Corresponding author. Tel.: 31 71 5263447; fax: 31 71 5266743; e-mail: meskers@orthopaedicsurgery.medfac.leidenuniv.nl. LCS is then still possible using a stylus: a receiver moun- ted on a pointer. Little is known about the accuracy of the tracking devices and their susceptibility to metal distortion. An et al. (1988) investigated the usefulness of the Polhemus AC tracking device. Milne et al. (1996) investigated the accuracy and susceptibility to metal dis- tortion of the &&Flock of Birds'' (F.O.B) DC electromag- netic tracking device. It was concluded from both studies that with the tracking devices su$cient accuracy could be gained, providing there was no metal nearby. How- ever, the issue of calibration is not dealt with and the accuracy of the combination of tracking device and stylus has not been investigated. The goal of this study was to investigate the accuracy of measurements performed with a stylus in workspace large enough to perform position measurements of both the right and the left shoulder. A calibration procedure is carried out. Finally, the accuracy of palpation measurements using the FOB is estimated from in vivo recordings of 3D shoulder positions. 0021-9290/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 1 - 9 2 9 0 ( 9 9 ) 0 0 0 1 1 - 1