A simple and rapid method for electromagnetic field distortion correction when using two Fastrak sensors for biomechanical studies

The article presents a simple and rapid method for the correction of electromagnetic distortions when using electromagnetic Fastrak (Polhemus, USA) sensors. It is based on the minimization of objective functions composed of derivative polynomial functions, hence estimating the distortion of the electromagnetic field. The polynomial functions composing the objective function each contain 35 deformation coefficients. These coefficients are then used to correct the electromagnetic measures in position and orientation. Preliminary results on the efficacy of the method are presented for two subjects who walked on a treadmill, and for whom relative movement of the lower leg with respect to the thigh was recorded using two Fastrak sensors. The corrected Fastrak measurements were compared with optoelectronic measurements (Vicon, USA), which are not affected by distortions as electromagnetic sensors are. Results showed that after 3 min of calibrating a volume of approximately 1 m3, the method proved to be efficient in correcting errors in orientation (56% (2.72–1.12°), 78% (4.4–0.89°), and 56% (2.25–0.90°) of error reduction in the respective flexion/extension, ab/adduction and tibial internal/external rotation) and position (53% (18.9–8.9 mm), 21% (6.6–4.6 mm), and 48% (15.9–8.1 mm) of error reduction in the respective medial/lateral, anterior/posterior and proximal/distal translations) (values are overall means for two subjects and four calibration procedures). That amount of correction compared favorably with values presented in the literature.