Comparison of an Electromagnetic and Optical System During Dynamic Motion

Abstract

Few studies have concurrently investigated the accuracy and repeatability of an optical and electromagnetic (EM) system during dynamic motion. The purposes of this study were to: (1) assess the accuracy of both an EM and optical system when compared to a gold standard and (2) to compare the intra- and inter-day repeatability during 3D kinematic motion of both systems. The gold standard used for accuracy assessment was a robot programmed to manipulate a carbon fiber beam through pre-defined motions within the capture volume of both systems at 30, 45 and 60°/s. A total of 12 healthy young adults were tested for intra- and inter-day repeatability of hip, knee and ankle joint angles during a sit-to-stand movement. Marker trajectories were captured using an 8-camera Motion Analysis system and a Polhemus Liberty system. Optical markers for both portions of the study were precisely marked to allow for digitization by the EM system, with collections taken at 120 Hz. Accuracy and repeatability were assessed using the RMS error and coefficient of multiple correlations (CMC), respectively. The optical system demonstrated a 1–2.5° lower RMS error in tracking the robot movements in the transverse and sagittal planes when compared to the EM system. However, it was possible that metal interference affected the accuracy of the EM system. High intra-day and inter-day repeatability was demonstrated by both systems during the sit-to-stand task. The optical system did demonstrate slightly higher CMC values for between day trials, though skin motion artifact might have affected the EM system to a greater extent. Overall, both systems demonstrated an adequate ability to track dynamic motion.

Comparing optical and electromagnetic tracking systems to facilitate compatibility in sports kinematics data

“…Accuracy in EM systems is dependent on the distance between transmitter and receivers, number of receivers in use, and ferromagnetic interference in the capture volume. Similar studies comparing the accuracy of EM and OMC systems have been conducted utilizing markers/receivers applied to a robotic, articulated arm where the true range of motion was known (Hassan et al 2007 ; Lugade et al 2015 ). Hassan et al ( 2007 ) concluded that accuracy of EM and OMC systems were comparable for measuring simulated upper extremity kinematics when appropriate post-hoc filtering and corrections were applied.…”

“…The authors did not assess repeatability of the tracking systems using an in vivo model. Lugade et al ( 2015 ) reported sagittal plane lower limb joint angles during a sit-to-stand activity, finding that OMC had lower RMS errors and higher intra- and inter-day CMC values (0.99 ± 0.001; 0.97 ± .025) than EM (0.97 ± 0.05; 0.94 ± 0.038) but concluded that both systems were adequate for tracking dynamic motion (Lugade et al 2015 ).…”

“…Few in-vivo studies have compared the performance of these systems for compound motions under dynamic conditions. Lugade et al ( 2015 ) examined intra- and inter-day repeatability of EM and OMC measurements during a sit-to-stand task, finding high coefficients of multiple correlation (CMC) for sagittal plane hip, knee, and ankle joint angles and no differences between systems in joint range of motion (Lugade et al 2015 ). There are no publications that assess these technologies in more complex, higher speed, whole-body motions, such as rowing.…”

Tracking Joint Angles During Whole-Arm Movements Using Electromagnetic Sensors

“…Some studies have placed sensors at known distances to the transmitter or to each other, often using assessment phantoms such as grid boards [1-4, 6, 8, 35]. Other studies have characterized the accuracy of EM systems by comparing EM measurements to a standard, such as a robot [36] or materials testing device [8], optoelectronic motion capture system [5,36,37], inclinometer [15], pendulum potentiometer [38], inertial-ultrasound hybrid motion capture system [39], or linkage digitizer [7].…”

Effects of Internal Fixation for Mid-Shaft Clavicle Fractures on Shoulder Kinematics During Humeral Elevations

“…Even though the measurement accuracy of the magnetic sensors has been assessed by bone pin data (Karduna et al, 2001), it is noted that the skin movement may be limited by the bone pins (Ramsey et al, 2003;Cappozzo et al, 2005). Electromagnetic tracking devices are also susceptible to nearby metal interference (Lugade et al, 2015). IMUs have been assessed for accuracy by using an optoelectronic system during functional tasks, giving RMS errors of less than 3.6 • (Cutti et al, 2008), which may be attributed to the so-called inertial sensor drift (Luinge et al, 2007) and linear acceleration interference (Zhang et al, 2012) apart from skin movement artifacts.…”