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

Abstract

Electromagnetic (EM) tracking has been used to quantify biomechanical parameters of the lower limb and lumbar spine during ergometer rowing to improve performance and reduce injury. Optical motion capture (OMC) is potentially better suited to measure comprehensive whole-body dynamics in rowing. This study compared accuracy and precision of EM and OMC displacements by simultaneously recording kinematics during rowing trials at low, middle, and high rates on an instrumented ergometer (n=12). Trajectories calculated from OMC and EM sensors attached to the pelvis, lumbar spine, and right leg were highly correlated, but EM tracking lagged behind ergometer and OMC tracking by approximately 6%, yielding large RMS errors. When this phase-lag was corrected by least squares minimization, agreement between systems improved. Both systems demonstrated an ability to adequately track large dynamic compound movements in the sagittal plane but struggled at times to precisely track small displacements and narrow angular ranges in medial/lateral and superior/inferior directions. An OMC based tracking methodology can obtain equivalence with a previously validated EM system, for spine and lower limb metrics. Improvements in speed and consistency of data acquisition with OMC are beneficial for dynamic motion studies. Compatibility ensures continuity by maintaining the ability to compare to prior work.

Keywords: Optical motion capture; electromagnetic motion tracking; kinematics; rowing.

Figure 1.

Diagram of OMC marker and EM receiver layout during rowing trials. Top-down view of sensors affixed to the instrumented ergometer (left). Anterior and posterior views of sensors as applied to subject anatomical landmarks (right). Relative position of key stroke occurrences (catch, mid-slide, release) is indicated to the left of the slide rail

Figure 2.

Difference from ground truth position of static marker/receivers in A/P plane for OMC and EM systems. Similar patterns were found for M/L and S/I directions

Figure 3.

A/P plane displacements (mean ± 95% CI) for OMC and EM systems across all stroke rates. (a) Before phase-lag correction, EM system capture latency is visible as a rightward shift in mean displacement for handle and seat trajectories. (b) After phase-lag correction using least-squares fitting, differences between systems decreased, as indicated by smaller RMSE values

Figure 4.

Seat width (mean ± SD), calculated as the M/L distance between markers/receivers fixed to left and right sides of ergometer seat. The OMC system slightly overestimates seat width throughout the stroke while the EM system was less consistent, underestimating seat width near the catch and release positions and overestimating seat width during mid-slide. Catch occurs at 0% and 100%. Release occurs at ~40%

Figure 5.

Co-localized marker tracking (mean ± 95% CI) in three dimensions with and without phase correction. These show similar trends despite EM capture latency, with each mean trajectory falling within well overlapped 95% confidence intervals. Agreement between systems in A/P displacements shows the most improvement with phase-lag correction. M/L and S/I displacements show more apparent magnitude differences between systems, but without a clear trend in over/under estimation of one system relative the other

Figure 6.

Simultaneous joint center tracking using the three methods (mean ± 95% CI). A/P and S/I displacements show improved trajectory agreement with phase-lag correction. S/I hip joint movement and A/P ankle joint movement show confidence intervals, which do not substantially overlap, with both having large magnitude discrepancies. Confidence of agreement in M/L displacements is ambiguous due to larger relative standard deviations, particularly at knee joint center

Figure 7.

Bland-Altman comparison of bias and limits of agreement (mean ± 95% CI) during dynamic apparatus tracking for A/P handle displacements between OMC and EM systems before (OMC-EM) and after (OMC-CorEM) phase-lag correction. After phase-lag correction the cyclic trend collapses and the limits of agreement contract