Wearable sensor validation of sports-related movements for the lower extremity and trunk

Inertial Measurement Units (IMUs), an alternative to 3D optical motion capture, are growing in popularity to assess sports-related movements. This study validated an IMU system against a "gold-standard" optical motion capture system during common sports movements. Forty-nine healthy adults performed six movements common to a variety of sports applications (cutting, running, jumping, single leg squats, and cross-over twist) while simultaneously outfitted with standard, retroreflective markers and a wireless IMU system. Bias, RMSE, precision, and maximum absolute error (MAE) were calculated to compare the two systems at the lower extremity joints and the trunk in all planes of movement and for all activities. The MAE difference between fast and slow activities for the sagittal, transverse, and frontal planes were 11.62°, 7.41°, and 5.82°, respectively. For bias, the IMU system tended to report larger angles than the optical motion capture system in the transverse and frontal planes and smaller angles in the sagittal plane. Average intraclass correlation coefficients for the sagittal, transverse, and frontal planes were 0.81±0.17, 0.38±0.19, and 0.22±0.37, respectively. When calculating a global bias across all three planes, the IMU system reported nearly identical angles (< 3.5° difference) to the optical motion capture system. The global precision across all planes was 2-6.5°, and the global RMSE was 7-10.5°. However, the global MAE was 11-23°. Overall, and with suggestions for methodological improvement to further reduce measurement errors, these results support current applications and also indicate the need for continued validation and improvement of IMU systems.