Agreement between an inertia and optical based motion capture during the VU-return-to-play- field-test

The validity of an inertial sensor-based motion capture system (IMC) has not been examined within the demands of a sports-specific field movement test. This study examined the validity of an IMC during a field test (VU®) by comparing it to an optical marker-based motion capture system (MMC). Expected accuracy and precision benchmarks were computed by comparing the outcomes of a linear and functional joint fitting model within the MMC. The kinematics from the IMC in sagittal plane demonstrated correlations (r²) between 0.76 and 0.98 with root mean square differences (RMSD) < 5∘, only the knee bias was within the benchmark. In the frontal plane, r² ranged between 0.13 and 0.80 with RMSD < 10∘, while the knee and hip bias was within the benchmark. For the transversal plane, r² ranged 0.11 to 0.93 with RMSD < 7∘, while the ankle, knee and hip bias remained within the benchmark. The findings indicate that ankle kinematics are not interchangeable with MMC, that hip flexion and pelvis tilt higher in IMC than MMC, while other measures are comparable to MMC. Higher pelvis tilt/hip flexion in the IMC can be explained by a one sensor tilt estimation, while ankle kinematics demonstrated a considerable level of disagreement, which is likely due to four reasons: A one sensor estimation, sensor/marker attachment, movement artefacts of shoe sole and the ankle model used.