This is the "System validation" dataset. It contains motion data collected with two motion capture systems simultaneously (IMUs & optical). Information about the dataset cen be found here:

Warmerdam et al. "Proposed Mobility Assessments with Simultaneous Full-Body Inertial Measurement Units and Optical Motion Capture in Healthy Adults and Neurological Patients for Future Validation Studies: Study Protocol". Sensors 2021, 21(17), 5833 https://doi.org/10.3390/s21175833

This motion_systemvalidation dataset is made available: https://doi.org/10.6084/m9.figshare.20238006.v2