Ground Reaction Forces
Six force plates (B1 and B2, type 9281B, 400×600 mm, B3 and B4, type 9285, 400×600 mm, B5, type 9281C, 400×600 mm and A1, type 9287B, 600×900 mm, 2000 Hz; (Kistler, Winterthur, Switzerland) aligned with the walkway, were used to measure the ground reaction forces (GRFs). These force plates were decoupled from the surrounding floor through their installation on an isolated concrete foundation (mounted directly on the ground floor below) to eliminate signal noise caused by ground vibration due to movement of the fluoroscope. The staircase and the ramp used for downhill walking were equipped with two mobile force plates (C1 and C2, type 9286AA, 400×600 mm, 2000 Hz; Kistler, Winterthur, Switzerland). To obtain the exact location of the origin, as well as the orientation of the mobile force plates, the position of calibration markers plugged into the force plates were captured.
All force plates were additionally calibrated to improve the estimation of the centre of pressure (CoP) with an in-situ point of force application calibration method using a calibration grid. While the procedure is reported comprehensively elsewhere (List et al., 2017a), we provide a brief description here: forces with known directions were applied to either 78 or 98 calibration positions on the surface of each plate, allowing the CoP to be calculated using the 8 channel signal outputs. The positions of the calculated CoPs were then compared with the real force application point, resulting in a calibration matrix that can be used for correction of the calculated CoP. As a result of the procedure, the mean error of the determined CoP was thereby reduced from 0.8 to 19.8mm before correction to within a range of 0.04 to 2.2mm.