The influence of deformation height on estimating the center of pressure during level and cross-slope walking on sand

- The GRFML varied between limbs in cross-slope condition when walking on sand.
- The COP difference is magnified by increased deformation height when walking on sand.
- The COP differences are significant between AP and ML directions.
- The COP difference is more sensitive for downhill limb in cross-slope sand condition.
- The thresholds are 20 and 10 mm for level and cross-slope condition before correction.

Force plates are frequently used to collect the ground reaction forces (GRF) and center of pressure (COP) during gait. The calculated COP is affected by the material type and thickness covering the top surface. If the surface is deformable, these effects can be significant. The purpose of this study is to simulate and evaluate the effects of deformation height when calculating the COP in a deformable surface during gait. The GRF and COP data during normal gait were collected from 20 healthy adult males on sand in two conditions (level and cross-slope of 10°). The COP differences in the anteroposterior (AP) and mediolateral (ML) directions were modeled for constant deformation heights (10-50 mm, 10 mm increments). The results showed the magnitude of COP changes in the AP and ML directions were different in both level and cross-slope conditions. A significantly larger COPML difference was shown for the cross-slope condition than level condition for the same deformation height. The COP was more sensitive to the deformation height for the downhill limb than uphill limb in the cross-slope condition. The results of this study suggest that the maximum allowable deformation height before a correction for surface deformation is needed is 20 mm for level condition and 10 mm for cross-slope condition, where 3 mm difference in COP is considered as the tolerance limit. Surface deformations beyond these thresholds may lead to an inaccurate interpretation and evaluation of joint kinetics during gait on deformable surfaces.