Contribution of center of mass-center of pressure angle tangent to the required coefficient of friction in the sagittal plane during straight walking

In human bipedal walking, the peak value of the traction coefficient (i.e., the ratio of the shear force component to the vertical force component exerted on the floor) produced shortly after heel contact is termed as the required coefficient of friction (RCOF). Based on a bipedal inverted pendulum model, with a whole-body center of mass (COM) and reaction forces applied at the center of pressure (COP) of standing feet, RCOF in the sagittal plane (RCOFy) can be expressed as the sum of the tangent of the COM-COP angle and a residual term (RT), mainly comprising the moment around COM. In this study, we investigated the contribution of the tangent of the COM-COP angle to RCOFy during straight walking. The study involved four healthy young adult males. The participants were asked to walk on a 5-m long carpeted walkway. Each participant performed nine trials, i.e., three walking speeds (1, 1.4, and 1.9 m/s) × three step lengths (0.55, 0.75, and 0.95 m). COM was estimated using motion capture. COPs for the left and right feet were measured using eight force plates embedded in the walkway. RCOFy was calculated from the anterior-posterior and vertical ground reaction force components measured using the force plates. We found that the tangent of the COM-COP angle accounted for 91%-124% of the RCOFy value. This percentage tended to decrease with increasing walking speed (p < 0.05). The magnitude of RT accounted for only 5.3%-24% of RCOFy. These results suggest that the tangent of the COM-COP angle dominantly determines RCOFy.