Modulation of sagittal-plane center of pressure and force vector direction in human standing on sloped surfaces.

Abstract The multi-joint coordination responsible for maintaining upright posture in the standing human manifests in the pattern of variation of the support-surface force (F). Assessment of both the translational and rotational kinematics in the sagittal-plane requires understanding the critical relationship between the direction and location of F. Prior work demonstrated that band-pass filtered F direction and center-of-pressure (CoP) covary in time such that the F vector lines-of-action pass near a fixed point called an intersection point (IP). The height of that IP (IPz) varies systematically with the frequency of the pass band. From F measurements in able-bodied humans (n =17) standing on various pitched surfaces, the present study also found the emergent property of an IP, with IPz located above the center of mass (CoM) at frequencies