Experimental Measurement of the Ski Boot Stiffness in Sagittal and Frontal Planes

In the present paper a novel device and a post-processing procedure are proposed to evaluate the mechanical behavior of a ski boot stiffness. The originality of the methodology consists in the following elements: (1) the stiffness is evaluated not only in plantar/dorsi flexion direction, but also in inversion/eversion one; (2) the loads are applied automatically and along programmable paths; and (3) the stiffness is calculated as the full jacobian of the applied moments. The testing apparatus is a XY Cartesian robot equipped with two linear potentiometers, a 6-axes load cell and a cardanic/prismatic joint for resembling the behavior of the knee complex. The experiments consisted of a cyclic horizontal displacement applied to the knee, with a period of 1 s in variable directions, and the measurement of the consequent boot reaction force. The chosen directions of load were straight lines passing through the neutral position of the boot; the lines changed their angle in respect to the ski when three displacement cycles were completed. The range of variation of flexion and inversion angles were 7° and 5°, respectively. The device showed a high level of repeatability, in comparison with the level of accuracy typical for the biomechanical evaluations, that permitted estimation of the stiffness of the tested boot as a 3rd order polynomial. The results achieved could be useful for the estimation of the reaction forces corresponding to ankle joint angles measurable during the skiing.