Individual flexion stiffness of ski boots

Abstract

Objectives: Ski boots are designed to transfer forces from skier to ski. This transfer is among others affected by the flexion stiffness (FS) and so effects safety and skiing performance. Previous studies have used devices with prosthetic legs to evaluate FS, however, influencing factors like the foot and lower leg shape or individual buckle closure are not considered. The purpose of the study was to (i) develop a device to measure the individual flexion stiffness (IFS) of ski boots worn by skiers, to (ii) determine the repeatability of the measurement, and to (iii) compare the IFS with the nominal flex index of the manufacturers.

Methods: 21 subjects were tested twice to assess repeatability. The IFS of 135 subjects were measured on ski slopes and compared with the nominal flex indices.

Results: Repeated measurements revealed a correlation r_p of 0.98 (p<0.001) and a relative standard error of SEM_rel=3.0%. The correlation between IFS and nominal flex index was moderate with r_s=0.64 (p<0.001). Post hoc analysis showed no statistical differences between flex index 80 and 90 (p=0.29) and flex index 100 and 110 (p=0.60).

Conclusion: The determination of IFS was sufficiently repeatable. Considerable differences were found between IFS and the nominal flex indices of the manufacturers. The introduction of a measurement standard may improve the comparability among the manufacturers. Our method is not suitable as standardization method due to the measurements with subjects. However, the data collected may provide a valuable baseline for a future standardization.

Keywords: Alpine skiing; Individual flexion stiffness; Nominal flex index; Ski boot.