Please cite this article in press as: L. Immler, K. Schindelwig, D. Heinrich, et al.. Individual flexion stiffness of ski boots. J Sci Med Sport (2019), https://doi.org/10.1016/j.jsams.2019.01.015 ARTICLE IN PRESS G Model JSAMS-2026; No. of Pages 5 Journal of Science and Medicine in Sport xxx (2019) xxx–xxx Contents lists available at ScienceDirect Journal of Science and Medicine in Sport journal homepage: www.elsevier.com/locate/jsams Individual flexion stiffness of ski boots Lorenz Immler a,b,∗ , Kurt Schindelwig b , Dieter Heinrich b , Werner Nachbauer a,b a Research Center Snow, Ski, and Alpine Sports, Austria b Department of Sport Science, University of Innsbruck, Austria a r t i c l e i n f o Article history: Received 19 April 2018 Received in revised form 16 January 2019 Accepted 22 January 2019 Available online xxx Keywords: Alpine skiing Ski boot Individual flexion stiffness Nominal flex index a b s t r a c t 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 stan- dardization method due to the measurements with subjects. However, the data collected may provide a valuable baseline for a future standardization. © 2019 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. 1. Introduction In Alpine skiing the incidence rate for severe injuries of the ligamentous structures in the knee has been tripled 1 over the last 40 years while ankle and tibia injuries have been reduced significantly. 2 According to expert stake holders the “ski-plate- binding-boot” system is one out of the top five categories for severe injuries in alpine ski racing. The others are “changing snow con- ditions, physical aspects of the athletes, speed and course setting aspects as well as speed in general”. 3 The ski boot is an essential piece of the ski-plate-binding-boot system, which acts as a con- nection between skier and ski through the binding with the main purpose to transfer forces and torques from the skier to the ski. The transfer of forces and torques is among other parameters affected by the flexion stiffness (FS) of the ski boot. In particular, a ski boot with high FS leads to a more direct force transmission and con- sequently a more aggressive ski-snow interaction that has been associated with the causation of severe knee injuries. 4 In addition, ∗ Corresponding author at: Research Center Snow, Ski, and Alpine Sports Fürsten- weg 185, 6020 Innsbruck, Austria. E-mail address: Lorenz.Immler@uibk.ac.at (L. Immler). higher FS may lead to an upright or backward lean position which causes higher loads at the knee 5,6 and higher FS has also been sug- gested as an important risk factor for ACL injuries occurring during jump landing maneuvers. 7–9 Ski boots with low FS may effect safety issues. If the FS of a ski boot is too low to limit the motion in the ankle joint, excessive dorsiflexion above 40 ◦ may occur resulting in ankle injuries. 10 A low FS may also impair binding release at the heel piece of the binding since a low FS may delay the needed upward force for its proper release. There is a lack of studies dealing with the effect of the FS of ski boots on skiing performance. Practical experience indicates that boots with high FS reduce the range of motion at the ankle joint and thus as an example the necessary dorsiflexion to initiate a turn. Too soft boots delay the force transfer from the skier to the skis impairing the steering of the ski. It is thought that an optimal flex- ion stiffness exists in dependence of skie´ rs anthropometrics, snow conditions, and boot temperature among others. As an example, competitive skiers experiment with cooling and warming of their boots depending on the ambient temperature. The FS of ski boots is determined by the torque around the flex- ion axis of the boot. Boot manufacturers use in-house measurement methods and stiffness parameters to describe FS. These parame- ters are called nominal flex indices which range from 50 (soft) to https://doi.org/10.1016/j.jsams.2019.01.015 1440-2440/© 2019 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.