Comparison of alpine ski boot temperature-dependent flexural behaviour

Introduction & Purpose The flexural behaviour of alpine ski boots is one of the most important characteristics for selecting ski boots regarding performance and comfort. Serving as the mechanical interface, the stiffness of the ski boot controls the transmission of forces from the athlete to the ski. Considering this, it is remarkable that no standardized laboratory approach for the mechanical characterization (Flex Index) of ski boots has yet been established. In addition, the flexural behaviour of ski boots is mainly influenced by the viscoelastic properties of the polymeric materials and is therefore highly dependent on environmental temperature (Colonna et al., 2013; Nicotra et al., 2015; Petrone et al., 2014). Differences in boot stiffness have been shown for both  field and laboratory conditions with substantial effects (Petrone et al., 2013). To the best of our knowledge, however, no study has systematically compared the flexural behaviour of different ski boots across varying environmental temperatures. Therefore, the objective of this study is to compare the torques required to bend two pairs of alpine ski boots, differing in their nominal Flex Index, to predefined flexion angles at two temperatures in a laboratory environment. Methods Two pairs of ski boots (Atomic Redster: Flex Index 110 and Flex Index 170) were tested using a Flex Checker device (Figure 1). The boots were mounted in a 17° forward lean reference position. From there, 10 loading cycles consisting of forward as well as backward bending movements (±5°, 20 mm/sangular velocity) were performed at two temperatures (-3 °C, 23 °C).  In order to cool the ski boots to the desired temperature, they were placed in a freezer together with a thermometer prior to testing. During all measurements, angular displacements and bending  torpues were recorded. Positive bending torque peaks for each loading cycle were subsequently identified using a custom peak detection algorithm. The torque peaks of the left and right ski boot were combined, after which the peak bending torques were calculated as mean over cycles and boots for both boot Flex and temperature conditions (20 cycles each). Consecutive loading cycles (n = 10) were treated as repeated technical measurements.  A 2 × 2 repeated-measures ANOVA was performed with Flex Index (110, 170) and Temperature (23°C, −3°C) as within-subject factors. For additional insights into the material behaviour across the different conditions, a Sole Bend Test (Figure 1) for the left boot was performed, by measuring the applied force as a function of displacement (0–10 mm) at two temperatures (+23°C and -3°C) and Flex Index (110 and 170) conditions. Results The two-factor repeated-measures ANOVA revealed significant main effects of Flex Index (F(1,19) = 1207.77, p < .001) and temperature (F(1,19) = 1437.42, p < .001) on peak torque. No significant Flex × temperature interaction was observed (F(1,19) = 2.46, p = 0.133).The load–displacement curves obtained from the Sole Bend Test differed between conditions, additionally indicating variations in loadingresponse depending on the Flex Index as well as temperature. Discussion The present study demonstrates that both Flex Index and temperature significantly influence the flexural response of alpine ski boots, quantified by peak torque. From a mechanical perspective, the observed temperature effect can be attributed to the viscoelastic behaviour of the polymeric materials used in ski boots. In contrast, the shifts in torque response across Flex Index conditions primarily reflect structural and material stiffness differences of the boots. The main effects observed in the ANOVA analyses indicate that Flex Index and temperature independently affect the mechanical bending characteristics, which is consistent with previous studies reporting effects of boot stiffness (Petrone et al., 2013) and environmental temperature (Petrone et al., 2014). This is also supported by the Sole Bend Tests, which revealed condition-dependent displacement-load behaviour, with higher loads required for a defined displacement at lower temperatures and for boots with higher Flex Indices. The absence of significant interaction effects suggests that the influence of temperature on mechanical response is similar across different Flex Indices. Even though alpine skiing is typically performed under low-temperature conditions, the present results indicate that differences in Flex Index can also be reliably assessed under ambient laboratory conditions. This may represent an important contribution towards the development of standardized methods for quantifying ski boot Flex and may also be of practical relevance for ski boot selection as well as tuning processes. A limitation of the present analysis is that the repeated-measures structure was based on consecutive loading cycles of a standardized laboratory protocol rather than on independent ski boot samples. Consequently, the statistical inference primarily reflects the consistency of the observed effects across repeated technical measurements within the tested setup. Therefore, the results should be interpreted as evidence for the influence of Flex index and temperature on the mechanical response of the investigated boot configurations rather than as population-level estimates for all ski boots. A further limitation is the small number of tested ski boots and the fact that testing has not been conducted under fully standardized temperature-controlled environmental conditions. However, due to the relatively short testing duration and the thermal inertia of the ski boot materials, temperature changes during testing were assumed to occur gradually. Nevertheless, the actual boot temperature was not continuously monitored during testing. In addition, the flexural response has only been assessed in a single loading direction, which may not fully represent the complex, multi-directional movements occurring during alpine skiing. These factors may limit the generalizability of the findings and future work should include a broader range of ski boot models with differing Flex Indices, standardized temperature environments as well as more realistic loading scenarios. Conclusion The results of this study indicate the main effects of Flex Index and temperature on the flexural loading response of ski boots. The lack of evidence for an interaction effect suggests that ski boots may also be assessed for their flexural stiffness under ambient laboratory conditions. Beyond their practical significance for boot manufacturers, these results provide an important foundation for the development of standardized methods for measuring and evaluating the flexural characteristics of ski boots as a basis fot their selection and tuning.   Declaration of conflicting interests The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.   References Colonna, M., Nicotra, M., & Matteo, M. (2013). Materials, Designs and Standards Used in Ski-Boots for Alpine Skiing. Sports, 1, 78-113. https://doi.org/10.3390/sports1040078 Nicotra, M., Moncalero, M., & Colonna, M. (2015). Effect of the visco-elastic properties of thermoplastic polymers on the flexural and rebound behaviours of ski boots for alpine skiing. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 229(3), 199-210. https://doi.org/10.1177/1754337114564481 Petrone, N., Marcolin, G., Cognolato, M., Hofer, P., & Nachbauer, W. (2014). The Effect of Buckle Closure and Temperature on the In-vivo Flexibility of Ski-boots: A Pilot Study. Procedia Engineering, 72, 630-635. https://doi.org/https://doi.org/10.1016/j.proeng.2014.06.108 Petrone, N., Marcolin, G., & Panizzolo, F. A. (2013). The effect of boot stiffness on field and laboratory flexural behavior of alpine ski boots. Sports Engineering, 16(4), 265-280. https://doi.org/10.1007/s12283-013-0133-z