Individual flexion stiffness of ski boots.

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 rp of 0.98 (p<0.001) and a relative standard error of SEMrel=3.0%. The correlation between IFS and nominal flex index was moderate with rs=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.

Safety recommendations of winter terrain park jumps into airbags.

OBJECTIVES: In winter terrain parks special airbags are used for skiers and snowboarders to practice jumps and achieve safe landings. However, in 2010 two skiers landed at the end of oval airbags. One suffered fatal, the other severe, injuries. The aim of this study was to identify parameters that lead to jumping over the airbag and to suggest preventive measures. DESIGN: Simulation study. METHODS: For the calculation of the flight distance the equation of motion was solved for the jumper's approach and flight phase. Measured data of five jumps into an airbag employed in a similar geometry and conditions as in the second accident case were used to validate the simulation and to measure typical takeoff velocities. The effect of approach and takeoff parameters on the flight distance for oval and flat airbags was analyzed with the simulations. RESULTS: In both accident cases a too long approach led to a too high takeoff speed, which was the cause for landing at the end of the oval airbags. The effect of flight distance is considerably more sensitive to approach and takeoff parameters with oval versus flat airbags. CONCLUSIONS: Three measures are recommended to prevent jumping over an airbag. An approach corridor with top and lateral fences has to be set up and the approach should be steep. Flat airbags are preferable to oval airbags. Airbags should be equipped with a heightening at the end.

On the measurement of ski boot viscoelasticity.

OBJECTIVES: Since the polymeric materials commonly used for ski boots feature viscoelastic properties, the results of ski boot flexion tests are expected to be influenced by flexion velocity. Devices testing at all skiing specific ankle angular velocities are currently not available. Therefore, the aims of this study were to (i) develop a system allowing the testing of ski boots at high ankle angular velocities, (ii) quantify the effect of ankle angular velocity on viscoelasticity and (iii) determine the repeatability of the system. DESIGN AND METHOD: A test bench and a lower limb prosthesis were developed to determine tibia angle and applied torque. To assess the effect of angular velocity, two pairs of ski boots were tested at 5°/s, 50°/s, 75°/s and 100°/s. To assess stiffness variation and measurement repeatability, ten different used ski boots of different manufacturers were tested twice. RESULTS: Four ski boot flexion stiffness parameters and two energy dissipation factors were reported. The repeatability of the stiffness and the energy dissipation parameters was better than 4% and 3%, respectively. Stiffnesses and dissipation factors increased with increasing angular velocity. CONCLUSION: In the present study a reliable system facilitating the testing of ski boots at velocities of up to 100°/s was developed. To comprehensively characterise the viscoelastic properties of ski boots, we propose to report four ski boot stiffness parameters and two energy dissipation factors. An ankle angular velocity above 50°/s was recommended to perform mechanical tests of ski boots if employed in slalom-like skiing.

Does Avalanche Shovel Shape Affect Excavation Time: A Pilot Study.

In Europe and North America, approximately 150 fatalities occur as a result of avalanches every year. However, it is unclear whether certain shovel shapes are more effective than others in snow removal during avalanche victim recovery. The objective was to determine the performance parameters with a developed standardized test using different shovel shapes and to determine sex-specific differences. Hence, several parameters were determined for clearing the snow from a snow filled box (15 men, 14 women). A flat (F) and a deep (D) shovel blade with the shaft connected straight (S) or in clearing mode (C) were used for the investigation of the shovel shapes FS, DC and the subsequent use of DC&DS. Mean snow mass shifted per unit time increased significantly from 1.50 kg/s with FS to 1.71 kg/s (14%) with DS and further to 1.79 kg/s (4%) with DC&DS for all participants. Snow mass shifted per unit time was 44% higher (p < 0.05) for men than for women. In excavation operations, the sex-specific physical performance should be taken into account. The results were limited to barely binding snow, because only with this snow did the tests show a high reliability.

An approximate simulation model for initial luge track design.

Competitive and recreational sport on artificial ice tracks has grown in popularity. For track design one needs knowledge of the expected speed and acceleration of the luge on the ice track. The purpose of this study was to develop an approximate simulation model for luge in order to support the initial design of new ice tracks. Forces considered were weight, drag, friction, and surface reaction force. The trajectory of the luge on the ice track was estimated using a quasi-static force balance and a 1d equation of motion was solved along that trajectory. The drag area and the coefficient of friction for two runs were determined by parameter identification using split times of five sections of the Whistler Olympic ice track. The values obtained agreed with experimental data from ice friction and wind tunnel measurements. To validate the ability of the model to predict speed and accelerations normal to the track surface, a luge was equipped with an accelerometer to record the normal acceleration during the entire run. Simulated and measured normal accelerations agreed well. In a parameter study the vertical drop and the individual turn radii turned out to be the main variables that determine speed and acceleration. Thus the safety of a new ice track is mainly ensured in the planning phase, in which the use of a simulation model similar to this is essential.

Distribution of injury mechanisms and related factors in ACL-injured female carving skiers.

While ACL injury mechanisms in skiers using traditional skis are well studied, no study has yet investigated the distribution of injury mechanisms in carving skiers. In traditional skiers, the backward twisting fall seems to be the dominant injury mechanism, especially in female skiers. Female recreational skiers have a threefold higher risk to sustain an ACL injury than male skiers; therefore, it is important to determine if carving skis influence the distribution of injury mechanisms and the related frequencies of ACL injuries in female skiers. We investigated the frequencies of injury mechanisms and related factors in 65 ACL-injured female carving skiers by questionnaire. The forward twisting fall was the most reported ACL injury mechanism with about 51%, followed by the backward twisting fall within 29% of cases. Catching an edge of the ski (59 vs. 24%, P = 0.03) when executing turns (69 vs. 41%, P = 0.053) was a more frequent cause for forward twisting falls than for the other types of falling. While 29% of bindings released during a forward twisting fall, only 3.1% released during the remaining mechanisms. In contrast to traditional skiers, the forward twisting fall was the dominant injury mechanism in female carving skiers with ACL injury.