The altitude differential of the specific mechanical energy, diff (e(mech)), is used to evaluate skiing performance. It is defined as the negative differential between the skier's total specific mechanical energy (e(mech)) and the altitude of the skier's center of mass (COM). Till now, emech was obtained upon a mass--point (MP) model of the skier's COM, which neither considered the segmental energies of their relative movements to the COM, nor their rotational kinetic energies. The aims of the study were therefore: (a) to examine the deviations in diff(e(mech)) between the MP and a more complex linked segment (LS) skier model consisting of 15 rigid bodies, which encountered the aforementioned defectiveness, (b) to compare the energy fluctuations of the two skier models, and (c) to investigate the influence of the gate setup on (a) and (b) in giant slalom. Three--dimensional whole--body kinematics of nine skiers was measured using a global navigation satellite system and an inertial motion capture system while skiing on a predefined course divided into a turny and open gate setup. Mechanical energies including their altitude differentials were calculated for the LS and MP models. There were no significant differences in emech and diffemech ski turn averages, as in individual data points, between both skier models for both analyzed gate setups. The energies additionally considered by the LS model presented a negligible part regardless of the gate setup. In conclusion, the MP skier model is sufficiently accurate for the evaluation of the skiing performance with diff(e(mech))

This article presents a study of torsion stiffness of a specific racing cross-country ski boot for the skating technique conducted by means of numerical analyses and measurements. Its aim is to determine relative torsion stiffness contributions, relative mass contributions, and the ratios between them (torsion stiffness/mass ratios) for individual boot components of the torsionally most deformable part of the boot. These are basic data for torsion stiffness–mass optimization of the boot. For acquiring these data, a complex finite element model of the cross-country ski boot and an artificial silicone foot were created in different versions after an existing device designed for measurement of stiffness properties of the boots. Material properties employed in the model were acquired experimentally. The confirmed model was used for determination of the torsion deformation contributions of different regions of the boot. Focus was given to the middle region, whose contribution is the largest. The aforementioned data were determined for individual boot components of this region. The soles, which contribute the most to the middle region’s torsion stiffness, turned out to have the highest potential for torsion stiffness–mass optimization, because of their lowest torsion stiffness/mass ratio and highest mass contribution. The shoe-upper has the highest torsion stiffness/mass ratio and is, therefore, the most worthwhile to be enlarged. The torsion stiffness/mass ratio of the strengthening bands is lower than expected and could likely be increased by their positioning.

In hydraulics, threaded plugs are used to close various manufacturing holes and other fluid channels. They are preloaded to ensure sufficient sealing force. Since the range of recommended thread and underhead friction coefficients for preloaded threaded connections in the literature is very wide, they are not suitable for accurate determination of the preload–torque relationships of plug–valve connections. In the study, two non-standard plugs with metric threads were equipped with strain gauges and repeatedly tightened three times in valve housings under lubricated and unlubricated conditions. The preload and tightening torque were measured. (1) Although the plug–valve connections had a similar geometry with the same surface roughness of the contacting surfaces, the average overall friction coefficient (uniform thread and underhead friction coefficient) and torque coefficient differed between the two connections in the unlubricated and lubricated conditions by 16% and 18%, respectively. This indicates that even small geometrical differences can have a considerable influence on these coefficients. The overall friction and torque coefficients were between 8% and 17% higher in the unlubricated condition than in the lubricated condition (not statistically proven). (2) The overall friction and torque coefficients decreased with repeated tightening under lubricated conditions. This influence decreased with the number of tightening repetitions. (3) Consideration of the minimum and maximum thread and underhead friction coefficients given in VDI 2230 would lead to an error in the estimated preload of −15% to +86%. In conclusion, for accurate determination of the preload–torque relationship of the plug–valve connections, measurements considering repeated tightening are crucial. These should be performed for each type and size of plug–valve connection separately. To minimize the repeated tightening influence, it is recommended to re-tighten the connections several times before leaving production.

For greater energy efficiency of sports footwear, mass needs to be minimized while preserving other favourable characteristics. In this article, an analysis of the flexion stiffness of the foot region, precisely its middle region, of a specific racing cross-country ski boot for the skating technique regarding its mass was performed. On the basis of a complex finite element model of the ski boot and an existing boot stiffness measuring set-up, flexion stiffness portions, mass portions and flexion stiffness/mass portion ratios were determined for individual boot components regarding the middle boot region. These values were determined for the shoe-upper with strengthening bands and shoelaces (altogether S-U), the sole, the midsole and the glue layer between. The S-U turned out to contribute a high flexion stiffness portion to the boot’s middle region’s flexion stiffness and also its flexion stiffness/mass portion ratio turned out to be the highest. The midsole and the sole present the highest potential for flexion stiffness/mass optimization due to their lowest flexion stiffness/mass ratios and highest mass portions. In order to increase the flexion stiffness/mass ratio of the middle boot region, the sole’s and the midsole’s size portions should be reduced, while the S-U’s size portion should be increased. Beside these findings, other suggestions in order to increase the flexion stiffness/mass ratio of the boot’s middle region are also given.

This case study examines the impact of boot longitudinal flexural stiffness on the total external mechanical work of a skier’s centre of mass per distance travelled in the forward direction ([ ] _EX (J/m)) and on running economy during skate roller-skiing under submaximal steady-state conditions. Moreover, it analyses time derivatives of total W _EX, of W _EX performed by the roller-skis and poles, respectively, and of the directly useful mechanical work (the sum of the work to overcome centre of mass’ gravity and rolling resistance) within a typical roller-skiing cycle. Multiple roller-skiing trials (G3 technique) were performed by one subject on an inclined treadmill with boots of soft, intermediate, and stiff flexural stiffness. The orientation and magnitude of the roller-ski and pole ground reaction forces, body kinematics, VO₂, and lactic acid concentration were monitored. The stiff boots had 13.4% ( p < 0.01) lower [ ] _EX compared to the intermediate boots, and 20.7% ( p < 0.001) lower [ ] _EX compared to the soft boots. Regarding running economy, the soft boots had 2.2% ( p < 0.05) higher VO₂ compared to the intermediate boots, but the same VO₂ compared to the stiff boots. In conclusion, the soft boots had significantly higher [ ] _EX and running economy, while stiff boots had significantly lower [ ] _EX and intermediate boots significantly lower running economy. Moreover, [ ] _EX appears to be a better indicator of the boot flexural stiffness impact on energy efficiency than running economy.

For a detailed biomechanical analysis of roller-ski skating a precise measurement of ground reaction forces is needed in addition to the measurement of the kinematic parameters. An innovative portable force measuring system was developed...