The aerodynamics of the skier’s equipment and the effect of postural changes on the aerodynamic forces acting on the skier during downhill speed racing have been studied theoretically. The aerodynamic characteristics of skier and equipment have been determined by a source panel method based on the velocity potential theory. The calculations indicate that the skier’s torso should be slightly lifted from the tangential direction of downhill during skiing, thus causing a lift force and reducing the friction between the skis and snow. The drag of the torso—tilted by a few degrees—will remain almost the same as the drag of the torso in strict tangential direction. The force acting on the skier’s legs can be directed according to individual needs. The shape of the leg spoilers will give the wanted drag/lift ratio. The optimum shape of the helmet depends on the skiing style. The results introduced here are obtained from theoretical calculations, and their validity should first be tested in a wind tunnel and finally during the normal skiing performance. The calculated drag forces, which are based on the velocity potential theory, do not include the base drag of the skier’s body.
Thomas A. Stoffregen, Karen Adolph, Esther Thelen, Kathleen M. Gorday and Yang-Yi Sheng
This study was undertaken to determine whether young children, after only a few weeks standing experience, could respond adaptively to the dynamical constraints imposed by different support surfaces. The spontaneous postural motions of young children (13-14 months old) were observed as they stood on surfaces that differed in length, friction, and rigidity. There were no externally imposed perturbations to stance. Children's postural control was remarkably adaptive: There were few falls on any of the surfaces. Moreover, the children showed surface-specific utilization of manual postural control (holding onto wooden poles), suggesting that manual control is an adaptive strategy for postural control. Finally, kinematic analysis suggested that, in some instances, children were able to employ independent control of the hips, contrary to previous models which had suggested that hip motions could not be controlled before the age of 3 years. Small, slow hip movements useful in controlling spontaneous sway (unperturbed stance) may serve as a basis for the development of larger, faster hip movements that are associated with imposed perturbations.
Martin D. Hoffman, Philip S. Clifford, Božo Bota, Michael Mandli and Gregory M. Jones
A theoretical analysis was used to evaluate the effect of body mass on the mechanical power cost of cross-country skiing and roller skiing on flat terrain. The relationships between body mass and the power cost of overcoming friction were found to be different between cross-country skiing on snow and roller skiing. Nevertheless, it was predicted that the heavier skier should have a lower oxygen cost per unit of body mass for roller skiing, as is the case for snow skiing. To determine whether the theoretical analysis was supported by experimental data, oxygen consumption measurements were performed during roller skiing by six male cross-country ski racers who spanned a 17.3-kg range in body mass. The theoretical analysis was supported by the experimental findings of decreases in oxygen consumption for each kg increase in body mass of approximately 1.0% for the double pole technique, 1.8% for the kick double pole technique, and 0.6% for the VI skate technique.
Isabelle Schöffl, Thomas Baier and Volker Schöffl
After a pulley rupture, most climbers regain the full function of their previously uninjured fingers. However, in some cases of pulley rupture, a persistent inflammation of the tendon sheath is observed. In this study, 16 cadaver fingers were loaded until pulley rupture and then studied for the rupturing mechanism. In addition, two patients with this pathology were investigated using ultrasound and MRI, and received surgery. In 13 fingers, a rupture of one or several pulleys occurred and almost always at the medial or lateral insertion. In one finger, a capsizing of the pulley underneath the intact tendon sheath was observed, leading to an avulsion between tendon and tendon sheath. A similar pathology was observed in the ultrasound imaging, in MRI, and during surgery in two patients with prolonged recovery after minor pulley rupture. In cases of prolonged tenosynovitis after minor pulley rupture, a capsizing of the pulley stump is probably the cause for constant friction leading to inflammation. In those cases, a surgical removal of the remaining pulley stump and sometimes a pulley repair may be necessary.
Glenn S. Wunderly and Maury L. Hull
A new approach to ski binding design is advanced. It begins with a release locus derived from injury mechanics research and knowledge of the expected loading conditions and then incorporates these into the final binding design. A mechanical ski binding designed by following the new approach is presented. This binding offers a number of performance features not found in commercially available designs. One feature is the ability to eliminate the axial force supported by the tibial shaft from affecting release in forward bending. A second feature is the binding’s ability to release according to virtually any preprogrammed locus of the combination of moments in both bending and torsion. A third feature is a release mechanism that is insensitive to the common frictional forces that affect the release consistency of conventional heel/toe bindings. In addition to these features, the binding offers a variety of operational conveniences. The presentation of the binding not only describes the design details but also evaluates the release performance (i.e., locus and consistency) based upon laboratory tests under quasistatic loading.
Mont Hubbard, Michael Kallay and Payam Rowhani
We have developed a mathematical model and computer simulation of three-dimensional bobsled turning. It is based on accurate descriptions of existing or hypothetical tracks and on dynamic equations of motion including gravitational, normal, lift, drag, ice friction, and steering forces. The three-dimensional track surface model uses cubic spline geometric modeling and approximation techniques. The position of the sled on the track is specified by the two variables α and β in the along-track and cross-track directions, differential equations for which govern the possible motions of the sled. The model can be used for studies involving (a) track design, (b) calculation of optimal driver control strategies, and (c) as the basis for a real-time bobsled simulator. It can provide detailed quantitative information (e.g., splits for individual turns) that is not available in runs at actual tracks. The model also allows for comparison of driver performance with the numerically computed optimum performance, and for safe experimentation with risky driving strategies.
Christina Åsan Grasaas, Gertjan Ettema, Ann Magdalen Hegge, Knut Skovereng and Øyvind Sandbakk
This study investigated changes in technique and efficiency after high-intensity exercise to exhaustion in elite cross-country skiers. Twelve elite male skiers completed 4 min submaximal exercise before and after a high-intensity incremental test to exhaustion with the G3 skating technique on a 5% inclined roller-ski treadmill. Kinematics and kinetics were monitored by instrumented roller skis, work rate was calculated as power against roller friction and gravity, aerobic metabolic cost was determined from gas exchange, and blood lactate values indicated the anaerobic contribution. Gross efficiency was the work rate divided by aerobic metabolic rate. A recovery period of 10 min between the incremental test and the posttest was included to allow the metabolic values to return to baseline. Changes in neuromuscular fatigue in upper and lower limbs before and after the incremental test were indicated by peak power in concentric bench press and squat-jump height. From pretest to posttest, cycle length decreased and cycle rate increased by approximately 5% (P < 0.001), whereas the amount of ski forces did not change significantly. Oxygen uptake increased by 4%, and gross efficiency decreased from 15.5% ± 0.7% to 15.2% ± 0.5% from pretest to posttest (both P < .02). Correspondingly, blood lactate concentration increased from 2.4 ± 1.0 to 6.2 ± 2.5 mmol/L (P < .001). Bench-press and squat-jump performance remained unaltered. Elite cross-country skiers demonstrated a less efficient technique and shorter cycle length during submaximal roller-ski skating after high-intensity exercise. However, there were no changes in ski forces or peak power in the upper and lower limbs that could explain these differences.
Andrea Monte, Francesca Nardello and Paola Zamparo
The effects of different loads on kinematic and kinetic variables during sled towing were investigated with the aim to identify the optimal overload for this specific sprint training.
Thirteen male sprinters (100-m personal best: 10.91 ± 0.14 s) performed 5 maximal trials over a 20-m distance in the following conditions: unloaded and with loads from 15% to 40% of the athlete’s body mass (BM). In these calculations the sled mass and friction were taken into account. Contact and flight times, stride length, horizontal hip velocity (vh), and relative angles of hip, knee, and ankle (at touchdown and takeoff) were measured step by step. In addition, the horizontal force (Fh) and power (Ph) and maximal force (Fh0) and power (Ph0) were calculated.
vh, flight time, and step length decreased while contact time increased with increasing load (P < .001). These variables changed significantly also as a function of the step number (P < .01), except between the 2 last steps. No differences were observed in Fh among loads, but Fh was larger in sled towing than in unloaded. Ph was unaffected by load up to +20%BM but decreased with larger loads. Fh0 and Ph0 were achieved at 20%BM. Up to 20%BM, no significant effects on joint angles were observed at touchdown and takeoff, while at loads >30%BM joint angles tended to decrease.
The 20%BM condition represents the optimal overload for peak power production—at this load sprinters reach their highest power without significant changes in their running technique (eg, joint angles).
Dionne A. Noordhof, Carl Foster, Marco J.M. Hoozemans and Jos J. de Koning
Speed skating posture, or technique, is characterized by the push-off angle or effectiveness (e), determined as the angle between the push-off leg and the ice; the preextension knee angle (θ 0); and the trunk angle (θ 1). Together with muscle-power output and environmental conditions, skating posture, or technique, determines velocity (v).
To gain insight into technical variables that are important to skate efficiently and perform well, e, θ 0, θ 1, and skating v were determined every lap during a 5000-m World Cup. Second, the authors evaluated if changes (Δ) in e, θ 0, and θ 1 are associated with Δv.
One camera filmed the skaters from a frontal view, from which e was determined. Another camera filmed the skaters from a sagittal view, from which θ 0 and θ 1 were determined. Radio-frequency identification tags around the ankles of the skaters measured v.
During the race, e progressively increased and v progressively decreased, while θ 0 and θ 1 showed a less consistent pattern of change. Generalized estimating equations showed that Δe is significantly associated with Δv over the midsection of the race (β = −0.10, P < .001) and that Δθ 0 and Δθ 1 are not significantly associated with Δv.
The decrease in skating v over the race is not due to increases in power losses to air friction, as knee and trunk angle were not significantly associated with changes in velocity. The decrease in velocity can be partly ascribed to the decrease in effectiveness, which reflects a decrease in power production associated with fatigue.
Vagenas * Blaine Hoshizaki * 2 1992 8 1 11 29 10.1123/ijsb.8.1.11 A Perturbation Study of Lower Extremity Motion during Running Wilbert Van Woensel * Peter R. Cavanagh * 2 1992 8 1 30 47 10.1123/ijsb.8.1.30 Friction Measurement in Tennis on the Field and in the Laboratory Bart Van Gheluwe * Eric