Search Results

You are looking at 31 - 40 of 154 items for :

Clear All
Restricted access

Maarten F. Bobbert, Han Houdijk, Jos J. de Koning and Gert de Groot

To gain a better understanding of push-off mechanics in speed skating, forward simulations were performed with a model comprising four body segments and six muscles. We started with a simulated maximum height one-legged jump, obtained by optimization of muscle stimulation time histories. The simulated jump was very similar to one-legged jumps produced by a human, indicating that the model was realistic. We subsequently studied how performance was affected by introducing four conditions characteristic of speed skating: (a) We changed the initial position from that in jumping to that at the start of the push-off phase in skating. This change was accommodated by a delay in stimulation onset of the plantar flexors in the optimal solution. (b) The friction between foot and ground was reduced to zero. As a result, maximum jump height decreased by 1.2 cm and performance became more sensitive to errors in muscle stimulation. The reason is that without surface friction, the foot had to be prevented from slipping away, which constrained the solution space and reduced the tolerance to errors in stimulation. (c) We introduced the requirement to maintain the upper body in a more or less horizontal position. This change could be accommodated by a delay in stimulation onset of the hamstrings, which inevitably caused a reduction in maximum jump height by 11.6 cm. (d) We increased the effective foot length from 16.5 cm, representative of jumping, to 20.5 cm, representative of skating with klapskates. At the 20.5-cm foot length, rotation of the foot did not start during the buildup of plantar flexion moment as it did at smaller foot lengths, but was delayed until hip and knee extension moments decreased. This caused an unbalanced increase in segment angular velocities and muscle shortening velocities, leading to a decrease in muscle force and muscle work and a further decrease in maximum jump height by approximately 5 cm. Qualitatively, these findings help clarify why and how performance of speed skaters depends on the location of the hinge of their skate.

Restricted access

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.

Restricted access

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.

Restricted access

Sauli Savolainen

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.

Restricted access

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.

Restricted access

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.

Restricted access

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.

Restricted access

Paul R. Geisler and Todd Lazenby

Clinical practice in sports medicine is often guided by axioms or paradigms of practice, some of which have persisted over time despite a lack of objective evidence to support their validity. Evidence-based practice compels practicing clinicians to not only seek out and produce evidence that informs their decision-making, but also to challenge existing paradigms of thought and practice, especially when favorable treatment outcomes remain elusive. Insidious, load induced lateral knee pain around the iliotibial band in runners, cyclists, military personnel, rowers, and other athletes has for decades now been conceptualized as iliotibial band friction syndrome, a biomechanically based and unsubstantiated paradigm based on Renne’s 1975 theory that the iliotibial band slips back and forth over the lateral femoral epicondyle during flexion and extension movements of the knee, primarily irritating the underlying bursa and even the iliotibial band itself. Newer evidence about the anatomy and biomechanics of the iliotibial band, the physiology of the condition, and interventional outcomes is now available to challenge that long-held paradigm of thought for iliotibial band related pathology. Given this plethora of new information available for clinical scientists, iliotibial band impingement syndrome is proposed here as a new, evidence-informed paradigm for evaluating and treating this problematic overuse syndrome.

Restricted access

Jules Woolf, Jess C. Dixon, B. Christine Green and Patrick J. Hill

Christiaan Jacobs is the new Dean of Student Affairs at the University of South Central Ontario, which puts him in charge of the Department of Athletics and Recreation. Jacobs has learned that the hypercompetitive environment established by the athletic director, Nathan Scott, has been causing friction in many areas of the department, potentially resulting in the resignation of several long-term employees. As part of an organizational audit, he interviewed many employees and had them complete the Competing Values Framework questionnaire, the results of which were troubling. How should Jacobs lead this department forward and can he count on Scott to be supportive of the direction that he wants it to go? The purpose of this case is to introduce students to the importance of organizational culture and challenges to organizational change. Students will learn about the Competing Values Framework, change management, and have the opportunity to analyze qualitative and quantitative data in formulating responses to the case-guiding questions. This decision-focused case is suitable for use with upper division undergraduate and graduate sport management students in courses such as Organizational Behavior, Strategic Management, Collegiate Athletics Administration, and Critical Issues in Sport.

Restricted access

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.