Gert de Groot

Hydrodynamic drag and lift forces on human hand/arm models

Forces acting on the forearm and hand during swimming can be decomposed into drag forces and lift forces. In this study drag and lift forces were measured on two models of a human hand and forearm when towed in a towing tank. To compare the results of models with different size at different velocities force data were normalized to drag and lift coefficients (Cd and Ct). Influence of the orientation of the model with respect to the flow, velocity, size of the model and the relative contribution of the hand and forearm on Cd and Ct were studied. The orientation of the model with respect to the line of motion was varied by rotating the models around three axes, and quantified using the angle of pitch (AP: the angle between the hand plane and flow) and the sweep-back angle (SB: the orientation of the flow vector when projected on the hand plane). Cd was maximal when the palm of the hand is almost perpendicular to the flow (AP = 65 degrees, SB = 342 degrees). Ct shows maximal values at two different orientations: with the hand in a thumb-leading position, AP = 31 degrees, SB = 358 degrees, and with the hand in a little finger-leading position, AP = 48 degrees, SB = 193 degrees. The orientation of the hand was very critical in generating lift forces. By contrast, the influence of velocity and size of the model on the values of Cd and Ct was limited.(ABSTRACT TRUNCATED AT 250 WORDS)

Optimisation of Sprinting Performance in Running, Cycling and Speed Skating

SummarySprinting performances rely strongly on a fast acceleration at the start of a sprint and on the capacity to maintain a high velocity in the phase following the start. Simulations based on a model developed in which the generation of metabolic power is related to the mechanical destinations of power showed that for short-lasting sprinting events, the best pacing strategy is an all out effort, even if this strategy causes a strong reduction of the velocity at the end of the race. Even pacing strategies should only be used in exercises lasting longer than 80 to 100 seconds.Sprint runners, speed skaters and cyclists need a large rate of breakdown of energy rich phosphates in the first 4 to 5 seconds of the race (mechanical equivalent > 20 W/kg) in order to accelerate their body, and a power output of more than 10 W/kg in the phase following the start to maintain a high velocity. Maximal speed in running is mainly limited by the necessity to rotate the legs forwards and backwards relative to the hip joint. The acceleration phase, however, relies on powerful extensions of all leg joints. Through a comparison of the hindlimb design of highly specialised animal sprinters (as can be found among predators) and of long distance animal runners (as found among hoofed animals), it is illustrated that these 2 phases of a sprint rely on conflicting requirements: improvement of maximal speed would require lower moments of inertia of the legs whereas a faster acceleration would require the involvement of more muscle mass (not only of the hip and knee extensors but also of the plantar flexors).Maximal speed in cycling and speed skating is not limited by the necessity to move leg segments but rather on air friction and rolling or ice friction. Since the drag coefficients found for speed skaters and cyclists (about 0.8) are considerably higher than those of more streamlined bodies, much progress can still be expected from the reduction of air friction. Speed skaters and especially cyclists show much smaller accelerations during the start than do sprint runners. Skaters might try to improve their very first push off by developing a start technique that allows a much more horizontally directed propulsive force. The small propulsive force at the onset of a cycling sprint is due to the gearing system. For sprint cycling (the 1000m time trail and the 4000m pursuit) much progress could be expected from the development of a gearing system that allows a considerably higher propulsive force at the onset of the race and that adapts itself automatically to the velocity.

Ice friction during speed skating

Abstract During speed skating, the external power output delivered by the athlete is predominantly used to overcome the air and ice frictional forces. Special skates were developed and used to measure the ice frictional forces during actual speed skating. The mean coefficients of friction for the straights and curves were, respectively, 0.0046 and 0.0059. The minimum value of the coefficient of ice friction was measured at an ice surface temperature of about −7°C. It was found that the coefficient of friction increases with increasing speed. In the literature, it is suggested that the relatively low friction in skating results from a thin film of liquid water on the ice surface. Theories about the presence of water between the rubbing surfaces are focused on the formation of water by pressure-melting, melting due to frictional heating and on the ‘liquid-like’ properties of the ice surface. From our measurements and calculations, it is concluded that the liquid-like surface properties of ice seem to be a reasonable explanation for the low friction during speed skating.

Effect of a triathlon wet suit on drag during swimming

The effect of a triathlon wet suit on drag was studied in 12 subjects (eight male, four female) swimming at different velocities (1.10, 1.25 and 1.50 m.s-1). The active drag force was directly measured during front crawl swimming using a system of underwater push off pads instrumented with a force transducer (M.A.D. system: 6). Measurements were made when swimming over the system with and without a wet suit. A 14% reduction in drag (from 48.7 to 41.8 Newtons) is found at a swimming velocity of 1.25 m.s-1, which is a typical swimming speed for triathlon distances. At 1.50 m.s-1 a reduction in drag of 12% was observed, which suggests that the wearing of such a suit might be beneficial in conventional swimming events. The reduction in drag can explain the higher swimming velocities observed in triathletes using a wet suit. The effect of the reduction is probably largely due to an increased buoyancy inducing less frontal resistance. However, since the effect of the suit on the lighter female swimmers was not different from the effect on the heavier male swimmers, a reduction in friction drag and drag coefficient may also be significant.

Coordination of leg muscles during speed skating

Abstract Five speed skaters of elite performance level and six speed skaters of trained level were subjected to an inverse dynamical analysis during speed skating. Push-off forces were registered by means of special skates. Myoelectric activity (EMG) of ten leg muscles and cinematographic data were recorded. Linked segment modelling yielded net joint moments and joint powers. The speed skating technique is characterized by a typical horizontal position of the trunk and a suppression of a plantar flexion during the push-off. This technique, necessary to reduce external friction, constrains the transfer of rotation in joints to translation of the mass center of the body. In spite of constrained push-off, the EMG levels of the leg muscles show a proximo-distal temporal order which to a certain extent is comparable to that previously found in an unconstrained vertical jump. This proximo-distal sequence is also reflected by the time courses of the net moment and net power output in hip, knee and ankle joints. The temporal sequence in activation levels of activated muscles is not different between elite and trained speed skaters. The difference in performance level between these groups obviously has an origin in the ability of the elite speed skaters to realise larger net joint moments. Differences in net joint moments and in kinematics result in a higher power output and a lower air frictional force for the elite than for the trained speed skaters.

Reproducibility of a Triaxial Seismic Accelerometer (DynaPort)

PURPOSE To examine the reproducibility of a triaxial seismic accelerometer under controlled conditions and real-life conditions. METHODS Instrumental reproducibility was examined using a shaker device. The accelerometers (DynaPort MiniMod; McRoberts B.V., The Hague, The Netherlands) were shaken at four frequencies (0.8, 2.1, 3.6, and 4.6 Hz) in x- and y-directions. The magnitude of acceleration ranged from 0 to 1.277g. Additionally, reproducibility under real-life conditions was examined in 55 adolescents (12-17 yr), with the accelerometer attached to the lower back. Each subject walked four short walking trials on level ground at preferred speed. To make this setting meet real-life conditions, we detached and reattached the accelerometer between trials 2 and 3. Detachment of accelerometer between trials 2 and 3 was done by either the same researcher or different researchers (four in total). Intra- and interobserver reproducibility were calculated. RESULTS Intra- and interinstrumental intraclass correlation coefficients (ICC) were 0.99 for both x- and y-directions. The intrainstrumental coefficients of variance (CoV) were lower than 1.13%. The interinstrumental CoV were lower than 1.37%. Intraobserver ICC was 0.97, and interobserver ICC was 0.88. CONCLUSION The reproducibility of the accelerometer is high under the controlled conditions of a shaker device and in walking at preferred speed.

From biomechanical theory to application in top sports: the Klapskate story

The development of the new skate design specifically the klapskate in a historical and scientific perspective is described. Reasons why it took so long for top athletes to use the new skate design is explained. The klapskate demonstrated its advantage over conventional skates and proved its benefits.

A power equation for the sprint in speed skating

An analysis of the start of the 500 m speed skating races during the 1988 Olympic Winter Games showed a remarkably high correlation between the acceleration of the skater in the first second of the sprint and the final time (r = -0.75). In this study a power equation is used to explain this high coefficient of correlation. The performance in speed skating is determined by the capability of external power production by the speed skater. This power is necessary to overcome the air and ice friction and to increase the kinetic energy of the skater. Numerical values of the power dissipated to air and ice friction, both dependent on speed, are obtained from ice friction and wind tunnel experiments. Using aerobic and anaerobic power production as measured during supra maximal bicycle tests of international-level speed skaters, a model of the kinetics of power production is obtained. Simulation of power production and power dissipation yields values of speed and acceleration and, finally, the performance time of the sprint during speed skating. The mean split time at 100 m and the final time at 500 m in these races, derived from simulation, were 10.57 s (+/- 0.31) and 37.82 s (+/- 0.96), respectively. The coefficient of correlation between the simulated 500 m times and the actual 500 m times was 0.90. From the results of this study it can be concluded that the distribution of the available anaerobic energy is an important factor in the short lasting events. For the same amount of anaerobic energy the better sprinters appear to be able to liberate considerably more energy at the onset of the race than skaters of lower performance level.

Delta efficiencies of running and cycling.

PURPOSE The purpose of the present study was to compare delta efficiencies of running with cycling, while several factors that can possibly influence delta efficiency were excluded. METHODS Twelve subjects performed a submaximal running and cycling test on subsequent days. Delta efficiencies of running and cycling were compared at equal metabolic intensities. Furthermore, rest periods were included in the protocol to avoid fatigue. Pedaling and stride frequencies were held constant during the tests. Finally, the influence of two ways of applying extra external load (inclination of treadmill and horizontal impeding forces) on the delta efficiency of running and cycling was investigated. RESULTS The results of the present study show that the mean delta efficiency of running (45.5%) is still significantly higher than the mean delta efficiency of cycling (25.7%). The way extra external load is applied does not influence delta efficiency. CONCLUSION The way of loading and the difference in metabolic intensity can be excluded as causes for the observed difference in delta efficiency between running and cycling. It is suggested that a different contribution in the metabolic load attributable to muscular activity of the arms and/or trunk that does not directly contribute to the work needed to overcome the amount of applied external load may be a relevant factor.

A new skate allowing powerful plantar flexions improves performance

To prevent the tip of the blade from scratching through the ice, the technique in speed skating requires that plantar flexion is largely suppressed during the gliding push off. This not only prevents the plantar flexors from contributing to external work but also causes the skater to lose contact with the ice long before the knee is fully extended. To prevent these disadvantages of the gliding technique, a new skate was developed that permits the shoe to rotate relative to the blade in a hinge between shoe and blade. In a case control study the progression between the 1993/1994 and 1994/1995 skating seasons of 11 male skaters from a regional junior selection who consented to switch to this new skate was compared with the progression of 72 skaters of this and all other regional and national male junior selections of The Netherlands. The experimental group appeared to improve their personal best times by 6.2 +/- 2.3%, which is a significantly (P < 0.001) larger progress than the 2.5 +/- 1.6% improvement of the control group. The new skate will therefore most likely add a new dimension to the art of speed skating.