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Sauli Savolainen and Reijo Visuri

In this review we analyze competition sports, particularly downhill speed skiing and cross-country skiing, in terms of physical performance. The effects of various forces/parameters on athletic performance are summarized, and metabolic energy cost and mechanical power output are reviewed. The primary factors contributing to energy loss in the athlete are drag, friction between athlete and ground, and gravitational force (i.e., the movement of body segments in the gravitational field). According to previous reports the latter is the most significant factor. However, estimated levels of energy expenditure, occurring as a direct result of gravitational force, vary considerably depending on the method used in the analysis. We also demonstrate the importance of changes in friction and drag in athletic performance, using practical examples from skiing.

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Abderrehmane Rahmani, Alain Belli, Tomasz Kostka, Georges Dalleau, Marc Bonnefoy and Jean-Rene Lacour

This paper describes the application of the inverse dynamic method developed by Bosco and colleagues for measuring the mechanical properties of knee extensor muscles under ballistic conditions. Twenty elderly men performed a series of maximal ballistic leg extensions at different loads. Accurate measurements of friction and inertial effects during the movement were used to calculate the torque and power produced during extension. The error of the results was 6.2–45.3% when the friction and inertial effects were neglected. The torque-angular velocity relationships were linear (r = 0.92 to 0.99, p < .001). The peak measurements obtained were in agreement with published isokinetic values. This new ergometer allows assessment of movements similar to those performed in day-to-day activities and can be used. without training, by elderly or disabled subjects.

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Gertjan J.C. Ettema, Steinar Bråten and Maarten F. Bobbert

A ski jumper tries to maintain an aerodynamic position in the in-run during changing environmental forces. The purpose of this study was to analyze the mechanical demands on a ski jumper taking the in-run in a static position. We simulated the in-run in ski jumping with a 4-segment forward dynamic model (foot, leg, thigh, and upper body). The curved path of the in-run was used as kinematic constraint, and drag, lift, and snow friction were incorporated. Drag and snow friction created a forward rotating moment that had to be counteracted by a plantar flexion moment and caused the line of action of the normal force to pass anteriorly to the center of mass continuously. The normal force increased from 0.88G on the first straight to 1.65G in the curve. The required knee joint moment increased more because of an altered center of pressure. During the transition from the straight to the curve there was a rapid forward shift of the center of pressure under the foot, reflecting a short but high angular acceleration. Because unrealistically high rates of change of moment are required, an athlete cannot do this without changing body configuration which reduces the required rate of moment changes.

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Craig A. Williams, Eric Doré, James Alban and Emmanuel Van Praagh

This study investigated the differences in short-term power output (STPO) using three different cycle ergometers in 9-year-old children. A total of 31 children participated in three cycle ergometer sprint tests of 20 s duration: a modified friction braked Monark, a modified friction braked Ergomeca cycle ergometer, and a SRM isokinetic ergometer. Common indices of peak and mean power, peak pedal rate, time to peak power, and pedal rate were recorded. Indices of peak power 1 s for the Monark, Ergomeca and SRM ergometer were found to be 299 ± 55, 294 ± 55, 297 ± 53 W and mean power 20 s to be 223 ± 40, 227 ± 43 and 216 ± 34 W, respectively. The time to peak power was found to be 3 ± 2, 6 ± 2, 5 ± 3 s, respectively. The standard error of measurement was lower in mean 20-s power compared to 1-s peak power. Despite instrumentation and protocol differences these results demonstrate reproducibility in 9-year-old children that will allow researchers confidence in comparing STPO data obtained from different ergometers.

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Victoria H. Stiles and Sharon J. Dixon

Research suggests that heightened impacts, altered joint movement patterns, and changes in friction coefficient from the use of artificial surfaces in sport increase the prevalence of overuse injuries. The purposes of this study were to (a) develop procedures to assess a tennis-specific movement, (b) characterize the ground reaction force (GRF) impact phases of the movement, and (c) assess human response during impact with changes in common playing surfaces. In relation to the third purpose it was hypothesized that surfaces with greatest mechanical cushioning would yield lower impact forces (PkFz) and rates of loading. Six shod volunteers performed 8 running forehand trials on each surface condition: baseline, carpet, acrylic, and artificial turf. Force plate (960 Hz) and kinematic data (120 Hz) were collected simultaneously for each trial. Running forehand foot plants are typically characterized by 3 peaks in vertical GRF prior to a foot-off peak. Group mean PkFz was significantly lower and peak braking force was significantly higher on the baseline surface compared with the other three test surfaces (p < 0.05). No significant changes in initial kinematics were found to explain unexpected PkFz results. The baseline surface yielded a significantly higher coefficient of friction compared with the other three test surfaces (p < 0.05). While the hypothesis is rejected, biomechanical analysis has revealed changes in surface type with regard to GRF variables.

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Glenn M. Street and Robert W. Gregory

While the scientific literature has confirmed the importance of high maximal aerobic power to successful cross-country skiing performance, the same cannot be said of skiing technique or gliding characteristics of skis. The purpose of this study was to determine whether glide speed was related to Olympic race performance. Male competitors in the 50-km freestyle event were videotaped during the 1992 Winter Olympic Games. Glide speeds of the entire field were measured through a 20-m flat section at the bottom of a 150-m, 12° downhill. A significant correlation (r = -.73) was found between finish time and glide speed, showing that the more successful competitors tended to have faster glide speeds through this section of the course. A predictive model of glide speed suggested that the faster glide speeds were due primarily to differences in friction. There was little evidence to suggest that differences in air drag, body mass, or initial speed accounted for the major differences in glide speeds.

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Cheryl D. Pierson-Carey, David A. Brown and Christine A. Dairaghi

The purpose of this study was to determine the effects of limiting ankle motion on pedal forces. Sixteen adults pedaled an instrumented ergometer against constant cadence and frictional load while wearing hinged braces. Ankle motion was limited under four randomly assigned conditions: both braces unlocked (UL), only the preferred leg (PL) brace locked, only the nonpreferred leg (NPL) brace locked, and braces on both legs (BL) locked. Measurements of pedal force, crank, and pedal angles were sampled at 200/s for 20 s. With both braces locked, resultant force mean magnitude decreased during the downstroke, due to reduced radial crank force. Asymmetry between PL and NPL decreased during the power phase when only PL was braced but increased when only NPL was braced. It was concluded that constrained ankle motion, as may occur with ankle injury or hemiplegia, reduces the ability to transmit power during the downstroke while enhancing ability during the upstroke.

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Jos J. de Koning, Gert de Groot and Gerrit Jan van Ingen Schenau

The purpose of this study was to describe the intermuscular coordination and power production for the constrained asymmetrical movement during skating the curves. Seven elite male speed skaters took part in the experiments. The speed skaters were simultaneously filmed from frontal and sagittal views. EMGs were obtained telemetrically and push-off force was registered with special skates. Inverse dynamic analysis yielded power production data, which differed for left and right leg. Marked differences were also found in intermuscular coordination of each leg. The activation patterns of the muscles were influenced by the asymmetrical nature and the typical body position during the speed skating movement. External power output was determined by three methods. The mean joint power output for left and right leg showed similar values as the external power output calculated from air and ice friction. These values were lower than the values predicted with a geometrical model for skating the curves.

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Matt R. Cross, Matt Brughelli, Pierre Samozino, Scott R. Brown and Jean-Benoit Morin

Purpose:

To ascertain whether force-velocity-power relationships could be compiled from a battery of sled-resisted overground sprints and to clarify and compare the optimal loading conditions for maximizing power production for different athlete cohorts.

Methods:

Recreational mixed-sport athletes (n = 12) and sprinters (n = 15) performed multiple trials of maximal sprints unloaded and towing a selection of sled masses (20–120% body mass [BM]). Velocity data were collected by sports radar, and kinetics at peak velocity were quantified using friction coefficients and aerodynamic drag. Individual force–velocity and power–velocity relationships were generated using linear and quadratic relationships, respectively. Mechanical and optimal loading variables were subsequently calculated and test–retest reliability assessed.

Results:

Individual force–velocity and power–velocity relationships were accurately fitted with regression models (R 2 > .977, P < .001) and were reliable (ES = 0.05–0.50, ICC = .73–.97, CV = 1.0–5.4%). The normal loading that maximized peak power was 78% ± 6% and 82% ± 8% of BM, representing a resistance of 3.37 and 3.62 N/kg at 4.19 ± 0.19 and 4.90 ± 0.18 m/s (recreational athletes and sprinters, respectively). Optimal force and normal load did not clearly differentiate between cohorts, although sprinters developed greater maximal power (17.2–26.5%, ES = 0.97–2.13, P < .02) at much greater velocities (16.9%, ES = 3.73, P < .001).

Conclusions:

Mechanical relationships can be accurately profiled using common sled-training equipment. Notably, the optimal loading conditions determined in this study (69–96% of BM, dependent on friction conditions) represent much greater resistance than current guidelines (~7–20% of BM). This method has potential value in quantifying individualized training parameters for optimized development of horizontal power.

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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.