RUNDELL, KENNETH W.
Several studies have investigated the effects of wind resistance on external energy expenditure in cycling(4,10,14-17), running(5,14,20), cross-country skiing(2,24,25), and speed skating(6,27). It has been shown that the majority of total energy output during high-velocity sports such as cycling and speed skating is required to overcome air resistance(6,15). At speeds above 28 km·h-1, air resistance accounts for more than 90% of the total forces a cyclist has to overcome (14). This estimation is probably consistent with speed skating, since the coefficient of friction (μ) for rolling resistance of a racing bicycle has been determined to be similar to that of speed skates on ice (16,27). Aerodynamic drag (D) is expressed as the square of the velocity, such thatequation 1
where CD represents the drag coefficient, p the air density, Ap the projected frontal area, and v the velocity. Since CD can be considered constant and Ap does not change appreciably with speed, at constant temperature and barometric conditions, the equation can be expressed equation 2
where k is the constant 1/2CDpAp. The external power against aerodynamic drag (WD) at a given v can be expressed as equation 3
This equation tells us that a small increase in speed requires a disproportionate increase in external power output. Thus, one athlete drafting behind another should save considerable energy in high-velocity sports.
Because of this enormous energy requirement to overcome wind resistance, the benefits of drafting behind a leader have been investigated by several researchers. Kyle (14) identified a decrease in air drag of drafting cyclists by more than 40%. This corresponded to a reduction in power output of about 30%. A 10% reduction in the heart rate response of track cyclists who were in the drafting position at 30 km·h-1 was noted by Caru et al. (4). Moreover, Hagberg and McCole(10) and McCole et al. (17) found that the benefit of drafting in an eight-rider pack resulted in an approximate 39% reduction in energy cost over riding alone. The benefits of drafting are quite apparent in cycling, where it is extremely difficult for a solo rider to escape from the pack.
The magnitude of energy cost to overcome air drag is so great that even at relatively low velocities benefits are realized. Pugh(20) found a decrease in total energy expenditure of a drafting runner at 6 m·s-1 by approximately 6%. It was estimated by Pugh (20) and Davies (5) that this energy savings calculated to a 1-s gain in 400 m. Kyle(14) also proposed a significant gain in running performance of about 1.7 s per 400-m lap by drafting with a 2-m space between runners. To date, only two studies have been done evaluating the effects of drafting using cross-country skiers. Spring et al. (24) found a decrease in drag of about 25% when a skier on roller skis paces within 2-3 m of a lead skier. Bilodeau et al. (2) found significantly lower heart rates (154 vs 163) when skiers drafted 2 m behind a leader at a fixed speed (5.6 m·s-1) for 2 km. Street(25) estimated that a 6% reduction in mechanical power could be achieved skiing at 5.5 m·s-1 when no head wind was apparent. These studies clearly indicate that drafting a leader can result in considerable energy savings, even though the speed is not great.
Only two studies have investigated the effects of drafting in speed skating. Using wind tunnel experiments, Di Prampero et al.(6) quantified the energy spent against wind as being equal for running, skating, and cycling. From these experiments, he concluded that energy spent against other forces resulted in the different speeds attained in these exercises for equal power outputs. Van Ingen Schenau(27) found a decrease in drag of 16% and 23% when a skater was shielded 2 m and 1 m behind another skater in a wind tunnel. The full benefits of drafting cannot be achieved in long-track speed skating, since drafting is not permitted. However, during the required lane changes in long-track skating, it would benefit the skater in the back position to draft for as long as possible during the change. The potential advantages offered by drafting in pack-style races, such as marathon skating, are immediately obvious. Until this study, no research evaluating the effects of drafting in short-track speed skating has been done. Since short-track speed skating involves racing in a pack of five to seven skaters at high speeds (sometimes exceeding 40 km·h-1), drafting could result in considerable energy savings during a 3000-m event. The energy conserved during the early stages of the race could potentially be realized during the finishing laps. However, the tight 111-m oval track could limit the benefits of drafting because of the potentially large internal power loss due to the enormous forces that the skaters must overcome during cornering. Thus, the purpose of this study was to determine; 1) the effects of drafting on heart rate responses and blood lactate concentrations during controlled-pace 4-min skating trials, and 2) if shielding benefits can be realized in sprint time performance after a controlled-pace 4-min drafting trial.
Eighteen short-track speed skaters (10 National Team members, 8 developmental athletes; 14 males, 4 females) who skate at the international and/or national level participated in this study. Subject characteristics(mean ± SD) were 20.3 ± 4.1 yr of age, 65.8 ± 9.4 kg body wt., 7.9 ± 2.8% body fat. All skaters were technically proficient and familiar with skating in a drafting situation. Informed and written consent was obtained from each skater prior to their participation in this study.
Testing occurred on a regulation 111-m “short track” set on an enclosed hockey rink during a 2-wk period. Two series of experiments were performed. Series I was designed to determine heart rate and blood lactate responses of one skater drafting behind another (N = 18). Series II was designed to evaluate the influence of drafting on sprint performance after a 4-min skating trial (N = 6). Series II was performed to determined whether the metabolic benefits obtained by drafting would be realized in a typical end-of-race sprint situation.
Series I. Skaters were paired for body size and skating ability. Each subject skated two randomized 4-min trials on separate days, one drafting and one leading. Drafters were encouraged to mimic the stride of the leader and to skate as close as possible to the leader. Distance between drafter and leader was generally within 1 m. Lap times were equal for draft and lead trials and held constant at ≈12.6 s (≈8.8 m·s-1). Skaters were informed of their pace each lap to allow for minor adjustments in skating speed for the 4-min trial.
Heart rates (HR) were continuously monitored and stored in the receiver's memory at 5-s intervals during all skating trials (Polar Vantage XL heart rate monitors). Upon completion of the trials, HR data was transferred through a computer interface for analysis. The highest 1-min average HR data during each skating trial were considered as the steady-state HR.
Blood samples were taken via finger stick 1 min after the completion of each trial for blood lactate (LA) determination (YSI model 2300 STAT Plus Lactate analyzer).
Series II. The effect of drafting on performance was evaluated by comparing 3-lap sprint results immediately after completion of two separate 4-min trials at a constant pace (≈9.2 m·s-1) to a 3-lap sprint with no prior skating trial. During one trial, the athlete performed the pre-sprint 4-min piece alone. During the other trial, the athlete was drafted in a pack of four other skaters who created a vortex in an attempt to establish the best draft possible. The rested 3-lap sprint was used as a performance reference for the two sprints with prior skating trials. This design afforded the best opportunity to observe any performance improvements provided by drafting over leading or skating alone. The individual trials, the pack-drafting trials, and the rested sprint trials were randomized and performed over a 4-d period to reduce any possible effects due to fatigue. The pre-sprint 4-min trials were skated at a pace of approximately 12 s per lap(9.2 m·s-1). The higher pace than Series I was used to better simulate race conditions. Upon completion of the 4-min trial (immediately prior to the 3-lap sprint) and 1 min post-sprint, an arterialized blood sample was taken via finger stick for later lactate determination. The 4-min pre-sprint trial ended at the opposite side of the rink from the start of the 3-lap sprint and blood sampling was done at the 3-lap sprint start area. The 3-lap sprint began from a standing start. Timing was performed by two individuals experienced at timing, using handheld stop watches. The average of the two times was used as the sprint time.
Variables were compared between trials using ANOVA. Differences between performance trials were compared using paired t-tests. Comparisons between performance trials and rested sprint trials were made using repeated measures ANOVA. Tukey's post-hoc analysis was performed when significant F-ratios were found. A P value of <0.05 was considered significant for all comparisons.
Lap times and speed of the skaters during 19 laps on a 111-m course were similar during leading and drafting (12.57 ± 0.19 s·lap-1, and 8.8 ± 0.13 m·s-1). Ice conditions between trials were maintained by local resurfacing with water.
Heart rate response during the highest 1-min average of the 4-min trial was significantly lower during drafting than during leading (174 ± 2.1 vs 180 ± 2.1; Fig. 1A, P < 0.05). Percent of peak HR (determined during maximal cycle ergometry) was significantly lower during drafting than during leading (88.9 ± 3.9 vs 92.2 ± 4.4, P < 0.05). HR standard deviation for the individual 12 5-s values during the highest 1-min recording was 1.25 ± 0.63 beats during leading and 1.39 ± 0.50 beats during drafting. HR deltas between drafting and leading trials ranged from 0.8 to 12.4 bpm.
Figure 1-Heart rate ...Image Tools
Mean blood lactate concentrations 1-min post skating trial were significantly lower during drafting than during leading (5.56 ± 0.51 vs 7.75 ± 0.52; Fig. 1B, P < 0.05), and were 39.9 ± 13.2 and 56.7 ± 18.3% of peak lactate determined during maximal cycle ergometry. LA deltas between drafting and leading trials ranged from -0.18 to 5.37 mM.
Lap times and speed of the skaters during 19 laps on a 111-m course were similar (≈9.2 m·s-1) during skating alone (unaided) and drafting (12.08 ± 0.214 s·lap-1, and 12.07 ± 0.175 s·lap-1, respectively). Ice conditions between trials were maintained by local resurfacing with water.
The effects of drafting during a 4-min trial on 3-lap sprint performance are presented in Table 1. Heart rates at a pace of≈9.2 m·s-1 were significantly lower when the athlete drafted than when the athlete skated the 4-min trial unaided. Blood sampling time(recovery time) between the 4-min paced trial and the 3-lap sprint trial was not different between unaided and drafting pieces. Heart rates at the beginning of the 3-lap sprint were significantly different for all trials(P < 0.05). Although starting HR for the 3-lap time trial was significantly lower for the drafting trial than for the unaided trial, the percent drop during the recovery time was similar (≈10%). Peak HR at the completion of the sprint were not different between trials(Fig. 2A).
Blood lactate levels at the beginning of the 3-lap sprint were different for all trials (Table 1, P < 0.05). One-min post-sprint lactate levels were not different between the rested 3-lap sprint and the 3-lap sprint preceded by the 4-min drafting piece. Post-sprint lactate for the 3-lap sprint preceded by the 4-min unaided piece was significantly higher than the other two bouts (Fig. 2B, P< 0.05). However, no difference was found between lactate deltas between any trials.
Three-lap sprint time was not significantly affected when the sprint was preceded by an approximate 33 km·h-1 4-min skating piece where the skater was drafted by four other skaters. When the 3-lap sprint was preceded by 4 min of 33 km·h-1 skating unaided by drafting, sprint time was significantly slower than when the sprint was done in a rested state or when the skaters drafted during the 4-min piece before the sprint trial (P < 0.05).
It is well documented that pacing behind a leader during high-velocity sports such as cycling results in considerable savings in external energy expenditure. As found by others, drafting enables an athlete to maintain a given pace at lower oxygen uptake(2,4,5,10,15,17,19,24) and lower blood lactate levels (2) than when performing unshielded from air resistance. Our study confirmed and extended these findings by including post-drafting performance measures. Even though the benefit in aerobic energy expenditure appears to be lower than expected at the high velocities of this study, the observed large differences in blood lactate production could be critical to performance.
The short course may in fact compromise the benefits of drafting because of internal power losses to overcome high forces required to skate the tight corners. In this light, the attenuated physiological stress and enhanced performance observed because of drafting during short-track speed skating is a novel finding. Our results indicate that most short-track speed skaters reduced their energy requirements for skating at a constant pace if they drafted another skater. Drafting resulted in a mean 6 bpm reduction in heart rate for the 18 skaters in this study. This difference corresponded to an approximate 5-5.5% reduction in oxygen uptake as determined by regression analysis of data obtained during inline skating on a treadmill(21). This is less than expected, since our skaters were traveling at approximately 32 km·h-1 and the coefficient of friction for skating is similar to that of cycling (6), where drafting at a similar speed resulted in an approximate 31% reduction in power output from leading (14). Other studies using cyclists have also demonstrated much greater differences between drafting and leading at a similar speed. Caru et al. (4) observed a 17 bpm difference in HR when track cyclists drafted at 30 km·h-1. Likewise, McCole et al. (17) found an 18% reduction in oxygen uptake when cyclists drafted at 32 km·h-1. Unfortunately, neither researcher reported data as percent of peak HR or ˙VO2, so the relative intensities cannot be compared to our study. The mean heart rates in our study at approximately 32 km·h-1 were 92.2 ± 4.4 and 88.9 ± 3.9% of peak HR for leading and drafting, respectively. Bilodeau et al. (2) reported data from leading and drafting cross-country skiers as 83 ± 5 and 78 ± 6% of maximum HR, respectively. The 9 bpm difference he noted in heart rate between leading and drafting is in agreement with our results. However, comparison of Bilodeau's data with ours should be done with caution, since the skiers in his study were skiing at a much slower pace at a lower percent of HRmax than skaters in our study (5.6 m·s-1 vs 8.8 m·s-1). The smaller (yet significant) difference in HR we observed between drafting and leading could be due to the difficulty in drafting in the corners and/or the high force imposed by the tight-radius corners. Calculations using the “skating” radii of the corners in short (111 m) track and long (400 m) track (approximately 10 m and 28 m, respectively), and the 1000-m short-track American record time (1:28.6 min) and 1000-m time for long-track from the Calgary Olympics (1:14.5 min) provide an estimate of cornering forces for the two disciplines. A centripetal force of 866 N was calculated for the American Short Track 1000-m record by a 68-kg male, while the cornering force determined for a 75-kg male performing the Calgary 1000-m long-track event was 482 N. The energy requirement to overcome the high cornering forces of short-track could explain why drafting benefits observed in this study were not as great as those observed in cycling.
The diminished oxygen uptake and higher blood lactates observed for a given HR when in the skating posture (21,22), are critical issues that could compromise the benefits of drafting during speed skating. Rundell and Pripstein (22) found blood lactate values to be 53% higher during low walking than during cycle ergometry at 86% of ˙VO2peak. Recently, Rundell (21) found blood lactates to be about 3-fold higher at similar oxygen uptakes when comparing treadmill inline skating in the low skating position to skating in an upright position. These findings are supported by the high blood lactate levels recorded in our study (Fig. 1B, Table 1). The reduction in blood lactate values of 2.3 mM at 8.8 m·s-1 and 3.8 mM at 9.2 m·s-1 when drafting indicates an attenuated metabolic stress that can be attributed to lower aerodynamic drag.
Heart rate and blood lactate data by skaters in Series II, in combination with data obtained from these skaters during treadmill inline skating(21), can be used to estimate savings in external power output against aerodynamic drag (PD,equations 1-3) by drafting. Regression analysis provides ˙VO2 values of 47.8 and 45.4 for respective lead and drafting heart rates (r = 0.81). Since the skaters accelerated to speed(9.2 m·s-1) at a controlled, even pace, an assumption can be made that the change in blood lactate occurred at an even rate over the 4-min paced skate. Given this, an O2 equivalent for blood lactate production can be estimated at 5.7 and 2.6 ml·kg-1·min-1 for leading and drafting, respectively. These values can then be added to respective ˙VO2 values from the regression analysis and provide power output estimates of 245 and 220 W for leading and drafting. Calculated PD for these skaters at 9.2 m·s-1 is approximately 118 W, and is in accordance with estimated values by others(27,28). The 10% difference in estimated power output from heart rate and lactate data between leading and drafting resulted from a 21% decrease in power output against aerodynamic drag due to drafting.
External power can be estimated from air friction force and ice friction force (Fice = μN, with μ the ice friction coefficient and N the normal force) using the calculated cornering force during short-track and reported values for long-track skating (≈4 N on the straights and ≈6 N in corners (27)). Since approximately 50% of skating distance in short-track is cornering, total Fice = [(866N ÷ 482N·6N] + 4 N = 7.4 N. At 9.2 m·s-1, external power = 0.15 v3 + 7.4 v = 184.9 W. From these equations, one can see if air friction force is reduced by 21% from drafting, a decrease of 13% in total power can be realized. These calculations are consistent with our physiological data and in agreement with values of Van Ingen Schenau(27), who calculated decreases in drag of 16% and 23% when the shielded skater followed 2 m and 1 m behind a lead skater. The discrepancy between PD and energy expenditure in this study is also accordant with cycling data (14) where a 38% decrease in PD resulted in a 31% drop in power output at 8.9 m·s-1. McCole et al. (17) noted an 18% drop in ˙VO2 when shielding at 8.9 m·s-1. The smaller difference in this study could be the result of a higher energy expenditure in the tight-radius corners and/or a change in trunk angle when cornering. The latter would increase frontal area and effectively reduce the benefits of drafting.
The wide range of HR and LA deltas (0.8-12.4 and -0.18-5.37, respectively) imply that some individuals were more efficient at drafting than others. This observation is consistent with those of Bilodeau et al.(2) who found a 33-36% difference in drafting heart rates when paired skiers of different sizes drafted one another. Kyle(14) found that a reduction of spacing from 30 cm to 15 cm by drafting cyclists resulted in an additional 2% decrease in wind resistance. The reason for the variance we observed is not readily apparent, since the skaters were matched in size and drafted as close as physically possible. Although the larger skater would, of necessity, require a greater force output during cornering, and project a larger frontal area, no relationship was observed between HR or LA and body weight or size during any of the skating trials. The answer could perhaps lie in cornering technique, where the effective drafting space is not directly behind the skater, or the skaters were not technically matched according to stride length and stride rate. Video analysis of drafting pairs could shed light on this phenomenon.
The improved short-term sprint performance realized when skaters drafted during a prior 4-min skating bout demonstrates that drafting could be an important race strategy. Previous studies have demonstrated benefits when comparing paced times versus non-paced times. For example, Pugh(20) found about 1 s per 400 m improvement in run time when a runner is effectively drafting, and Kyle (14) pointed out the difference between the individual and team 4000 m pursuit times because of drafting. However, our study is the first to examine sprint performance immediately after an exercise bout where the athlete drafted a leader. The skaters in this study demonstrated significant improvement(P < 0.05) in 3-lap (333 m) sprint time after 4 min of drafting in a pack of four other skaters versus skating the 4-min pre-sprint trial individually. Although heart rates and delta lactates were not significantly different after the all-out sprint, heart rates and blood lactate concentrations were significantly lower after the 4-min trial when skaters drafted (Table 1). During the non-drafting trial, the skater began the sprint with significantly higher heart rate and blood lactate concentration than when they drafted. Although our skaters began their post-drafting sprint with blood lactate levels of 5.22 ± 1.18 mM, post-drafting sprint time was not different from rested sprint time where starting blood lactate levels were significantly lower (2.75 ± 0.96). Elevated blood lactate levels at the beginning of intense exercise may be related to fatigue (3,7,11-13) and are inversely related to work capacity (11). Fatigue attributed to concomitant increases in lactate and H+ may be the result of diminished glycolytic rate due to phosphofructokinase inhibition(26), interference with the calcium binding site of troponin (8), or an effect on the creatine kinase equilibrium, which reduces phosphocreatine concentration(18). Accordingly, acidosis resulting from increased lactate production has been correlated with a decrease in sprint performance(12). Ainsworth et al. (1) found a significant relationship between decreased power output during a repeated 45-s all-out cycling test and pre-existing blood lactate. Bogdanis et al.(3) suggested that elevated blood lactate levels by prior arm exercise resulted in a 10% drop in peak power output during a 30-s cycle ergometer sprint. Our results confirm and extend these observations to a race situation where the reduced metabolic stress due to drafting increases post-exercise sprint capacity. Albeit recovery time between pre-exercise trials and sprint trials was less than 40 s, phosphocreatine concentration may increase exponentially with a half time of 25 s (9). Sargeant and Dolan (23) found that recovery of maximal short-term power output followed a half time of approximately 32 s. This similarity to the kinetics of phosphocreatine resynthesis suggests that the 3-lap sprint performance in this study could have been affected by the recovery time between pre-exercise trials and sprint trials. Nonetheless, the attenuated lactate accumulation due to drafting remains a strong candidate for the faster sprint times observed in this study.
The obvious importance of our results is the application of drafting to short-track racing strategy to conserve energy for the final laps of the race where the pace often exceeds 40 km·h-1. The importance of maintaining a controlled even pace during short duration (2.5-3 min) exercise was emphasized by Foster et al. (7). In this study, speed skaters performed a 2-km time trial on a windload-simulated bicycle using five pacing strategies. The athletes were required to ride the first km at a pace ranging from very slow (≈55% of best 2-km time) to very fast (≈48% of best 2-km time). The results demonstrated that by initially attenuating metabolic stress by controlling pace, 2-km time could be improve by ≈7%. In pack-style racing this strategy is not always practical, since the pace is often dictated by the pack. However, our results indicate that the same strategy, beginning a race at a lower metabolic stress, can be achieved by drafting and may lead to improved performance during the late stages of a race.
In summary, since short-track speed skating involves pack-style racing, drafting could be an important race strategy. This study demonstrates that drafting during short-track speed skating results in attenuated heart rate and blood lactate responses, and these benefits can be realized in post-drafting sprint performance. This could provide an advantage in the final sprint to the finish during a race. The individual variability in the heart rate and lactate responses suggest that some skaters are inherently better drafters than others. Therefore, proper drafting technique should be emphasized in training to optimize the advantages afforded by drafting.
1. Ainsworth, B. E., R. C. Serfass, and A. S. Leon. Effects of recovery duration and blood lactate level on power output during cycling.Can. J. Appl. Phys.
2. Bilodeau, B., B. Roy, and M. R. Boulay. Effect of drafting on heart rate in cross-country skiing. Med. Sci. Sports Exerc.
3. Bogdanis, G. C., M. E. Nevill, and H. K. A. Lakomy. Effects of previous dynamic arm exercise on power output during repeated maximal sprint cycling. J. Sport Sci.
4. Caru, B., L. Mauri, M. Knippel, and F. Carnelli. Effects of air resistance on heart rate of track race cyclists. Int. J. Sports Cardiol.
5. Davies, C. T. M. Effects of wind assistance and resistance on the forward motion of a runner. J. Appl. Physiol.: Respirat. Environ. Exerc. Physiol.
6. Di Prampero, P. E., G. Cortili, P. Mognoni, and F. Saibene. Energy cost of speed skating and efficiency of work against air resistance. J. Appl. Physiol.
7. Foster, C., A. C. Snyder, N. N. Thompson, M. A. Green, M. Schrager, and M. Foley. Effect of pacing strategy on cycle time trial performance. Med. Sci. Sports Exerc.
8. Fuchs, F., Y. Reddy, and F. N. Briggs. The interaction of cations with the calcium-binding site of troponin. Biochim. Biophys. Acta
9. Harris, R. C., R. T. H. Edwards, E. Hultman, L. O. Nordesjo, B. Nylind, and K. Sahlin. The time course of phosphorylcreatine resynthesis during recovery of the quadriceps muscle in man.Pflügers Arch.
10. Hagberg, J. M. and S. D. McCole. The effect of drafting and aerodynamic equipment on energy expenditure during cycling. Cycling Sci.
11. Hermansen, L. and J. B. Osness. Blood and muscle pH after maximal exercise in man. J. Appl. Physiol.
12. Jones, N. L., J. R. Sutton, R. Taylor, and C. J. Toews. Effect of pH on cardiorespiratory and metabolic responses to exercise.J. Appl. Physiol.
13. Karlsson, J. and B. Saltin. Lactate accumulation in the working muscle. Acta Physiol. Scand.
14. Kyle, C. R. Reduction of wind resistance and power output of racing cyclists and runners traveling in groups.Ergonomics
15. Kyle, C. R. Mechanical factors affecting the speed of a cycle. In: Science of Cycling
, E. R. Burke (Ed.). Champaign, IL: Human Kinetics, 1986, pp. 123-136.
16. Kyle, C. R. and E. R. Burke. Improving the racing bicycle. Mech. Eng.
17. McCole, S. D., K. Claney, J-C. Conte, R. Anderson, and J. M. Hagberg. Energy expenditure during bicycling. J. Appl. Physiol.
18. Noda, L., S. A. Kuby, and H. A. Lardy. Adenosine triphosphatecreatine transphorylase. IV. Equilibrium studies. J. Biol. Chem.
19. Pugh, L. G. C. E. Oxygen intake in track and treadmill running with observations on the effect of air resistance. J. Physiol.
20. Pugh, L. G. C. E. The influence of wind resistance in running and walking and the mechanical efficiency of work against horizontal or vertical forces. J. Physiol.
21. Rundell, K. W. Compromised oxygen uptake in speed skaters during treadmill inline skating. Med. Sci. Sports Exerc.
22. Rundell, K. W. and L. P. Pripstein. Physiological responses of speed skaters during treadmill low walking and cycle ergometry.Int. J. Sports Med.
23. Sargeant, A. J. and P. Dolan. Effect of prior exercise on maximal short-term power output in humans. J. Appl. Physiol.
24. Spring, E., S. Savolainen, J. Erkkila, T. Hamalainen, and P. Pihkala. Drag area of a cross-country skier. Int. J. Sport Biomech.
25. Street, G. M. Biomechanics of cross-country skiing. In:Winter Sports Medicine
, M. J. Casey, C. Foster, and E. G. Hixson(Eds.). Philadelphia: F. A. Davis Co., 1990, pp. 284-301.
26. Trivedi, B. and W. H. Danforth. Effect of pH on the kinetics of frog muscle PFK. J. Biol. Chem.
27. Van Ingen Schenau, G. J. The influence of air friction in speed skating. J. Biomech.
28. Van Ingen Schenau, G. J., J. J. De Koning, and G. De Groot. A simulation of speed skating performances based on a power equation.Med. Sci. Sports Exerc.
SKATING; SHIELDING; LACTATE; HEART RATE; ENERGY COST