INTERNATIONAL JOURNAL OF SPORT BIOMECHANICS, 1987, 3, 69-79

Biomechanical Aspects of Push-off Techniques in Speed Skating

the Curves

Ruud W. de Boer, Gertjan J.C. Ettema, Hans van Gorkum, Gert de Groot, and Gerrit Jan van lngen Schenau

Characteristics of stroke mechanics of elite and trained speed skaters were measured during the skating of curves. Film and vidw analysis from the 5000-meter races at the Dutch National Championships yielded biomechanical variables that were correlated to performance. There are fundamental differ- ences in push-off mechanics between skating the straight parts and skating the curves. The left stroke shows a more powerful push-off in the curve, caused by a greater push off angle compared to the right leg. The high speed and power output of the better skaters is a result of a high amount of work per stroke, caused by a short and effective directed push-off. These results strongly support the previous finding that skaters of different performance levels can be distinguished by differences in amount of work per stroke and not by differences in stroke frequency.

Performance in speed skating is the result of an optimum in external power production and a minimum of power losses due to air and ice friction (Ingen Schenau, 1982). The external power delivered by the athlete is equal to the amount of work per stroke (A) times stroke frequency (0 (Ingen Schenau & Bakker, 1980). A previous study (Ingen Schenau, de Groot, & de Boer, 1985) showed that the amount of work per stroke (A) is a constant property, independent of speed, for a skater of a given performance level. In skating the straight parts of the ice rink, the skater can choose the stroke frequency to deliver a certain power output. Re- cently de Boer, Ettema, Gorkum, de Groot, and Ingen Schenau (1986b) found that in skating the curves the stroke frequency is dependent on speed, work per

The authors are with the Dept. of Functional Anatomy and Workinggroup of Exer- cise Physiology and Health, Interfaculty of Physical Education, Free University, Amster- dam, The Netherlands.

Direct all correspondence to Ruud W. de Boer, Workinggroup of Exercise Phys- iology and Health, Academisch Medisch Centrum, Meibergdreef 15, 1105 AZ Amster- dam, The Netherlands.

70 de BOER, EnEMA, van GORKUM, de GROOT, van INGEN SCHENAU

stroke, and radius of the curve. In contrast to the straight sections of the rink, the stroke frequency and thus the power production in the curves cannot be chos- en freely by the skater. It is therefore to be expected that differences in push-off mechanics among skaters of different performance levels will prove to be most pronounced in speed skating the curves.

The small amount of experimental data concerning speed skating techniques in the curves is striking. Although from both a biomechanical point of view and from the perspective of speed skating coaches the curve is an important topic, most studies focus only on the techniques used in the straight parts. The stroke mechanics of skating the straight parts is extensively described by Ingen Schenau et al. (1985) and by de Boer, Schermerhorn, Gademan, de Groot, and Ingen Schenau (1986a). For a proper speed skating technique, then, it is essential that during the stroke the push-off skate continues to glide forward. The push-off takes place while the point of application of the push-off force is continuously displaced forward. One of the most important aspects of speed skating is the direction of the push-off at right angles to the gliding direction of the skate (Figure 1). The sideward push-off results in an increase of kinetic energy of the center of gravity (CG) and a change of direction of CG with respect to the ice (Figure 1). The amount of useful external work per stroke is dependent on the component of the push-off force in the direction perpendicular to the gliding skate (Figure 2).

Figure 1 - Trajectory of the center of gravity (CG, broken line) and of the right skate (solid line) in speed skating. The ve- locity increment Av, of the CG with respect to the point of a p plication of the push-off force results from sideward push-off. vy represents CG velocity just before push-off, v represents v e locity just after push-off. The push-off results in an increase of kinetic energy and causes a change of direction a.

72 de BOER, ETTEMA, van GORKUM, de GROOT, van INGEN SCHENAU

Literature on stroke mechanics in speed skating the straight parts (Ingen Schenau & Bakker, 1980; Ingen Schenau, de Groot, & Hollander, 1983; Ingen Schenau et al., 1985; de Boer et al., 1986a) shows that among groups of elite skaters, performance is related to (a) a large amount of work per stroke, (b) large gliding time followed by a horizontally (and thus effective) directed push-off, and (c) high knee extension velocity (found for males only). Apart from discus- sions by Haase (1954), the only experimental data considering speed skating tech- niques in the curves are published by Lier (1975) and Ingen Schenau (1981), who measured push-off forces, by Kuhlow (1976), who measured velocity changes, and by Ingen Schenau et al. (1985), who measured stroke frequency in the curves.

Purpose

The purpose of this study was to investigate the push-off technique of the right and left leg in speed skating the curves with biomechanical parameters similar to those used in the study of speed skating technique at the straight parts (de Boer et al., 1986a). Also investigated were the biomechanical parameters that deter- mine the difference in speed (and thus in performance) among subjects speed skat- ing the curves.

Methods

Data were collected during the 5000-meter races at the National Dutch Champi- onships (NC) for men at Alkmaar, The Netherlands, in 1985. The athletes were divided into two groups based on their performance times on the 5000-meter. The elite group (n = 14,79.7 f 6.3 kg body weight and 1.84 f 0.04 m height) had times between 7'27" and 7'48". The trained group (n = 10, 76.6 f 6.7 kg body weight and 1.80 f 0.09 m height) made times between 7'58" and 8'10".

Measurements

During the 5000-meter race the outer curve was used for data collection. In this way, six laps could be measured for each athlete. The speed skaters were filmed in sagittal and frontal planes. The camera perpendicular to the sagittal plane (Bo- lex Paillard, 16 mm, 60 fps) was placed outside the track at the length axis of the speed skating oval (distance between camera and skater was about 18 m). The second camera (Teledyne DBM 55, 16 rnm, 51 fps) was positioned at right angles to the optical axis of the other camera; the distance between this camera and skater varied from 20 to 30 m. Internal timing lights and timing light genera- tors pulsing at a frequency of 2 Hz provided a basis for the exact film speeds. At least one push-off of the left leg and one successive push-off of the right leg per lap were filmed by both cameras.

Both films were analyzed frame by frame using a NAC motion analyzer, which was coupled to an Apple I1 microcomputer. From the film of the sagittal plane, knee angle (Ok) and trunk position with respect to the horizontal (el) of each right and left push-off were obtained (Ingen Schenau, 1982). From the film of the frontal plane, the push-off angle 4 (angle between the push-off leg and the vertical, de Boer et al., 1986a) of both legs was obtained (see Figure 2). Both films were synchronized the instant the push-off leg was lifted from the ice. The angles derived from film analysis were smoothed using a digital filter (low pass

SPEED SKATING THE CURVES 73

Butterworth, fourth order, cutoff frequency = 12.4 Hz; Winter, 1979). By means of angle (6, mean speed, and position of the skater in the curve, the angles in the sagittal plane were corrected for distortion. Angular velocity was obtained with a Lanczos 5-point differentiating filter (Lees, 1980). The time phases of a speed skating stroke were defined as follows according to de Boer et al. (1986a): B = beginning of the stroke, S = start of the push-off, and E = end of the push-off.

Video recordings of the subjects throughout the laps provided speed (v) and stroke frequency (f) measurements per lap (r) and during the curve (c). Differ- ence in speed between the end and the beginning of the curve (Av3 was obtained from the video registrations. Power necessary to overcome friction (Pk) was cal- culated using a model of air and ice friction losses (Ingen Schenau, 1982). This model predicts power as a function of trunk position, knee angle, body weight, body length, and speed of CG. The ice friction coefficient p was estimated at 0.006 as a mean value for the whole lap (Kobayashi, 1973). To diminish the er- ror in determining the angles (Ingen Schenau et al., 1985), values of the parame- ters were averaged per subject over at least four laps.

Statistics

Differences in stroke parameters between elite and trained male speed skaters were tested for significance using a student t test or unpaired comparison (p < 0.05; two-tailed). Pearson's correlation coefficients between external power and speed on the one hand and the technical parameters on the other hand were de- rived and tested for significance (null hypothesis : correlation coefficient r = 0, p < 0.05).

Results

Both the speed during the whole lap (v,) and the speed in the curve (vJ was higher for the elite group than for the trained group (Table 1). During the races the skaters

Table 1

Mean Values (M) and Standard Deviations (SD) of Selected Variables For the Trained and Elite Male Groups

Males (NC) Elite (n = 13) Trained (n = 9) M SD M SD

(m s-') 11.02 0.16 10.19 0.15 'r "fr ON kg-') 3.71 0.24 3.38 0.17 'r (J kg-') 2.60 0.17 2.45 0.19 fr (S-'1 1.43 0.05 1.39 0.06

(m s - I ) 10.76 0.16 10.15 0.27 "c *

AV, (m S-') - 0.58 0.17 -0.66 0.26 fc (S-'1 1.66 0.14 1.60 0.08

'Significant differences between the groups (p < 0.05).

74 de BOER, ETTEMA, van GORKUM, de GROOT, van lNGEN SCHENAU

experienced a strong opposing wind in the measured curve, which resulted in a decline of speed in the curve. The difference in speed between the end and the beginning of the curve (Av,) was negative for all the skaters (Table 1). It was not possible to detect a significant relationship between decline in speed (AvJ and performance variables.

The power Pf, (calculated by means of speed per lap) is higher (3.71 + 0.24 W kg-') for the elite group than for the trained group (3.38 + 0.17 W

kg-', p < 0.05). This difference (Table 1) is the result of a slightly higher work per stroke (A) and a slightly higher stroke frequency (f,) for the elite group com- pared to the trained group. The most important differences in stroke parameters of speed skating technique between the groups are shown in Table 2 (left stroke) and Table 3 (right stroke). Figure 3 shows the greater push-off angle for the elite group compared to the trained group during the whole stroke. The differences between the elite and trained group are most pronounced in the stroke of the left leg. The elite group showed a higher push-off angle (4 B) and a shorter stroke time (t B-E) compared to the trained group (Table 2). For the left leg, the push- off angle (4B and 4E), the knee extension range in the gliding phase (4k B-S), and the push-off and stroke times (t S-E and t B-E) were significantly correlated to performance (see Table 2).

For the right leg the picture was less pronounced. Stroke time (t B-E) and push-off angle (q5 S) were correlated to performance variables (see Table 3). Note

Table 2

Mean Values (M) and Standard Deviations (SD) of Variables Derived From Film Analysis of a Left Stroke for the Male Elite

and the Trained Groups

Males (NC) Elite (n = 13) Trained (n = 9) Lefi stroke M SD M SO

$1 5i (degr.1 16.0 4.8 17.5 5.1 Ok (degr.) 105.0 3.4 107.3 7.0 8, S (degr.1 117.3 5.5 115.5 6.6 8, B-S (degr.) 11.9 5.6 9.4 6.9 + !k (degr.) 152.5 6.0 154.2 4.4 8, max (degr s-*) 485.5 60.8 464.7 57.3 dJ B (degr.1 12.5 3.1 9.6 2.4 " + dJ s (degr.1 26.0 5.3 25.6 3.9 6 E (degr.1 41.2 1.4 40.3 3.1 + t B-S (s) 0.37 0.05 0.39 0.06 - t S-E (S) 0.17 0.04 0.19 0.05 - t B-E (S) 0.54 0.05 0.60 0.05 * -

8, is trunk angle, 8, is knee angle, ik is knee angular velocity, dJ is push-off angle. B, S, and E = beginning of stroke and start and end of push-off phase, respectively. t B-S: gliding phase, t S-E: push-off phase, t B-E: stroke phase. * = significant differences between groups, p < 0.05. Significant correlation coefficients, p < 0.05, with performance parameters (P and v) indi- cated with the sign of the correlation coefficient.

SPEED SKATING THE CURVES

Table 3

Mean Values (M) and Standard Deviations (SD) of Variables Derived From Film Analysis of a Right Stroke*

Males (NC) Right stroke

Elite (n = 14) Trained (n = 9) M SD M SD

0, S 0, Ok s 0, B-S

ek 0, max cp B cp s cp E t B-S t S-E t B-E

(degr.1 (degr.) (degr.1 (degr.1 (degr.1 (degr s-') (degr.1 (degr.1 (degr.1 (s) (s) (s)

Significant correlation coefficients, p < 0.05, with performance parameters indicated with the sign of the correlation coefficient. *See note under Table 2.

Figure 3 - Mean curves of elite and trained male speed skaters of knee angle (Bk), knee angular velocity (ak), and push-off angle (4) against time. The left part shows a left stroke in the curve, the right part shows a right stroke in the curve. Vertical bars indicate the beginning (J5) of a stroke, and the start (S) and end (E) of the push- off. Note: T i e B-S: gliding phase. Time S-E: push-off phase.

76 de BOER, ETTEMA, van GORKUM, de GROOT, van lNGEN SCHENAU

the sign of the correlation coeff~cients: A better performance correlates with a shorter gliding phase, a shorter push-off phase, and a shorter stroke time. For both the left and the right leg stroke, no differences in trunk position (el B) and knee angle (83 between the elite and trained speed skaters were found (Tables 2 and 3).

Discussion

The decline in speed found in the curve (Table 1) can be explained in mechanical terms only if the power produced by the athletes is less than the power lost due to air and ice friction. A recently developed geometrical model of speed skating the curves (de Boer et al., 1986b) calculates the power delivered by the skater from work per stroke, and speed and radius of the curve. In a study of elite fe- male skaters, the increase of kinetic energy of CG in the curve, as a result of the increase in velocity, could be explained by the difference in power produc- tion and power losses with an error of only 3%. The influence of wind on air and ice friction is known (Ingen Schenau, 1982). From these relationships it could be calculated afterwards that in the present study the mean opposing wind velocity in the curve was 3 m s-'. It is possible that some differences in speed skating technique between the groups in this study (with a relatively small difference in performance level) are obscured by variations in opposing wind.

The measured differences in speed and power between the groups are due more to differences in work per stroke (trained group: 94% of the value of the elite group) than to differences in stroke frequency (trained group: 97% of the value of the elite group). This is in agreement with the studies of Ingen Schenau et al. (1985) and de Boer et al. (1986a). An optimal push-off technique resulting in a high amount of work per stroke is the first prerequisite for a good speed skating performance.

The push-off skate is lifted from the ice at a knee angle of about 154" (see Tables 2 and 3). This phenomenon occurs in a comparable way at the straight parts (for extensive discussion see Ingen Schenau et al., 1985, & de Boer et al., 1986a) and results from an anatomical and mechanical constraint related to the absence of plantar flexion in speed skating.

The increase of the knee angle during the gliding phase (ek B-S in Tables 2 and 3) was not found in elite and trained skaters at the straight parts (de Boer et d . , 1986a). This can be explained by the relation of ek and 4 and the position of the CG in the frontal plane (Figure 2). It is not necessary to move the CG from the lateral to the medial side of the skate, as it is in the straight part. The speed skater "hangs" in the curve during the whole stroke (4 B is positive, while at the straight part 4 B is about - 15"). The drop of the CG caused by the in- creasing push-off angle + can only be compensated by an increase of the knee angle ek. Unlike in the straight part, in the curve the first part of the gliding phase is bypassed. This also explains why the fJf, ratio (1.27 f 0.10 s-I and 1.21 f 0.09 s-I for the elite and trained group, respectively) is greater than 1.

The description of stroke parameters in the curve and the comparison be- tween the groups is found in Tables 2 and 3. It should be noted that there are no differences in trunk position S) and knee angle (ek S) at the start of the push-off. Although these aspects are important for a good skating technique (e.g., for a low air friction), these parameters are not sensitive enough to explain differ- ences between skaters with relatively small differences in performance level. The

SPEED SKATING THE CURVES 77

difference in knee extension velocity is not significant. The most pronounced differences between speed skaters at the straight parts are a longer gliding time and a greater push-off angle at the onset of push-off by the better skaters. De Boer et al. (1986a) concluded that the greater push-off angle is a direct conse- quence of the longer gliding time and might be related to the greater muscle mass of the elite skaters. In this study, for the left leg performance is not related to (b S. The gliding time of the elite group is even shorter. This is a direct result of the already discussed shorter gliding phase in the curve compared to the straight part. It is not necessary to glide for a long time to reach an optimal push-off an- gle 4 S. A larger gliding phase was found in the right leg than in the left leg. This parallels the situation at the straight parts and might therefore explain the positive correlation between 4 S and Pf, Fable 3).

In summary, the most pronounced differences in speed skating technique between groups are the shorter stroke and push-off times and the greater push- off angle during the whole stroke of the elite group compared with the trained group. This results in a better directed push-off (component F, in Figure 2).

Differences Between Right and Left Leg

The comparison between technical parameters of a right and a left stroke in speed skating the curve is shown in Figure 4. The left stroke is shorter (t E-B) com-

Figure 4 - Differences between a left stroke and a right stroke in speed skating the curves. Shown are knee angle (Bk), knee angular velocity (dk) and push-off angle (4) against time of male speed skaters. See also Note under Figure 3.

78 de BOER, E?TEMA, van G O R K U M , de G R O O T , van INGEN SCHENAU

pared to the right stroke, due to a shorter gliding phase. The push-off angle (4) for the left leg is larger during the whole stroke. The knee angle (Ok) during the left stroke is smaller than during the right stroke; the difference is greatest at the beginning of the stroke and grows smaller toward the end of the stroke. A higher maximal knee angular velocity during the left stroke was also observed (p < .05).

Most of these differences are related to the "leg over" technique used by speed skaters in the curve. During the left push-off, the right leg passes over the left leg and the right skate has to be placed more vertically and forward on the ice compared to the left skate. This explains the greater Ok and smaller 4 at the beginning of the right stroke. Compared to a right stroke, the shorter stroke time of the left leg is a consequence of the greater push-off angle 4 at time B; a larger part of the gliding phase is bypassed.

The higher knee extension velocity Ok max and greater + suggest a more powerful push-off of the left leg. Lier (1975) measured push-off forces and found a higher peak force value for the right leg. It is quite possible, however, that the effective component of the push-off force (F sin +, see Figure 2) is smaller for the right leg. Combined push-off force measurements and film analysis are necesary to provide more insight into this problem.

Conclusions

We conclude that the high speed and power output of the elite group is a result of higher work per stroke, caused by a short and effective directed push-off. The left stroke shows a more powerful push-off in the curve, caused by the greater push-off angle compared to the right leg.

References

de Boer, R.W., Schemerhorn, P., Gademan, J., de Groot, G., & Ingen Schenau, G.J. van (1986a). Characteristic stroke mechanics between elite and trained male speed skaters. International Journal of Sport Biomechanics, 2: 175-185.

de Boer, R.W., Ettema, G.J.C., Gorkum, H. van, de Groot, G., & Ingen Schenau, G.J. van (1986b). A geometrical model of speed skating the curves. Manuscript accepted for publication in Journal of Biomechanics.

Haase, H. (1954). Zur Entwicklung des Eisschnellaufes-Unsere Laufer lernen von sow- jetischen Freunden, Teil 11. Theorie und Praxis der Korperkultur, heft 1.

Ingen Schenau, G.J. van (1981). A power balance applied to speed skating. (Doctoral dissertation, Free University, Amsterdam, 1982). Free University, Amsterdam: Rodopi.

Ingen Schenau, G.J. van (1982). The influence of air friction in speed skating. Jouml of Biomechanics, 15:449458.

Ingen Schenau, G. J. van, & Bakker , K. (1980). A biomechanical model of speed skating. Journal of Human Movement Studies, 6: 1 - 18.

Ingen Schenau, G.J. van, de Groot, G., & de Boer, R.W. (1985). The control of speed in elite female speed skaters. Journal of Biomechanics, 18:91-96.

Ingen Schenau, G.J. van, de Groot, G., & Hollander, A.P. (1983). Some technical, physiological and anthropometrical aspects of speed skating. European Journal of Applied Physiology, 50:343-354.

SPEED SKATING THE CURVES 79

Kobayashi, T. (1973). Studies of the properties of the ice in speed skating rinks. Ashrae Journal, 15:51-56.

Kuhlow, A. (1976). Running economy in long-distance speed skating. In P.V. Komi (Ed.), Biomechanics V-B, International Series on Biomechanics, Vol. 1B (pp. 291 -298). Baltimore: University Park Press.

Lees, A. (1980). An optimized film analysis method based on finite difference techniques. Journal of Human Movement Studies, 6:165-180.

Lier, A. (1975). Fraskyvsstrukturen i hurtiglop pa skoyter. Norges idretshogskole, NIH- NORA, Oslo, 37.

Winter, D.A. (1979). Biomechanics of human movement. New York: Wiley.