A.V. Gursky, associate professor, Ph.D.
Smolensk State Academy of Physical Culture, Sport and Tourism, Smolensk

Keywords: technical skills of cross country skier, phase structure of skating strides, kinematics and dynamics of movement.

Introduction. The phase structure of simultaneous skating strides was developed on the basis of the unity of formulated principles and the application of complex instrumental methods of research of techniques. The method of biomechanics cinematography was used, based on the analysis of synchronous rapid video recordings of technique side-on and full-face during competitions (using two cameras at the same time). Film materials were processed and movements were analyzed using the semi-automatic film analyzer "Nac-Sportias" (Japan) with the printout of the digital data and their graphical construction in the form of motion paths. In addition, the dynamics of support reactions of the skier when moving using skating strides on special dynamographic skis with ski poles was registered synchronously with cinematography or video recording. This approach was used for objective, integrated study of the dynamics and kinematics of movements, periods, phases, and the structure of motor actions of cross country skiers.

The distinctive features of the technique of skating strides in comparison with the classical ski techniques detected in the study were its alternated and simultaneous nature. The interdisciplinary technology of formation and improvement of the motion pattern with regard to their specificity was developed.

The push-off from the gliding, stationary ski is a specific and fundamental distinctive feature of the mechanism of moving using skating strides. That is why, skating strides should be singled out into a special group of ski techniques with a slide stop.

Motor skills in skating strides are characterized by lack of specificity, unnaturalness of athlete's movements, which also largely distinguishes the classical ski techniques from the technique of skating strides.

There are significant differences in muscle work too (R.N. Dorokhov, V.V. Ermakov, 1985), which necessitated considerable redevelopment of the technology of technical training, search for new special-purpose means and development of speed-strength qualities, local development of muscles.

There are also individual differences in the technique of specific motor actions, which result from the main fundamental differences. In particular, we detected:

- diversity of pole planting and completion of the push-off with both poles (time, angle and repulsive force);

- occurrence of double-support gliding during the push-off with the leg;

- increase in the time of application of efforts when pushing off with the leg (two-threefold);

- two-component nature of the push-off with the leg (at the beginning - pressing, at the end - speed-strength);

- change of direction of the push-off with the leg and both poles (forward, sideward, backward).

Results and discussion. The analysis of the dynamic support reactions within the cycle of movements performed by a cross country skier when moving using skating strides reveals that, due to the fundamental differences in the mechanism and direction of the push-off with the leg using both poles (side stop, slide stop, backward, sideward), the nature of the increase of efforts, the dynamogram structure have their unique features. Unlike the classical ski techniques, during skating strides the efforts applied to push off with the leg increase gradually and begin with the heel switching to the toe.

It is characteristic that within the ski technique cycle the dynamics of support reactions of the first skating stroke differs from that of the second one. The amount of efforts, as a rule, is a little less during the first push-off, while distribution of pressure to the heel and toe is uniform. Application of force to the heel during the first step is shorter in duration than during the second step.

The analysis of dynamograms compared with the classical ski techniques testifies to significantly lower efforts applied when moving using skating strides ≈ 1.5 of an athlete's body mass, versus ≈ 2.0 of one in classic skiing, which suggests using the classical ski techniques for accomplishing the educational task of development of endurance, including strength endurance.

The diversity is due to the way of pole planting and pushing off with both poles, which coincide with the push-off with the leg. The skating stride dynamogram does not contain the curved line that fixates the ski stopping time before the push-off. The strength of the push-off with both poles (left, right) is non-uniform either. Pressure to the pole, opposite to the take-off foot, is, on the average, twice heavier.

V1 skate technique with hand repulsion in a step (Table 1).

There are two periods within this ski technique cycle. A skating stroke in the first period is shorter in duration (0.58 s), and the distance covered (2.5 m), slower in speed (4.5 m/s) than in the second period (0.75 s; 4.0 m; 5.3 m/s respectively). The moment of drawing the hips together is considered the beginning of the first phase - the push-off with a lunge and gliding. To a certain extent, the first period might be termed preparatory, as they perform the motor actions needed for the main, more powerful push-off with the leg with simultaneous hand repulsion. The task of the first phase is to enhance the speed of movement. It is accomplished thanks to the key moves - the push-off with the leg, swinging leg and arm extension.

Form the moment the hips are drawn together, the push-off with the leg starts owing to the shifting of the total body center of mass forward - sideward (downward trunk inclination angle is 55-60°) and knee and hip extension, ankle flexion. The push-off with the leg is performed first by means of the pressing movement, then - the speed-strength one, and lasts 0.3-0.35 s; the distance covered equals 1.5-2.2 m, the rate of speed is 5.6-6.0 m/s.

During the push-off, the swing-up leg is extended in the direction of travel and planted on the support. The lunge time (from the moment the push-off is started till planting of the foot on the support) is 0.14-0.18 s. That was the push-off with a lunge, which slips into the push-off with gliding. Such body position admonishes of the double-support gliding during the diagonal - classic stride, but happens with the push-off and is not considered a weak point. This position is marked by a great singularity, its duration is variative and ranges between 0.05-0.07 s.

Table 1. Structure of motion pattern in skating strokes, tasks and motor actions in the phases of V1 skate technique with hand repulsion in a step

Second phase - free gliding - starts from the moment of lifting of the take-off foot with the ski from the support. Task - to prepare for the push-off. The motor actions performed within this phase are gliding with extension of the support leg, pulling of the swing-up leg to the support one. The free gliding phase is rather variative and is shorter in duration by 2-2.5 times (0.15-0.25 s), compared with the first one. The distance covered ranges from 1 to 1.5 m. Due to the powerful final effort applied when pushing off with the leg at the end of the 1^st phase, as well as short duration of free gliding, its speed reduces only by the end of the 2^nd phase (5.5-5.7 m/s). The downward trunk inclination angle is 60-64°, which provides a steady position of a skier during the single-support gliding. During the period of free gliding a skier performs the following preparatory moves: pulls the swing-up leg to the support one, straightens the support leg, actively continues the swinging arm extension. That is when the first period of the cycle of movements ends, and the second one begins.

Third phase - the push-off with the leg with simultaneous hand repulsion and gliding. The boundary moment of the phase beginning is, like in the first phase, hip shifting. The task of this phase is to enhance the speed of movement to the maximum, which is achieved by means of performing of the push-off with the leg and both poles, swinging leg extension. The phase lasts 0.35-0.40 s and is distinguished by full activity due to performance of the key motor actions - simultaneous leg and hand repulsion. By the distance covered and the rate of speed it exceeds the previous two phases (2.5-2.7 m and up to 6 m/s). The pole planting angle (left, right) is slightly different (by 3-4°) and, on the average, equals 70-75°. Because of lengthening of the ski poles the push-off has become more vectored ahead and more effective. The duration of hand repulsion (right, left) is different: from 0.02 to 0.05 s and equals 0.3-0.4 s on the average.

Fourth phase - free gliding - starts from the moment of lifting of the take-off foot with the ski from the support and ends with drawing of the hips together. The main task of this phase - to prepare for the push-off. It may be accomplished by means of the qualitative performance of such movements as gliding on the support leg, pulling of the swing-up leg to the support one, swinging arm extension forward.

Although its name is identical to that of the 2^nd phase, there are certain differences in performance of particular motor actions. While in the 2^nd phase a skier completes the swinging arm extension forward and prepares for planting the pole on the support, in the 4^th phase he has already completed the push-off with both poles and started the arm extension forward. As a result, there are significant differences in the body position, which is 55° bent by the end of the fourth phase. The time of the 4^th  phase is on the average 0.23-0.25 s, distance covered - 1.45-1.60 m, speed - 6.0-6.29 m/s, the downward trunk inclination angle at the beginning of the phase is 46°, at the end -55°.

This ski technique is quite universal and is widely used at different sections of the track, but mainly on easy and mean steepness hills (up to 8°). The ski technique efficiency depends on the level of special physical fitness of a skier.

V2 skate technique with hand repulsion at each step (Table 2). This ski technique cycle also consists of two phases. Simultaneous leg and hand repulsion are the main motor actions that make it possible to accomplish the task set before the 1^st phase, and ensure the speed of movement during the whole period. On the average, it ranges from 5 to 6.5 m/s. It is characteristic that the time of hand repulsion in this technique is much shorter than in the skate technique with hand repulsion in a step, and on the average equals to 0.15-0.25 s and falls completely on the 1^st phase. Planting of the poles on the support happens simultaneously with the beginning of the push-off with the leg or slightly behind time (0.1-0.2 s), ends when the swing-up leg touches the support. It should be noted that all motor actions related to the arm action are performed very intensively (stroke planting of the poles on the support, active push-off and swinging extension). The time of the first phase amounts to 0.45-0.50 s, the distance covered - 2.9-3.3 m, the speed - 6.1-6.3 m/s. The downward trunk inclination angle at the beginning of the phase is 62-65°, at the end - reduces to 57-60°. Planting of the poles on the snow (left, right) differs by 2-4° and, on the average, is equal to 69-72°.

Table 2. Structure of motion pattern in skating strokes, tasks and motor actions in the phases of V2 skate technique with hand repulsion at each step

Second phase - free gliding with arm extension. The boundary moment is lifting of the take-off foot with the ski from the support. The task of this phase is to prepare for the push-off, and it is solved by means of performance of such motor actions as gliding with extension of the support leg, pulling of the swing-up leg to the support one, swinging arm extension with trunk stretching. The time of the second phase ranges between 0.35-0.38 s, distance covered - 1.0-1.2 m, speed - 2.3-2.6 m/s. The body is extended up to 58-62°.

Conclusions. In general, a skier passes 4.5-5.0 m per cycle (0.7-0.9 s) and the passive nature of the second phase significantly affects its spatial and temporal characteristics.

The simultaneous skating stride with repulsion at each step in the plain and when moving uphill 4-6° steep was applied. It is one of speed ski techniques. Sometimes the rate of movement of the skier reaches 100 or more steps per minute, which places high demands on speed and strength and functional training of such athletes.

References

1.  Gross, H.H. Correlation modeling of sliding step to determine the effectiveness and improve the skiing technique: abstract of Ph.D. thesis / H.H. Gross. - Moscow, 1967. - 23 P. (In Russian)
2.  Evstratov, V.D. Skating move? Not only ... / V.D. Evstratov, P.M. Virolainen, G.B. Chukardin - Moscow: Fizkul'tura i sport, 1988. - 128 P. (In Russian)
3. Melent’eva, N.N. Analysis of techniques and methodology of teaching skating strides: guide / N.N. Melent’eva, N.V. Rumyantsev. – Moscow: Sovetskiy sport, 2011. – 168 P. (In Russian)

Corresponding author: saksportclub@mail.ru