Summary
Purpose: 1) to determine a ratio between different types of hockey shots that are made in different professional ice hockey leagues (the KHL, the top division of Russian hockey; the VHL, the second division of Russian hockey; the MHL, the top Junior league of Russian hockey). 2) to investigate the level of correlation between shot speed, strength and speed-strength abilities of professional ice hockey players, and hockey sticks' stiffness.
Methodology: all the shots made by the players of the KHL team "Ak Bars" Kazan, the VHL team "Bars" Kazan and the MHL team "Irbis" Kazan during the first 40 games of the regular season 2018/19 were analysed using the Iceberg Sports Analytics Software.
Conclusion: 1) Most of the shots in the KHL, VHL and MHL leagues are performed on the move while both skates are in contact with the ice surface. 2) Strength of the upper hand holding the stick and broad jump performance are the best predictors of shot speed. 3) Different types of medicine ball throws don't predict shot speed. 4) Strong correlation was found between shot speed and hockey sticks' stiffness.

Abstract
The study determines the ratio of types of shots made in the professional hockey leagues KHL, VHL and MHL, reveals the degree of correlation between shots speed and strength, speed-strength abilities of professional hockey players, as well as flexibility of hockey sticks.

Introduction
Hockey is one of the most popular and dynamically developing sports. The rules are regularly modified [1], from year to year manufacturers, introducing new technologies into production, improve sports equipment [1]. As it is known, the prerequisite for the development of the highest possible results in elite sports is an extreme level of skills development combined with a high level of special physical preparadness, which, in turn, is based on the level of general physical fitness.

In support of this research, in various sports disciplines the relationship between strength, speed-strength abilities of athletes and their competition performance indicators. Thus, Forthomme et al. found a correlation between upper body strength and serve speed in volleyball [2]. Marques et al. revealed the relationship between the ball throw speed in handball and strength, as well as speed-strength indicators in barbell bench press [3]. Loturco et al. proved the existence of a correlation between the force of punches in boxing and strength, speed-strength abilities of elite boxers [4].

However, due to the complexity of such research in hockey, there is a gap of reliable information regarding parameters primarily affecting the puck's speed. The researchers mainly studied the design and properties of sticks and its effects on shots speed and accuracy. As a result, some researchers did not reveal a reliable relationship between sticks flexibility and shot speed [5, 6], while others, on the contrary, claim a strong correlation [7, 8]. However, a serious negative aspect of the above studies is the lack of data on strength and speed-strength abilities of the subjects, which increases the likelihood of insufficient reliability of the results.

There is no doubt that the highest puck speed is reached by using the slap shot [5, 6, 7, 8]. This is achieved due to the shock-absorbing properties of the stick. When performing the slap shot, the stick bends, and released energy transfers to the puck while the stick returns to its original state. The disadvantage of this technique is lower accuracy in comparison with wrist shot, which is achieved due to a longer contact time between the blade of the stick and the puck [9]. According to A. Hache, on average the blade contacts with the puck for 0.3-1.5 m before release [9].

From the point of view of the author of this article, the most reliable study on this topic was done by J. Bezak and V. Pridal [10]. The researchers revealed a strong correlation between wrist shot speed and power output in 50 kg barbell bench press. A moderate relationship was found between the last exercise and slap shot speed. In addition, bench press 1 repetition maximum has a moderate correlation with wrist shot speed and weak - with slap shot.

Despite these results, many questions arise:
1) the authors refer to the results of studies by Emmert [11] and Pan et al. [12] on muscle activity during various types of shots in hockey. According to these works, the wrist shot in hockey is performed because of the activity of the following muscles: Triceps Brachii and Latissimus Dorsi; slap shot involves the same muscles, as well as in addition to them m. Pectoralis Major, the anterior bundle of Deltoid muscle and m. Biceps Brachii. Studies of the EMG activity during bench press exercise indicate that this movement is performed mainly due to contraction of Pectoralis Major muscle, Triceps Brachii, as well as the anterior bundle of Deltoid muscle [13]. Comparing the results of these studies, it would be logical to assume that there should be a strong correlation between bench press performance and slap shot speed, but not the wrist shot. The results of the study by J. Bezak, V. Pridal [10] indicate the opposite. This means that there is a possibility that shot speed, in addition to the muscles described above, is achieved by involvement of other muscles. This fact, which the authors of the article agree with, suggests the need for further searches for the most informative methods of studying tests that affect speed of the puck the most;
2) in the aforenamed research the relationship of the shot speed exclusively with upper body strength was studied. This raises doubts about validity of the results as most of the shots in hockey are made in motion. During their execution a large number of muscles of the kinetic chain from the feet to the hands are involved [14]. On the other hand, authors made an attempt to minimize the influence of lower body muscles by organizing the testing in a way where shots were performed in a standing on the ground position. However, even in this position the abdominal and muscles of lumbar spine region can have a great influence on the speed of the puck (Rectus Abdominis m., Internal and External Oblique Abdominal m., Transverse Abdominal m., Quadratus Lumborum m.) [14]. Based on this, it seems logical to search for such testing exercises, which will be more similar to the shots in hockey from biomechanical and muscle activity standpoint.

I. An analysis of a ratio between different types of shots in KHL, VHL, MHL

Methodology
The first 40 games of the regular season 2018/19 were were filmed using three FLIR PointGrey GrassHopper3 U3 28S4C video cameras, each of which filmed one of the three zones of the hockey field throughout the game. All the shots made by the players of the KHL team "Ak Bars" Kazan, the VHL team "Bars" Kazan and the MHL team "Irbis" Kazan were analysed using the Iceberg Sports Analytics Software.

The results were interpreted in accordance with the classification of the shots in hockey accepted in the Russian literature by V.P. Savin [16] and Yu.V. Nikonov [17]:

• Wrist;
• Snap;
• Slap;
• Saucer;
• Backhand;
• Deflection.

Results
943 shots of "Ak Bars" (KHL), 874 shots of "Bars" (VHL) and 1045 shots of Irbis (MHL) were analyzed. In all three leagues shots in motion predominate. Thus, Ak Bars players made 722 shots in motion (77%) against 221 while standing (23%); Bars - 720 in motion (81%) and 154 standing (19 %); Irbis - 851 in motion (81%) and 194 standing (19%) (Figure 1). This is quite logical, given that hockey is one of the fastest sports. At the same time, the lower percentage of shots while standing still in the VHL and MHL compared to the KHL can be explained by a more simplified manner of play, which is a consequence of a lower skill level of the players. In general, the data indicates that during the training process it is necessary to practice the shot more often in motion, rather than standing. The same is true for testing: in addition to the standing position, a test in motion should be present.

Shots in motion
In the KHL, snap shots, wrist shots, as well as slap shots are more often performed on one foot (Figure 2). From biomechanics standpoint, this allows to use the shock-absorbing properties of the stick more efficiently, which in turn leads to more powerful shots. On the other side, this places higher demands on player's balance. Since the best hockey players play in the KHL, this may explain why the situation is different in the other leagues.

Interestingly, in the VHL and MHL shots with two legs contacting the ice prevail (Figure 2). This can be explained by the fact that these techniques require more contact time of the puck with the blade and, accordingly, place increased demands on balance. Of course, it is much easier to maintain balance on two legs, especially when shooting from a standing position. This indicates the need for greater emphasis in the training process on mastering the technique of shots from one leg, during which a more effective transfer of body weight and greater use of the shock-absorbing properties of the stick are carried out, which will contribute to an increase in the puck speed.

Figure 2. Types of shots in motion

Shots while standing still
As it was mentioned above, shots while standing still place very high demands on the ability to maintain balance, and therefore in all three leagues shots in such situations are performed mainly from two legs (Figure 3).

Figure 3. Types of shots standing still

II. The relationship of various types of shots speed with strength and speed-strength abilities of professional hockey players

Subjects
15 professional KHL players aged 20-32 years were tested in the time frame from october 4, 2018 till december 16, 2018.

Methodology
The first 40 games of the regular season 2018/19 were were filmed using three FLIR PointGrey GrassHopper3 U3 28S4C video cameras, each of which filmed one of the three zones of the hockey field throughout the game. Speed of shots was analysed using the Iceberg Sports Analytics Software. The following indicators were registered:

• Average puck speed (km/h) of all types of shots by each player;
• The hardest snap shot (km/h) by each player;
• The hardest slap shot (km/h) by each player.

In addition, sticks flexibility was analyzed as a number of researchers found statistically significant relationship between shot speed and sticks flexibility [22]. The flexibility is indicated by all manufacturers on each stick. The stiffness of the sticks used in official games by the hockey players who took part in the study was taken into account.

Testing procedures
The following tests were performed in order to assess strength and speed-strength abilities of the subjects:

• Dynamometer hand grip strength;
• Medicine ball throws from different positions;
• Broad jump.

Dynamometer hand grip strength
The test is measuring the absolute (maximum) strength of the flexor muscles of the wrist and fingers [19], which are active during any type of shots in hockey [14]. The following indicators were analyzed:

• Mean score of five attempts (kg) - upper hand;
• Mean score of five attempts (kg) - lower hand;
• Best score of five attempts (kg) - upper hand;
• Best score of five attempts (kg) - lower hand.

For the left-handed shooters the right arm is upper one, as it is held on the top of the stick. For the right-handed players vice versa.

4.5 kg medicine ball throw [19, 20]
The main advantage of this test, performed in the "standing" position, is that it allows to engage "lower limbs - trunk - upper limbs" chain, simulating the shot action.
The following variations were used in the research (Figure 4):

• 4.5 kg medicine ball chest throw from standing position (m);
• 4.5 kg medicine ball throw in favourable direction (m);
• 4.5 kg medicine ball throw in unfavourable direction (m). For the left-handed shooters a medicine ball throw from left to right is considered as in favourable direction. For the right-handed players vice versa.
• 4.5 kg medicine ball chest throw from sitting position (m).

Figure 4. Medicine ball throw test variations

Broad jump (Figure 5) [19]
The following parameters are registered:

• jump distance (cm);
• power output (W), which is calcuted from subject's mass and jump distance using a special formula [21].

Figure 5. Broad jump

Results
The discriptive statistics and results of correlation analysis are reflected in tables 1 and 2.

Tests	1	2	3	4	5	6	7
Mean ± Standard Deviation	58.4±5.8	61.9±6.4	62.8±6.5	65.9±6.2	79.5±10.5	112.8±9.0	109.0±16.7
Tests	8	9	10	11	12	13	14
Mean ± Standard Deviation	5.1±0.6	7.9±0.6	10.9±0.9	11.0±1.0	268.6±15.4	2710.9±215.7	96.6±8.5

The Chaddock scale was used for interpretation of the correlation analysis results (Table 3)

Discussion
The analysis of the correlation matrix allowed us to draw a number of conclusions and assumptions. Very strong correlation was found between the best and mean score in hand grip strength of the same hand. In addition, an moderate to strong correlation is observed when comparing the best and average values of grip strength between hands. It indicates of hand grip dynamometry reliability and confirms the possibility of its daily use as athletes' readiness assesment tool [19]. Strong correlation of hand grip was found with stick stiffness, which is in agreement with other researchers [7, 8]. Combined with strong correlation with broad jump power, it indicates that most hockey players select stick stiffness correctly, relative to their strength and speed-strength abilities, height and prefered shots technique. Strong correlation was revealed between average puck speed and best score of the upper arm in hand grip; moderate relationship with best score of the lower arm, as well as with mean score of both hands. At the same time, the correlation value of the hand grip mean score of the upper arm is slightly higher than that of the lower one. This suggests that the upper hand that holds the top part of the stick has the greatest influence on the puck speed. In addition, these results confirm the assumption of the author of the book on physical training of hockey players [23] regarding the importance of developing forearm strength.

Moderate correlation between average puck speed and broad jump power is found, which is quite logical, given the "explosive" nature of the shot, as well as the fact that the energy to the puck is transmitted along a kinematic chain from the legs through the body, arms and stick. Moderate relationship of broad jump distance and power is in agreement with Popov [21]. The results of this study do not confirm the assumption that there is a "transfer" of the training effect from various types of medicine ball throws to hockey shots.

Conclusions

1. Most of the shots in the KHL, VHL and MHL leagues are performed on the move while both skates are in contact with the ice surface.
2. Strength of the upper hand holding the stick and broad jump performance are the best predictors of shot speed.
3. Different variations of medicine ball throws don't predict shot speed.
4. Strong correlation was found between the puck shot speed and hockey sticks' stiffness.

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