Ice-hockey is a team sport that requires the players to demonstrate comprehensive physical fitness. To be able to compete on ice in a permanent physical contact with opponents, they must attain high levels of speed, strength, and endurance (8). Inherent to this sport are high-intensity intermittent efforts related to starts, accelerations, stops, changes in skating direction, body checking, or other maneuvers (26,31). The players must therefore be characterized by substantial anaerobic endurance (6) allowing them to play with the required intensity for the whole duration of the game and by aerobic endurance enabling fast regeneration during short stoppages (36). The detailed knowledge of their activity and its changes over a typical match would help coaches to improve collaboration between players during technical-tactical combinations and optimize techniques and training to sustain performance at the highest level until the final minutes of play (17). The difficulty in establishing precisely the metabolic demands of the sport lies in the fact that ice-hockey players perform different movement activities and that the intensity and duration of their efforts are strongly varied.
Many attempts have been made to quantify the demands of ice-hockey with the time-motion analysis (TMA) (4,15,19,21,26,29). The TMA is frequently applied to evaluate the metabolic demands of team games such as soccer (3), volleyball (22), basketball (37), or rugby (12). Allowing the players' movement activities to be classified according to their intensity (low, moderate, and high) and their duration to be precisely recorded, the method offers a wealth of information about the demands team game places on the physiological efficiency of the metabolic systems (39). The use of the method is criticized particularly with regard to sports disciplines where effort intensity and duration are very irregular, such as ice hockey, futsal, or rugby (11). Some tactical movements the ice-hockey players perform (body checking, starts and stops, rapid changes in skating direction, or shots on goal, etc.) are too short for their duration to be recorded, so their numbers are only added up or their total duration is given (4,29). This approach distorts the information about their duration and intensity, resulting in inaccurate estimates of the demands on the efficiency of the players' physiological mechanisms.
The above criticism has caused researchers to try to make the estimates of players' exercise intensity during a game more precise by evaluating their heart rate (HR) and post-period blood-lactate (BLa) concentrations (5,15,27–29,34). However, on the basis of the available literature regarding evaluation of the ice-hockey players' exercise intensity, one cannot conclude that these methods provide complete information on the physiological demands of athletes. The BLa concentrations that Green et al. (15) established in blood samples taken 3–8 minutes after the final shift of a period ranged from 1.2 to 8.9 mmol·L−1 and did not provide information on the changes in game intensity. Noonan (27) took blood samples from Division I ice-hockey players during a game each time they were leaving the rink to measure their BLa concentrations to determine whether the exercise duration and the BLa level were related to each other. The study did not bring an unequivocal answer to this question because of the small number of participants examined. Noonan (27) observed, however, that in a shorthand situation, the players' BLa concentrations were the highest (ranging from 12.4 to 13.7 mmol·L−1). It is worth noting, however, that players' BLa can be measured only during the breaks, and taking blood samples is practically impossible when a game is in progress. Monitoring ice-hockey players' HR to estimate exercise intensity is also affected by some limitations. Individual differences in fitness levels and variations in exercise economy among the ice-hockey players may make the researcher using the method draw incorrect conclusions about exercise intensity (1). Furthermore, generalizing intensity zones determined from HRmax, over all players may not be the most accurate approach to determine the different intensity zones (18).
Being aware of the above limitations affecting the evaluation of the metabolic profile of exercise in team games performed using TMA, BLa, and HR, researchers have put forward a method that analyzes participants HRs recorded during a standard incremental exercise test performed to volitional exhaustion together with oxygen uptake (V[Combining Dot Above]O2), minute ventilation (V[Combining Dot Above]E), and carbon dioxide production (V[Combining Dot Above]CO2). Changes in V[Combining Dot Above]O2 and V[Combining Dot Above]E are used to determine 2 physiological points in gas exchange: the aerobic threshold/ventilatory threshold (V[Combining Dot Above]T1) and the anaerobic threshold/respiratory compensation point (RCP) (14). The HR recorded in participants during an incremental exercise test at intensities below V[Combining Dot Above]T1, between V[Combining Dot Above]T1 and RCP, and above RCP allowed dividing exercise intensity into low, moderate, and high zones, respectively. This approach has been found useful in evaluating the metabolism of athletes competing in individual sports, such as cross country running (13), skiing (32), or road cycling (23). As far as the team sports are concerned, the V[Combining Dot Above]O2-to-V[Combining Dot Above]E ratio determined during an incremental test for the purpose of evaluating exercise intensity during a game has been adapted for the needs of rugby union by Sparks and Coetzee (33). To our knowledge, no studies using this method to assess ice-hockey intensity have been carried out so far. The purpose of this study was to present a method that allows to determine the intensity during games played by junior elite players based on their HRs and the values of V[Combining Dot Above]O2max. The results may help coaches and players determine actual demands during both game and training conditions. The information provides a starting point for the programming of training activities in line with the physiological demands of players in a game situation.
Experimental Approach to the Problem
An experiment was performed during which the ice-hockey players performed an incremental exercise test on a cycling ergometer platform used to assess maximal oxygen uptake (V[Combining Dot Above]O2max) in a period directly preceding their participation in the International Ice Hockey Federation (IIHF) World Junior Championships. The results of the test allowed to indicate physiological markers and HRs necessary to determine 3 intensity zones. By juxtaposing each player's HR recorded during games played at the Championships with intensity zones obtained from the incremental exercise test, the length of time that individual players worked in each zone was calculated.
The experiment involved 16 ice-hockey players (11 forwards and 5 defensemen), members of the Polish male national junior under 20 years team (U20). For a participant to be considered in the study, they had to perform an incremental exercise test 2 weeks before the World Championships and to play in at least 2 periods of the 4 analyzed games their team played at the Championships. Each participant was asked to abstain from using any substances for 48 hours before the test and from any physical activity which might affect their physical or physiological responses to the test. Ice-hockey players who failed to meet any of the requirements were not considered for the study. The accepted participants were healthy, injury-free, fully rested, and adequately hydrated (hydration status was monitored by bioelectrical impedance vector analysis [model BIA-101; Akern-RJL Systems, Florence, Italy]). Written informed consent was obtained from all participants and parents after a brief but detailed explanation about the aims, benefits, and risks involved with this investigation. The research project was approved by the Bioethics Commission at the Regional Medical Chamber in Krakow as consistent with the institutional ethical requirements for human experimentation in accordance with the Helsinki Declaration. The characteristics of the participants are presented in Table 1.
Standard Incremental V[Combining Dot Above]O2max Test to the Point of Exhaustion
A standard incremental V[Combining Dot Above]O2max test was conducted in the laboratory by means of open-circuit spirometry and computerized instrumentation. Each participant performed the test until voluntary exhaustion on a cycling ergometer platform Cyclus 2 (RBM elektronik-automation GmbH, Leipzig, Germany). Before the test, the players warmed up on the ergometer for 5 minutes of steady ride at the power level of 1 W·kg−1 after which exercise intensity was increased to 4 W·kg−1 for duration of 10 seconds. The first 3 minutes of the test were completed at an intensity of 1 W·kg−1 of body weight, and then intensity was increased every 3 minutes by 0.5 W·kg−1 of body weight. Exhaled air was continuously sampled by an K4 b2 (Cosmed, Rome, Italy), and the rate of oxygen uptake (V[Combining Dot Above]O2), carbon dioxide production (V[Combining Dot Above]CO2), minute ventilation (V[Combining Dot Above]E), and the respiratory exchange ratio (RER) were calculated every 5 seconds by an online computer system. The K4 b2 was calibrated in accordance with the manufacturer's specifications at the beginning of each test day. The test was stopped if the participant wished so or if the V[Combining Dot Above]O2max criteria were met (e.g., RER greater than 1.10 at test termination; oxygen uptake reaching a plateau or starting to fall even though the work rate kept increasing or the maximal age-specific HR was reached) (10,24). Throughout the test, HRs were recorded every 5 seconds by means of a Fix Polar Heart Rate Transmitter Belt (Polar electro Oy, Kempele, Finland).
Ventilatory Threshold and Respiratory Compensation Point
Two physiological gas exchange points were identified. The V[Combining Dot Above]T1 was determined using the criteria of an increase in V[Combining Dot Above]E/V[Combining Dot Above]O2 with no increase in V[Combining Dot Above]E/V[Combining Dot Above]CO2 and departure from the linearity of V[Combining Dot Above]E (7). The RCP was taken at a point corresponding to an increase in both V[Combining Dot Above]E/V[Combining Dot Above]O2 and V[Combining Dot Above]E/V[Combining Dot Above]CO2 (7). Two independent experienced researchers visually detected V[Combining Dot Above]T1 and RCP. The different gas exchange phases were used to determine HRs corresponding to 3 exercise intensities (7). HRs at exercise intensities below V[Combining Dot Above]T1 were classified as the low-intensity HRs; HRs at exercise intensities between V[Combining Dot Above]T1 and RCP were classified as moderate-intensity HRs; and HRs at exercise intensities above RCP were classified as the high-intensity HRs (23).
Recording Participants Heart Rate During Ice-Hockey Games
Individual players' HRs were recorded at 5-second intervals during 4 games played at the IIHF World Championship Junior U20 using the Polar Team system (Polar Electro Oy). In the locker room, before a warm-up for the game to commence, a member of the research team helped each player to attach a Polar Team transmitter to their chests, making sure that the electrodes correctly contacted the skin at the lower sternum level. The transmitters were returned as soon as the game was over, and the researcher transferred the recorded HRs through the Polar ProTrainer software (v. 5) to the personal computer. The total time of the game was defined as including all stoppages, but excluding the regular breaks between periods.
Determination of Time Spent in Each Intensity Zone (Low, Moderate, and High)
The HR values recorded during the games were divided according to the 2 physiological gas exchange points obtained from the incremental exercise test and their respective HR values (detailed description of the procedure is included in the Ventilatory Threshold and Respiratory Compensation Point section). The data were organized in the Microsoft Excel 2010 worksheet (Microsoft Corp., Redmond, WA, USA). Furthermore, a special Visual Basic macro was applied to categorize the HR values into the previously established intensity zones. Considering that each HR value corresponded to the period of 5 seconds duration, we could calculate the total time spent in each of the intensity zones.
Descriptive statistics were calculated for each variable, i.e., mean values, minimum values, maximum values, and SDs. The statistical significance of the differences between the lengths of time the players spent in particular intensity zone was determined by means of a one-factor analysis of variance. When significant differences in the F ratio were found, the post hoc Tukey's test was applied. For all statistical analyses, α was set at 0.05. Calculations were performed with the Statistica 10 statistical software package (StatSoft, Tulsa, OK, USA).
Table 2 shows the values of parameters recorded during the incremental test and indicators derived from them according to the player's position. The mean values of V[Combining Dot Above]O2max and HRmax in the group of forwards were 56.6 ± 3.9 ml·kg−1·min−1 and 190.5 ± 10.4 b·min−1, respectively, and in the group of defensemen 55.3 ± 3.8 ml·kg−1·min−1 and 183.6 ± 10.1 b·min−1, respectively. In the first group, the gas exchange threshold, V[Combining Dot Above]T1, was determined at an average HR of 152.4 ± 9.1 b·min−1, a value corresponding to 80.0% of HRmax; for the defensemen, the value was 140.2 ± 12.8 b·min−1, an equivalent of 76.3% of HRmax. The forwards' RCP was established at an average HR of 176.0 ± 8.3 b·min−1, i.e., 92.5% of HRmax, whereas defensemen reached their RCP at an average HR of 163.4 ± 12.0 b·min−1, i.e., 80.7% of HRmax. Based on HR values corresponding to V[Combining Dot Above]T1 and RCP, 3 intensity zones called low, moderate, and high were determined: 143–151 b·min−1, 152–176 b·min−1, and 177–190 b·min−1 for the forwards, and 127–139 b·min−1, 140–163 b·min−1, and 164–184 b·min−1 for the defensemen.
Analysis of the Game
The mean values of HRs recorded in the forwards and defensemen during all analyzed matches were 159.1 b·min−1 (81.7% HRmax) and 150.7 b·min−1 (79.0% HRmax), respectively, what corresponds to medium intensity determined during the incremental test. However, the mean values of HRmax values recorded during the games in both the forwards and the defensemen (194.7 ± 8.32 b·min−1 and 190.7 ± 6.16 b·min−1, respectively) were greater than those derived from the HRmax obtained in the incremental test (190.5 ± 10.4 b·min−1 and 183.6 ± 10.1 b·min−1, respectively). In ice-hockey, the regular time of play spans 60 minutes of effective competition. From all games included in the analysis, it results that the forwards played an average of 1:17:47 ± 16:46 (h:mm:ss) and the defensemen 1:17:54 ± 19:19 (h:mm:ss). The detailed results summing up the analysis of the games are presented in Table 3.
Figure 1 shows the average time the forwards and defensemen spent in particular intensity zones as percentages of periods and of the entire match. As can be seen, in all 3 periods, both formations spent much more time in the low-intensity zone. Another finding is that the amount of time the forwards spent in this zone declines in time, from 61.44% in period 1 to 59.14% in period 2 and 55.98% in period 3. The percentage of the time the forwards spent in the low-intensity zone was on average greater than the defensemen did; in periods 1 and 2, the differences, 61.44% vs. 44.21% (p ≤ 0.001) and 59.14% vs. 47.23% (p ≤ 0.01), respectively, were statistically significant. In each period, the forwards stayed shorter in the high-intensity zone than the defensemen did, the differences in periods 1, 2, and 3 being 18.43% vs. 31.05% (p ≤ 0.01), 19.71% vs. 29.16%, and 19.83% vs. 29.84%, respectively. An analysis of the length of time the players in both formations spent in the moderate-intensity zone shows a similar pattern. For the offensive players, the time increases in successive periods, from 20.13 to 21.15% and 24.19%. Regarding the defensemen, in periods 1 and 2, the parameter's values are similar (24.74 and 23.61%), but in period 3, an increase to 26.27% is noticeable. An analysis of all games with respect to the lengths of time the offensive and defensive players spent in each of the intensity zones did not find the games to be significantly different.
Table 4 shows the mean lengths of the time that the forwards and defensemen spent in each of the intensity zones in particular periods and during the whole match. As can be seen, over the whole length of the game, the defensemen spent much more time in the moderate-intensity zone (20:57 vs. 17:43 mm:ss) and the high-intensity zone (24:13 vs. 15:34 mm:ss) than the forwards. At the same time, the forwards stayed longer in the low-intensity zone (47:24 mm:ss) than the defensemen did (35:55 mm:ss). Figure 2 shows the number of shifts for both formations during a match. The number increases with each period in both cases. Because the defensemen were fewer (n = 5), they were replaced in each period more often than the forwards (n = 11). The mean numbers of forwards' and defensemen's shifts in particular periods were 6.3 vs. 7.6, 6.5 vs. 8.2, and 8.3 vs. 9.3, respectively. It is interesting to note that the number of shifts was the highest for both forwards (max=12) and defensemen (max=15) in period 3.
According to some authors, fitness of the elite ice-hockey players decreases significantly over the competitive season (8,9,15,30). The decline may cause changes in players' metabolic profiles, but most of all, it may affect their performance during a match (25). At the same time, it is commonly thought that the level of players' fitness can be maintained or improved only when training loads and methods are selected in such a way as to activate game-specific muscle groups and energy systems. Numerous studies point to a discrepancy between training intensity and the intensity of physical activity in a game situation (8,15,28,34). What follows from the observation is that a prerequisite to programming a training process is the availability of a quick, simple and most of all reliable analysis of the actual impacts of training and game demands on athletes. The relevant method assigns the metabolic exercise characteristics determined from the ratio between the values of respiratory variables (V[Combining Dot Above]O2, V[Combining Dot Above]E, and V[Combining Dot Above]CO2) recorded during an incremental test (33) to HR values.
The main finding arising from this study is that ice-hockey players' HRs recorded during a game and those obtained from an incremental test can be applied to estimate exercise intensity. It was also found that between the start and end of each period, both forwards and defensemen spent most time in the low-intensity zone. In the first and second period, the forwards spent significantly more time in that zone than defensemen did, but in the third period, the difference was not statistically significant. This observation should not be surprising as the analysis used HR values recorded between the start and end of each period; thus, both game stoppages and bench time were included. It is noteworthy that in periods 1 and 2, the mean shift was higher for the defensive players, but in period 3, only a slight difference in mean shifts was noted (8.2 for forwards and 8.5 for defensemen). The results of this study are consistent with those obtained by Thoden and Jette (38) who found that the junior offensive players spent less time on ice in each period than did the defensemen by an average of 80 seconds. Even bigger differences were established for forwards in the professional National Hockey League, where the mean difference was 100 seconds. Léger (20) noted a similar pattern. Having applied the TMA to a group of 80 juniors, Léger (20) found that the forwards spent on ice an average of 30 minutes, of which 18 minutes were active time and 12 minutes were stoppages. The defensemen spent on ice an average of 32 minutes, i.e., by 7.2% more than the forwards did; their active engagement in the game was 19.4 minutes while stoppages accounted for 13 minutes.
In the course of this study, the percentage time the forwards and defensemen spent in the medium- and high-intensity zones were also established. In each period, the defensemen spent around 30% of their playing time in the high-intensity zone (HR exceeding 89.5% of HRmax) and approximately 25% in the medium-intensity zone (HR between 76.4 and 89.0% of HRmax). The total time they spent in these 2 zones was longer than the forwards'; in the latter case, the respective values were around 20% (HR higher than 92.9% of HRmax) and ca. 21% (HR from 80.0 to 92.4% of HRmax). The difference between the time the forwards and defensemen spent in the high-intensity zone was statistically significant only in period 1 (p ≤ 0.01).
As already mentioned in the Introduction, most authors studying exercise intensity during a game used TMA combined with observation of athletes' HRs, which were subsequently related to the HRmax recorded during a game or an exercise test, e.g., an incremental test. The pioneering analysis presented in the article, which was undertaken to quantify intensities in ice hockey, determines exercise intensity zones using physiological markers (V[Combining Dot Above]T1 and RCP) obtained during an incremental test. Its results are consistent with those published by other authors investigating the same problem. The results published by Green et al. (15), Montgomery (26), and Paterson et al. (28) show that the mean HR of ice-hockey players actively engaged in the game ranges from 85 to 90% of the HRmax, frequently exceeding 95%. Spiering et al. (34) reported that the intensity of play during women's national hockey competitions evaluated against the recorded HR values amounted to 90% of the HRmax. The researchers concur that the intensity of work performed by defensemen and forwards on ice is equal in both formations. Based on the data collected by Green et al. (15), differences were noted in HRs between the defensemen and forwards. In another study by Green et al. (16), there was no discrepancy in the average on-ice HRs between two varsity forwards and defensemen. Paterson et al. (28) reported similar HR data difference between the forwards and defensemen, except for a higher recovery HR for the defensive position. Peddie (29) who studied varsity ice-hockey players during games also found the forwards' and defensemen's HR to be equal, amounting to an average of 82.5% of the HRmax.
From the physiological perspective, an incremental test with a cycling ergometer may not be the best way to determine HR intensity zones for the purpose of quantifying exercise intensities in an ice-hockey game (2). Requiring the players to perform frequent accelerations, stops, and changes in skating direction, ice-hockey is extremely diverse in terms of its nature, intensity, and the duration of players' movements. Compared with that, cycling on an ergometer is a continuous activity performed with steady velocity (35). This implies that the metabolism of the muscles activated in a game situation is different than that during a cycling ergometer test, which explains why the means of the HRmax recorded during these two physical activities are different.
The telemetric measurement of the HR used to evaluate the aerobic demands on ice-hockey players has certain shortcomings that must be accounted for when the results are being interpreted. In a game situation, the players' HR may be affected by many factors, also other than those related to increased oxygen cost, such as emotions, static work of the torso muscles, varying demands of the play, torso temperature elevated by the player's outfit that reduces the amount of heat transferred from the body (26).
A method that uses the aerobic and anaerobic metabolism parameters to determine exercise intensity zones can significantly improve the reliability of assessment of the physiological demands of the game, which makes it a very useful tool for coaches in managing a sports training process. It can replace the approach where training intensity zones are derived from the HRmax or from intensities that have been assigned a priori to training loads. The method presented in this article provides coaches with a tool for creating an inventory of training loads and player's activities during a game, consistent with their motor preparation. This universal approach has already been tested for ice-hockey, a sport discipline characterized by intermittent physical activity of varying intensity, where the intervals are not long enough to ensure full rest.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
thresholds; game analysis; oxygen uptake