Introduction
Specific on-ice testing of aerobic fitness in hockey players first occurred nearly 40 years ago (7). However, most of the aerobic testing in hockey players since has employed the use of a cycle ergometer or a running treadmill (5,11,12). The first study involving on-ice testing required a heavy (i.e., 10 kg) gas collection apparatus to be worn by the subjects (7). Today's wireless metabolic systems allow subjects to wear a much lighter (i.e., <1 kg) measurement module. In addition, breath-by-breath expired air analysis systems allow for the determination of a ventilatory threshold (VT). A wireless metabolic system provides a useful tool for sport-specific testing, such as on-ice skating. On-ice testing is considered vital to providing an accurate physiological description of hockey players (18).
Montgomery (18) reported that nearly one-third of energy demands for hockey are met by aerobic metabolism, and this finding suggests that o2max, VT, and lactate threshold (LT) are important to monitor. A high o2max in hockey players has been reported to prevent fatigue (5,12,18). o2max has also been correlated to an increase in net scoring chances in Division I hockey players (12). Presumably, an increase in VT and LT would also be important to hockey players, but this hypothesis has not been thoroughly investigated. Furthermore, it is unknown if VT and LT correlate in hockey players. Several studies have shown that VT and LT do not occur at the same intensity for triathletes (1), cyclists (4), untrained subjects (8), or patients with McArdle syndrome (13,14). Ventilatory threshold has also been reported to occur at a lower intensity than LT during graded exercise (19). In contrast, some researchers have found no difference between VT and LT for cross-country skiers, aerobically active adults, sedentary adults, cyclists, and runners (9,16).
Testing o2max, VT, and LT in hockey players up to this point has been limited to men. Studies of women have primarily focused on physical characteristics, anaerobic fitness, and agility. Bracko (2) first compared elite and nonelite women during on-ice anaerobic power tests to show that elite women had greater on-ice anaerobic power. Bracko and George (3) then determined that the best off-ice test for predicting skating speed in female hockey players is the 40-yard dash. Finally, Geither et al. (10) tested elite female hockey players for differences based on the position that they play and showed that forwards had higher anaerobic power than either defensemen or goalies. These studies (2,3,10) did not employ expired air analysis or seek to determine how women may differ from men.
To the authors' knowledge, probing for aerobic gender differences has not occurred in hockey players. Women typically have o2max values 15% to 30% lower than those of men (17), but this finding has not been substantiated in hockey players. Therefore, the primary purpose of this study was to determine if gender differences exist for o2max, VT, and LT in college hockey players. A secondary goal was to determine if VT and LT occur at a different percentage of o2max or HRmax for each gender.
Methods
Experimental Approach to the Problem
A VmaxST wireless metabolic analyzer (SensorMedics, Yorba Linda, CA) was used to measure expired air. The metabolic analyzer was calibrated before each trial. Expired air analysis data were averaged for 10-second intervals. o2max values were obtained from the metabolic analyzer's highest measured o2max value relative to body mass (i.e., mL·kg−1·min−1). Similarly, VT was determined by the metabolic analyzer by using the dual-criteria method (i.e., an increase in VE/o2 without an increase in VE/co2) (21). A Polar heart rate strap (Polar Electro Oy, Kempele, Finland) was used in conjunction with the metabolic analyzer to obtain heart rate data. Metabolic and heart rate data were recorded to a laptop computer within the penalty box via the VmaxST telemetry system.
Blood lactate samples were obtained immediately following each stage of the graded exercise protocol. The skater came immediately to a table that was near the start and finish line to have a finger cleaned with an alcohol wipe, dried with sterile gauze, and pricked for a small blood sample. The blood sample was obtained approximately 20 seconds after completion of the previous stage. A LactatePro analyzer (Arkray, Inc., Kyoto, Japan) was used to analyze blood samples. The analyzer was calibrated prior to each testing session. The lactate analyzer was placed on the lactate collection table on the ice rink at least 30 minutes prior to the initial calibration, as recommended by the manufacturer.
Lactate threshold was determined by plotting lactate values for each stage and selecting the threshold as the stage previous to a >1.0 mmol·L−1 increase. Successful blood lactate samples were obtained at each stage for all 10 men and 7 of 10 women. Interestingly, the stage selected as the LT also corresponded to the stage before that of ≥4 mmol·L−1 blood lactate, or the onset of blood lactate accumulation (21), in 5 of 7 women and 9 of 10 men. The 3 subjects who did not meet the criterion of ≥4 mmol·L−1 had blood lactate concentrations between 3.6 and 3.7 mmol·L−1 in the stage following the one marked as the LT. Heart rate and o2 values corresponding to LT are averages for the last 20 seconds of that stage.
Subjects
Ten male (age, 20.8 ± 0.4 years; height, 179.1 ± 2.8 cm; mass, 83.7 ± 2.9 kg) and 10 female (age, 19.1 ± 0.4 years; height, 167.0 ± 1.5 cm; mass, 69.0 ± 1.5 kg) Division III college hockey players performed on-ice aerobic testing. Each player had completed between 4 and 8 games of the 2006-2007 hockey season prior to testing. Both the men's and the women's teams compete in 25 games per year, excluding postseason play. In addition, both the men's and the women's teams had similar recommendations for strength and conditioning routines. Off-season training recommendations were to strength train 2 or 3 days per week and aerobically train for 2 to 4 days per week, but this training was not required. In-season training included 2 days per week of strength training and 2 days per week of aerobic training in addition to normal practices and games. All subjects provided written informed consent prior to participation. Approval for testing was granted through The Human Research Committee of Michigan Technological University.
Procedures
The on-ice skating protocol used in this study was developed based on a general description of on-ice testing from a National Hockey League team. It was concluded during a preliminary test that the on-ice protocol should meet 4 requirements: Each stage is of equal duration. Velocity per stage increases linearly. Each stage requires the same number of starts and stops. Each stage starts and finishes in the same location to accommodate for blood lactate testing.
Each stage of the protocol involved 80 seconds of skating and 40 seconds of rest between each stage for lactate testing. Skaters started at the goal line on a “3, 2, 1, beep” played from a specially made compact disc (CD). This CD played background music with beeps every 10 seconds until reaching the end of the stage at 80 seconds. The 10-second beeps allowed the subjects to pace their skating speed. At the end of each stage was a 40-second silent period on the CD that corresponded to the time when blood lactate samples were obtained. Just before the beginning of the next stage, the “3, 2, 1, beep” would play so that the skater knew when to start the next stage.
Each stage involved skating out to a cone that was to be reached in 10 seconds as the beep sounded. The skater would stop at the cone and immediately skate back to the start line as the next 10-second beep sounded. The skaters came to a complete stop before changing direction. They would repeat this process for a total of 4 roundtrips to the cone and back to the start line, for a total of 80 seconds. For women, the first stage involved skating out to a cone that was 23 m from the start line, and for men, the first cone was 26 m from the start line. Each stage progressed to a cone that was 3 m further than that of the previous stage. Thus, women started skating at 2.3 m·s−1 and men at 2.6 m·s−1, with each successive stage increasing an additional 0.3 m·s−1. The initial stages were very easy to complete but provided skaters with an opportunity to accommodate to skating in sync with the beeps. This protocol is outlined in Figure 1. Participants continued skating until they voluntarily stopped due to fatigue or could no longer keep pace with the beeps on the CD.
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Figure 1: Each stage of the on-ice protocol required the subject to start at the goal line near the data collection table and to skate out to the nearest cone and back to the start line for a total of 4 roundtrips. The skater was required to stop each time before changing direction. The skater started on a beep played from a special compact disc that counted down “3, 2, 1” just before the starting beep. The skater was to arrive at the cone as the next beep sounded. A beep sounded every 10 seconds during an 80-second recording that included background music. Thus, each stage consisted of 80 seconds of skating and 40 seconds of rest between each stage for blood lactate sampling. The cone for the first stage was 23 m from the start for women and 26 m from the start for men. When the skater completed a stage, a cone would then be removed so that he or she would have to skate 3 m farther for each subsequent stage.
Testing was performed at the John MacInnes Student Ice Arena on the campus of Michigan Technological University. Permission was obtained prior to testing to measure and mark the location for each of the cones to be used during testing with a small dot of paint. All cones were put in place before a subject began skating. When the skater completed a stage, a research assistant would remove the cone corresponding to the completed stage. This would allow the skater to skate out and back to the nearest cone during the subsequent stage. The number of stages possible before reaching the far end of the ice rink was 12 for women and 11 for men. One additional stage was added by moving the start line back 3 m from the original start line. This stage was not necessary for any of the women but was necessary for 1 man.
Statistical Analyses
Data were analyzed by using SAS, version 9.1, statistical software (SAS Institute, Cary, NC). Significant differences were determined when p < 0.05. Analyses of o2max, HRmax, VT, and LT between genders were performed by using a 1-factor analysis of variance (ANOVA). Analyses of VT and LT within genders were performed using by paired t-tests. All results are presented as mean ± SE.
Results
The male hockey players had higher o2max values (52.7 ± 1.3 mL·kg−1·min−1) than the female hockey players (40.1 ± 1.0 mL·kg−1·min−1) (p < 0.01). In contrast, the men and women had similar HRmax values: 191.3 ± 2.5 b·min−1 and 185.8 ± 2.5 b·min−1, respectively. The women's VT occurred at a higher percentage of HRmax (77.3% ± 1.6%) than that of the men (72.6% ± 2.0%) (p < 0.02). This difference was also observed when VT was measured as a percentage of o2max, with the women at 67.3% ± 4.0% and the men at 52.7% ± 3.2% (p < 0.02). In contrast, LTs were similar between genders when expressed as a percentage of HRmax or o2max. Expressions of VT and LT as a percentage of HRmax and o2max are demonstrated in Figure 2.
Figure 2: The
ventilatory threshold (VT) for women occurred at a higher percentage of maximal heart rate (HRmax) and
o
2max than for men. *Significant difference between men and women (
p < 0.02).
Lactate threshold (LT) was similar for both genders. For each gender, LT occurred at a higher percentage of HRmax or
o
2max than VT (
p < 0.01).
For both genders, LT was higher than VT when expressed either as a percentage of HRmax or as a percentage of o2max. The men's LT occurred at 91.8% ± 1.0% of HRmax, and their VT occurred at 72.6% ± 2.0% of HRmax (p < 0.01). Similarly, the men's LT occurred at 83.1% ± 1.6% of o2max, and their VT occurred at 52.7% ± 3.2% of o2max (p < 0.01). The women demonstrated the same trend as the men, with their LT occurring at 90.8% ± 1.5% of HRmax and their VT at 77.3% ± 1.6% of HRmax (p < 0.01). Finally, the women's LT occurred at 85.2% ± 1.7% of o2max, and their VT occurred at 67.3% ± 4.0% of o2max (p < 0.01).
Discussion
The current investigation had 3 major findings. First, o2max is higher in male Division III hockey players than in female Division III hockey players. Second, VT is higher when expressed as a percentage of maximal intensity in female hockey players. Finally, VT and LT occur at different intensities for each gender, so VT should not be used to predict LT in hockey players.
The current study demonstrated that female hockey players have a higher VT than their male counterparts when expressed as a percentage of HRmax or o2max. This increase in VT may be a compensatory mechanism to offset lower o2max values. The male hockey players demonstrated higher o2max values than the female hockey players did. These findings suggest that female hockey players could reduce the effects of fatigue by improving o2max and that men could prolong the onset of fatigue by increasing VT. Improved aerobic fitness has been linked to improved net scoring opportunities by improving scoring chances for and reducing scoring chances against (12). With improved aerobic fitness, the player can increase his or her involvement in both the offensive and the defensive zones. It should be noted that women in the current study had o2max values approximately 24% lower than those of the men and that this result is considered typical (17). Higher aerobic fitness in men is primarily attributed to lower body fat percentage and higher hemoglobin concentration (17). Regardless of gender, it is recommended that aerobic training be performed during the season to avoid decreases in o2max (18).
The current study is in agreement with several others that report VT occurring at a lower percentage of maximal intensity than LT in triathletes (1), cyclists (4), untrained subjects (8), and patients with McArdle syndrome (13,14). In contrast, VT and LT have been reported to occur at the same percentage of maximal intensity when testing cross-country skiers, aerobically active individuals, sedentary adults, or cyclists and runners (9,16). It appears that the relationship between VT and LT (i.e., correlated versus uncorrelated) is dependent upon the mode of testing. Our results demonstrate that VT should not be used as a predictor of LT in hockey players during on-ice skating.
Hockey requires a high percentage of energy production from anaerobic metabolism (18). Whether differences in VT and LT are more prevalent in sports with a high anaerobic component, such as hockey, remains to be determined. Increases in LT have been found when training at intensities of 70% to 80% of o2max for as little as 2 weeks (8). Green et al. (11) estimated the on-ice intensity of college hockey players to be 70% to 80% of o2max. Additionally, Gaesser and Poole (8) reported that both continuous and interval training are necessary for increasing LT. It is possible that because playing hockey involves continuous and interval efforts and requires an intensity of 70% to 80% of o2max, increases in LT are beyond that of VT. However, the reasons underlying the differences in VT and LT remain unclear.
Finally, the graded on-ice protocol used in this study may serve as a useful tool for aerobically evaluating hockey players while employing specificity. Although o2max values are similar for hockey players tested on a cycle ergometer or a skating treadmill (6), treadmill skating has been found to increase stride rate, o2, and HR at submaximal velocities from values obtained on the ice (20). Cycle ergometry does not recruit muscles of the legs in the same way as skating, does not require movement of the upper limbs, and does not require the subject to support his or her entire body mass. Similar o2max values have also been found when comparing treadmill running to on-ice skating (15). Treadmill running recruits leg muscles differently than skating and does not employ the same equipment and environment as on-ice skating. Furthermore, cycle ergometry, treadmill running, and treadmill skating do not require the hockey player to change direction and accelerate or decelerate as he or she would on the ice. The on-ice protocol outlined within this study has a high level of specificity for muscle recruitment, equipment, and environment. Additionally, the on-ice protocol can also be used to emulate average shift duration. Eighty-second stages were chosen because they represent an approximate upper limit for shift duration (5,11,18).
The current study determined that o2max values are higher in male Division III hockey players than in female Division III hockey players. In contrast, the women's VT occurred at a higher percentage of o2max and HRmax than did that of the men. The LT was similar between genders when expressed as a percentage of o2max or HRmax. For each gender, LT was significantly higher than VT when expressed as a percentage of o2max or HRmax. Finally, the current study outlined a practical on-ice skating protocol that can be used for evaluating aerobic fitness in hockey players.
Practical Applications
The on-ice protocol in the current study can be useful to coaches. Coaches that have access to a wireless expired air analysis system can test o2max and VT on the ice. Stage duration can be specifically selected to mimic the shift duration of their players. The on-ice protocol allows for sport-specific determination of aerobic fitness levels in hockey players. The current results demonstrated gender differences in o2max and VT in Division III college hockey players. The difference in o2max between genders may indicate a specific need for increased aerobic conditioning for women. Because VT occurs at a lower percentage of maximal intensity in men than in women, male hockey players may benefit from interval training to improve VT. Results of on-ice testing can be used by coaches to address the specific aerobic training needs for each of their players.
Hockey players typically meet about one-third of energy demands via aerobic metabolism. Increasing aerobic fitness (i.e., o2max and VT) in hockey players could allow players to meet a higher percentage of energy demands aerobically. This may help to prevent lactate accumulation and fatigue. Coaches should consider increasing the focus on aerobic training for their players to potentially prevent fatigue and improve recovery between shifts, periods, and games.
Acknowledgments
We are grateful to Steve Ellison, RRT, from Portage Health, for his assistance with subject preparation and on-ice testing. We wish to thank Mitch Schuh and Christopher Plummer from the Department of Visual and Performing Arts at Michigan Technological University for developing the skating protocol music. We appreciate the cooperation of the Finlandia University hockey coaches, Joe Burcar and Chris Salani, and their players who volunteered for testing. We also appreciate the help of Dr. Casey Huckins from the Department of Biological Sciences and Dr. Tom Drummer from the Department of Mathematical Sciences at Michigan Technological University for their assistance with statistical analysis. This project was supported by a grant from the Portage Health Sports Medicine Institute.
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