Hypohydration (the state of decreased water content in the body) and gastrointestinal temperature (TGI) increases during exercise in hot humid environments have been extensively documented (3,4,5,8,9,11,13,14,16,20,22). Wet globe bulb temperature (WGBT) above 28° C present an increased risk for exertional heat illness secondary to increased TGI and hypohydration (3). Exertional heat illness occurs when the body is unable to effectively cool itself during exercise and lacks the ability to properly replenish the energy spent on exercise. Dehydration, measured by 1-2% of the pre-activity body weight, disturbs physiologic function and increases the risk of developing exertional heat illness (6). This level of dehydration is common in sports and can occur after just an hour of exercise; the rate of dehydration is more rapid if the athlete enters the exercise session dehydrated (3). Dehydration leads to an increased core temperature, nausea, dyspnea, and decreased athletic performance (3).
Hypohydration results in an increased core temperature in response to reduced blood volume, hyperosmolality, skin blood flow, and sweat rate. Increased cardiovascular strain occurs secondary to body water loss, hypovolemia, peripheral vasodilation, tachycardia, decreased venous return, and decreased stroke volume, resulting in a decreased capacity to perform submaximal endurance exercise (1,3). As body temperature increases, so does the strain on the visceral organs and central nervous system, potentially leading to failure and death (6). Although body temperature rarely rises to a dangerous level, when the core temperature increases beyond 39.4° C, it is a medical emergency yielding a 10% mortality rate (9).
An increase in core temperature and cardiovascular strain is directly related to the magnitude of dehydration accrued during prolonged exercise, and the optimal rate of rehydration approximates the rate of sweat production (1,3).
Anecdotal observation of the amount of sweating that occurs during a men's ice hockey practice and the visible evidence of evaporative cooling that occurs lead to the clinical question of identifying what changes occur in hydration status and core temperature in this population. The occurrence of exertional heat illness and hypohydration in atypical environments has not been widely investigated. Non-environmental risk factors that contribute to exertional heat illness include restrictive clothing, hypohydration, and an increase in core temperature (3). The only other studies investigating changes in core temperature and hydration status in male ice hockey players revealed significant increases in both factors (15,25).
Previous research regarding male ice hockey athletes has focused on neurological and musculoskeletal injury (10,12,17,26). Although these areas of inquiry are valuable to improving care for these athletes, the risk of dehydration in this sport could be easily overlooked.
The purpose of this study was to measure TGI, hydration status, sweat rates, and weight loss of male collegiate ice hockey players during 2 practice sessions. We hypothesize that ice hockey players approach dangerous dehydration and TGI thresholds comparable to football, cross-country, and other endurance event athletes.
Experimental Approach to the Problem
Studies investigating changes in core temperature and hydration of male ice hockey players revealed significant risks for exertional illness (15,25). A repeated measures design was used to assess if these risks exist in competitive collegiate ice hockey players. The dependent variables were gastrointestinal temperature (TGI) measured via a CorTemp ingestible sensor, urine-specific gravity (USG), mass, and sweat rate change. Independent variables included time (pre practice, 20 minutes, 40 minutes, and post practice) and group (defensemen or forwards). Data from both sessions were pooled across the two 110-minute practice sessions.
Seventeen, acclimatized, male, competitive, collegiate club ice hockey players (20.6 ± 1.1 years, height = 180 ± 5 cm, mass = 85.0 ± 7.9 kg) volunteered for this study. Four seniors, 4 juniors, 5 sophomores, and 4 freshmen participated. All participants had a mean of 7 years of competitive ice hockey experience. Data collection sessions occurred during mid-season (January) practice sessions. All subjects completed a physician-administered physical examination at the beginning of the men's club ice hockey season. A preparticipation orientation session was directed by a physician to review the contraindications to CorTemp (HQ, Inc., Palmetto, FL, USA) pill ingestion. This research was approved by the Biomedical Human Subjects Institutional Review Board, and all subjects were informed of the procedures to be used and signed a consent form before participating in this study.
CorTemp (HQ, Inc.) ingestible pills were used to monitor TGI temperature. The CorTemp transmits core temperature readings accurately within ±0.1° C to a handheld computer (7,18,23). Urine-specific gravity, a valid measurement of hydration status (1), was measured using urine reagent strips (DiaScreen; Hypoguard, Edina, MN, USA). A digital psychrometer (Mannix; Professional Equipment, Inc., Hauppauge, NY, USA) was used to record ambient air temperature and calculate the WGBT. CorTemp pills were administered approximately 7.5 ± 0.5 hours before the start of data collection. Subjects were then instructed to return to the arena 45 minutes before the practice session for baseline body weight and urine sampling. Core temperature measurements were taken before each participant took the ice and at 20 and 40 minutes into the 110-minute practice session. The practice sessions consisted of a moderate, 20-minute, skate warm-up; a 2-minute break to measure TGI, followed by four 20-minute periods of intense full ice drills, with 2-minute breaks separating these sessions; and ending with a 10-minute cooldown.
Ambient air temperature and WGBT were measured from center ice at the beginning of practice and at 20-minute intervals for the remainder of data collection. A final reading was taken once all subjects had left the playing surface. All data were collected by the same investigator.
Athlete hydration during this study followed the standard practice protocol. Water was freely and readily available to all athletes, and water breaks occurred as usual. To track the amount of water ingested during activity, each subject was issued a uniquely identified water bottle used for rehydration only. Water that was used for purposes other than drinking (e.g., rinsing out the mouth and spraying the face and head) was provided in separate bottles but was not tracked. An investigator tracked and recorded the amount of water that each subject consumed during each data collection session. Water consumption data and body weight changes were used to calculate sweat rate (6).
Core temperature readings, body weight, and USG samples were obtained immediately after the practice session. Subjects were not allowed to shower or drink while they removed their equipment and were asked to report to the data collection area immediately after equipment removal. These procedures were followed for 2 similarly structured practice sessions held 1 week apart.
A repeated measures analysis of variance was used to compare differences in TGI at 20, 40, and 110 minutes following baseline temperatures. Pre- and post-exercise mass, USG, and sweat rate measurements were analyzed using paired sample t-tests. An a priori probability level was set at p ≤ 0.05. Sweat rate was calculated using a formula recommended by the National Athletic Trainers' Association (NATA) position statement on fluid replacement using pre- and post-exercise body weight and the amount of fluid consumed during the practice sessions (6).
Statistically significant increases in TGI (F 3,99 = 56.70, p < 0.0001; 1 − β = 0.99; η2 = 0.632) were observed (Figure 1). Pairwise comparisons demonstrated that temperatures at 20, 40, and 110 minutes were significantly greater than the baseline (p < 0.05). TGI between 20 and 110 minutes were not statistically different (p > 0.05). Urine-specific gravity increased (t 1,33 = 7.58, p < 0.0001; pre = 1.009 ± 0.008; post = 1.020 ± 0.001) after the 110-minute sessions (Table 1). Post-exercise body weight (83.9 ± 7.6 kg) was statistically lower (t 1,15 = 5.92; p < 0.001) than the pre-exercise weight (85.0 ± 7.9 kg). On average, subjects lost 1.1% of their body weight. Sweat rate averaged 0.83 L·h−1.
TGI Changes Between Groups
TGI readings were similar between groups (Figures 2 and 3); however, a significant difference was noted when examining the amount of weight lost during activity (forwards = 0.71 kg, defensemen = 1.33 kg; Figure 3). Defensemen lost an average of 1.49% body weight compared with 0.86% for forwards.
Urine-Specific Gravity Readings
Our subjects entered the practice sessions well hydrated (USG = 1.009 ± 0.005) and ended the session with minimal dehydration (1.020 ± 0.007) (Table 2). Similar to previous research (13,14,16), we were unable to correlate any change in USG to TGI (Figure 4).
Significant changes were observed in TGI, USG, and body weight after a 110-minute intense ice hockey practice. Subjects demonstrated a significant TGI increase to 38.37° C after 40 minutes of exercise, nearly a 1°C increase over baseline. The maximum TGI temperature noted was 39.07°C, whereas the greatest increase by a subject was 2.18°C, both occurring 40 minutes into the data collection. These data support the findings of previous core temperature and hydration studies conducted on men's professional and junior ice hockey players (15,25). We feel that these changes are directly related to, and represent, the frequency, intensity, and duration of the practice sessions. Unlike game situations that involve frequent substitution of players, these practice sessions required longer continual activity bouts than what may be experienced under actual game conditions.
The type and nature of clothing can significantly impede the body's cooling mechanism (29). The combination of helmet, shoulder pads, gloves, pants, and shin pads inhibits the body's evaporative cooling mechanism. A football uniform inhibits evaporation by covering 60% of the body's surface area (3). An ice hockey player wears similar equipment but also covers the hands, legs, and arms. Although ice hockey equipment is of lighter weight, little attention has been directed to measuring these factors and their response in cold dry environments (21). Because of their less efficient thermoregulatory mechanisms, increased metabolic heat production and poor hydration habits increase the risk of heat illness in youth ice hockey players (2). Further investigation on the effects this equipment has on evaporative cooling should be conducted.
U.S. Army research concluded that the body can suffer from 70% exhaustion with a core temperature of 38.8° C (20). Although studies have concluded that hypohydration can contribute to increases in TGI, this was not demonstrated in our study. The average TGI did not rise above this threshold. However, some subjects met or exceeded this threshold, with the highest TGI reading at 39.07° C. The mean TGI 40 minutes into the practice sessions was 38.4° C, an increase of 0.92° C from baseline. Heat exhaustion thresholds may have not been achieved, but stresses on the system were subjectively evident among all subjects.
A recent study used protocols similar to ours when observing Ironman athletes during competition (16). In comparison to our study, final USG measurements were lower, indicating less dehydration among Ironmen. This comparison validates our hypothesis that male ice hockey players experience similar cardiovascular stresses as athletes competing in a warm environment (23° C, 60% humidity).
Fowkes-Godek et al. (14) observed changes in core temperature and hydration status among football and cross-country runners while exercising in the heat. Similar to our findings, the authors were unable to correlate core temperature to changes in USG (14,15). Significant differences in core temperatures exist during football practice when participants wore equipment compared with a no-pad practice. A U.S. Army study demonstrated that subjects exercising in combat gear had significantly higher core temperatures than those exercising in shorts and T-shirts (20). We observed similar results by noticing hyperthermia in athletes competing in restrictive clothing. We hypothesized that the same effects could be measured during a typical ice hockey practice (21).
We recognize that the reliability and validity of urine reagent strip testing compared with refractometry or hydrometry are debatable (28). Single measurements of USG using urine reagent strips compared with refractometry or hydrometry have provided varying results (28). However, pre-post measurements of urine reagent strips have not been considered in these comparisons. The time series comparison of pre-practice to post-practice reagent strip values conducted in our study provides evidence of change in urine reagent strip USG that has not been expressed in other studies using one-time measures of hydration status. Oppliger et al. (24) demonstrated that both USG and urine osmolality mimic acute changes in plasma osmolality during hypohydration protocols, and a USG value >1.020 is an indicator of hypohydration (6).
Sweat rate was observed to vary between subjects, mostly because of differences in the amount of water ingested during each practice session (Figure 5). On average, subjects lost 1.1% of their body mass and sweat rate was 0.83 L·h−1. Our sweat rate and body mass loss data are comparable to that observed in an investigation of junior hockey players (24). However, general conclusions were made when looking at differences in sweat rate by position. Similar to what was discovered when calculating the percentage of weight loss during activity, the defensemen had a much higher sweat rate when using the NATA formula (6). Although the defensemen group had a higher sweat rate, it seems that they were better equipped to handle the amount of stress, as their core temperature did not change as much as did the group of forwards.
The NATA position statement on exertional heat illness has correlated dehydration to a rise in core temperature (3). Urine-specific gravity rose in all subjects except one, indicating that the majority of our subjects did not consume enough water during the session to keep their hydration level constant. Weight loss for all subjects was 0.93 ± 0.62 kg, an average of 1.1% of the pre-exercise body weight. With a 1% decrease in body weight as a result of fluid loss, core temperature increases of 0.15-0.20° C have been observed in previous studies (19,27). In our study, the 1.1% decrease in body weight was coupled with an average 1° C increase in body temperature during the practice session. This is particularly concerning considering that our measurements were taken during a practice session and not during the more intense game competition. The percentage of weight loss of participants in this study reaches the midpoint of the danger range. It also indicates that hypohydration may be more prevalent among male ice hockey players than commonly assumed. In addition to thermal strain, ice hockey players may be more at risk for cardiovascular deficits such as decreased stroke volume, increased heart rate, and lower cardiac output associated with dehydration than previously suspected (6).
The discrepancy in the percentage of water weight lost between positions is also of concern when the amount of water ingested was considered. Defensemen averaged consuming 150 ml more of water during each practice but dropped more body weight. This difference may be explained by the fact that cardiovascular demands were much greater during this practice session for the defensemen than the forwards in amounts of ice time. This was validated when calculating each group's sweat rate, as the defensemen had more than 1.10 L·h−1, in comparison to 0.67 L·h−1 in the forwards group.
Participants in our study became hypohydrated during both sessions. This change in hydration status, combined with restrictive clothing and protective equipment, impeded appropriate heat dissipation and strained players' thermoregulatory systems manifested in an increased in gastrointestinal temperature. This risk may be greater during the preseason where players may not be in peak condition and acclimated and the ambient temperature and humidity are higher.
Preventative measures are needed to ensure adequate hydration for ice hockey players and individuals participating in cold environments before and after activity to prevent dehydration-related conditions. Strength and conditioning personnel, athletic trainers, physicians, and coaches should be aware of and implement hydration strategies such as those recommended by the American College of Sports Medicine and National Athletic Trainers' Association. Further research is recommended to investigate increases in core body temperature among athletes competing in cold environments.
This research was partially funded by the Ohio University Provost's Undergraduate Research Fund grant. No external funding was received for the conduct of this study.
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Keywords:© 2010 National Strength and Conditioning Association
hydration status; sweat rate; prevention; urine-specific gravity