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Active Dehydration Impairs Upper and Lower Body Anaerobic Muscular Power

Jones, Leon C1; Cleary, Michelle A2; Lopez, Rebecca M3; Zuri, Ron E4; Lopez, Richard4

Journal of Strength and Conditioning Research: March 2008 - Volume 22 - Issue 2 - p 455-463
doi: 10.1519/JSC.0b013e3181635ba5
Original Research

We examined the effects of active dehydration by exercise in a hot, humid environment on anaerobic muscular power using a test-retest (euhydrated and dehydrated) design. Seven subjects (age, 27.1 ± 4.6 years; mass, 86.4 ± 9.5 kg) performed upper and lower body Wingate anaerobic tests prior to and after a 1.5-hour recovery from a heat stress trial of treadmill exercise in a hot, humid environment (33.1 ± 3.1oC = 55.1 ± 8.9% relative humidity) until a 3.1 ± 0.3% body mass loss was achieved. Dehydration was confirmed by a significant body mass loss (P < 0.001), urine color increase (P = 0.004), and urine specific gravity increase (P = 0.041). Motivation ratings were not significantly different (P = 0.059), and fatigue severity was significantly (P = 0.009) increased 70% in the dehydrated compared to the euhydrated condition. Compared to the euhydrated condition, the dehydrated condition mean power was significantly (P = 0.014) decreased 7.17% in the upper body and 19.20% in the lower body. Compared to the euhydrated condition, the dehydrated condition peak power was significantly (P = 0.013) decreased 14.48% in the upper body and 18.36% in the lower body. No significant differences between the euhydrated and dehydrated conditions were found for decrease in power output (P = 0.219, power = 0.213). Our findings suggest that dehydration of 2.9% body mass decreases the ability to generate upper and lower body anaerobic power. Coaches and athletes must understand that sports performance requiring anaerobic strength and power can be impaired by inadequate hydration and may contribute to increased susceptibility to musculoskeletal injury.

1Athletics Department, Chicago State University, Chicago, Illinois; 2Department of Kinesiology and Leisure Sciences, University of Hawaii, Manoa, Honolulu, Hawaii; 3Department of Kinesiology, University of Connecticut, Storrs, Connecticut; 4Sport Science and Medicine Research Laboratory, Florida International University, Miami, Florida

Address correspondence to Michelle A. Cleary, mcleary@hawaii.edu.

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Introduction

Muscular strength and power are considered a basic component of physical performance and the factors affecting strength have been studied intensively (3,5,6,11,18,21,24,26,36). The effects of water depletion on physical performance have been well established; however, a lack of experimental evidence exists concerning the effects of dehydration on anaerobic muscular power. Exercise and sports participation in hot and humid climates have recently received considerable attention throughout sports-governing bodies, the athletic training community, and the media. Although certified athletic trainers and coaches strive to prevent injury and increase athletic performance, it is also necessary to understand how dehydration may adversely affect that goal. It is imperative for allied health care providers, coaches, and sports-governing bodies to be knowledgeable about the effects of dehydration and exercise in extreme temperatures.

Investigators have studied the detrimental effects of hypohydration (5,6,8,11,12,15,16,18,24,36,38) and the clinical manifestations of passively or actively induced hyperthermia (15,21,25,26,31,33) on exercise performance; however, discrepancies exist in the current research on dehydration and the effects it has on muscular strength and power. No significant differences between dehydrated (≥4% body mass loss) and euhydrated (normally hydrated) conditions have been demonstrated for peak torque during a maximal isometric voluntary contraction or for time to fatigue for knee extensors and elbow flexors (18). Further, no significant difference has been demonstrated between normothermic and hyperthermic conditions on a 1-repetition maximum bench press (18). On the contrary, leg press strength was significantly decreased after hyperthermic (30 minutes of sauna) exposure (21). Dehydration (amounting to 3.9% of body mass) during moderately intense prolonged exercise in the heat reduces the blood flow to the active skeletal muscles, elevates carbohydrate oxidation and lactate production, but does not impair either delivery of glucose and free fatty acids or the removal of lactate (16).

Athletes in sports requiring anaerobic power such as football do not always pay attention to thirst and become voluntarily dehydrated during competition, potentially affecting performance. A paucity of data exists on the effects of acute active dehydration by exercise in hot, humid environment on muscular power. Clearly, the literature does not provide definitive results dealing with the effects of dehydration on muscular power, and the variations of the dehydration procedures, the level of dehydration, and the influence of extraneous factors make comparisons difficult. Solid evidence to support clinical practice related to active dehydration on anaerobic muscular power has yet to be elucidated. Therefore, the purpose of this study was to examine the effects of acute dehydration by exercise in a hot, humid environment on upper body and lower body anaerobic muscular power.

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Methods

Experimental Approach to the Problem

The experimental design consisted of a test-retest within-subjects design. The independent variable was hydration (euhydration and dehydration), and the dependent variables were upper and lower body mean power, upper and lower body peak power, and upper and lower body decrease in power output. Upper and lower body Wingate anaerobic performance testing was performed in a euhydrated (normally hydrated) condition before a heat stress trial and again following recovery (about 1.5 hours of rest in a thermoneutral environment) in a dehydrated condition (3% body mass loss criterion) (11). The Wingate anaerobic test is a widely accepted measure of power output and anaerobic capacity (13,17,19,20,24,25,31). Our heat stress trial was designed (11) to go beyond the acute power enhancement resulting from a warm-up and actively induce hyperthermia in a manner similar to actual prolonged physical activity in the heat. The recovery period was provided to attenuate hyperthermia generated from the heat stress trial and reduce the effects of hyperthermia and fatigue (29). We elected to use an upper body Wingate anaerobic power test (27,33) in order to be able to attenuate fatigue as a mitigating factor since our heat stress trial protocol involved lower extremity treadmill exercise. Subjects performed the upper and lower body anaerobic power testing in a randomized and counterbalanced order.

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Subjects

Seven apparently healthy (mean age, 27.1 ± 4.6 years; mass = 86.4 ± 9.5 kg) men volunteered for the study. In order to facilitate the dehydration process during the heat stress trial, we recruited subjects who had been living active lifestyles in South Florida for at least 6 months, were resistance trained (anaerobic and aerobic workout at least 2-3 sessions per week), but were not highly aerobically fit or acclimatized to high levels of outdoor exercise in a hot, humid environment. We selected men to reduce the variability of ovarian hormone levels and substrate utilization between genders during exercise. We ascertained each subject's fitness level and health history via a health history questionnaire, and subjects were selected based on a history of no musculoskeletal injury or surgery, no predisposing cardiovascular or cardiorespiratory conditions, and/or no heat-related illness or injury within 1 year preceding data collection. Each subject was fully informed of the procedures and risks and signed an informed consent form. The study was approved by the Florida International University's Institutional Review Board.

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Experimental Protocols

Heat Stress Trial

The dehydrated condition was induced via heat stress trial with restricted fluid intake (11) consisting of 45-90 minutes of treadmill exercise in a hot, humid environment (ambient temperature = 33.1 ± 3.1°C, range = 28.5-40.5°C; relative humidity = 55.1 ± 8.9%, range = 40.7-68.1%). The treadmill exercise protocol commenced with a 5-minute warm-up at 3.0 mph. Treadmill speed was then increased, and the subjects exercised at 60% of their age-predicted heart rate range until a criterion of 3% body mass loss was achieved. A 60-second rest was administered every 15 minutes of exercise. All exercise was performed on a standard motor-driven treadmill (Proform; ICON Health & Fitness, Logan, UT) located outdoors in South Florida during the summer months. As safety precautions, mean arterial pressure, heart rate, and core body temperature were measured within the first 10 minutes of exercise and monitored every 15 minutes throughout heat stress trial and recovery. If core body temperature exceeded 39.0°C, the heat stress trial was terminated.

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Lower Body Wingate Anaerobic Test

To measure lower body anaerobic power, subjects performed the Wingate anaerobic test following standard procedures (13) before (euhydrated condition) and after (dehydrated condition) the heat stress trial. The 30-second protocol was performed on a cycle ergometer (Monark Ergomedic 818E; Monark Exercise AB, Vansbro, Sweden) following a 3-minute warm-up. The warm-up included 5-second sprints against no resistance after the first and second minutes. Subjects were allowed to maximize pedal speed approximately 2 seconds prior to test initiation in order to overcome the inertia of the flywheel. At the time of test initiation, a predetermined load equal to 0.075 kg per kilogram of body mass was applied to the flywheel and a count of pedal revolutions began. Subjects were asked to cycle as quickly and as forcefully as possible while remaining seated through the entire 30-second duration of exercise. Pedal revolutions were recorded at 5-second intervals using an electromagnetic counter (Red Lion CUB5; Nu-Metrics, Uniontown, PA). When the 30-second test was completed, the 0.075 kg per kilogram of body mass load was reduced and the subjects pedaled against no resistance to cool down. Mean power output was determined by averaging the six, 5-second mean power output values (31). Peak power was determined by number of pedal revolutions in the first 5 seconds of frictional load (31). Verbal encouragement was provided throughout the test.

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Upper Body Wingate Anaerobic Test

Upper body anaerobic power was measured by subjects performing the upper body Wingate anaerobic test before (euhydrated condition) and after (dehydrated condition) the heat stress trial. The upper body test was performed following the previously described Wingate protocol with only the exception of the cycle ergometer equipped with handles where the pedals are normally located. The cycle ergometer was placed securely on a 74.5-cm high plinth and weighted with 80 kg to prevent movement during testing. Subjects were seated comfortably in a chair placed behind the cycle ergometer so that their feet were flat on the floor and so that the cycle ergometer could be “pedaled” with no restrictions. At the time of test initiation, a predetermined load of 0.050 kg per kilogram of body mass was applied to the flywheel (27).

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Mood Ratings

Mood was assessed to identify confounding variables that may affect performance of the anaerobic power tests. Motivation level was determined by administering a visual analog scale prior to each administration of the anaerobic power test. The visual analog scale is designed to present to the respondent a rating scale with minimum constraints. The visual analog scale consisted of a 13-cm line with the left side being labeled “No Motivation at All” and the right side labeled “Highest Possible Amount of Motivation.” Subjects marked the location on the line corresponding to the amount of motivation perceived at that time. The mark on the line was measured from the left to the nearest 0.01 cm and recorded for data analysis. Fatigue severity was determined using a question reading “At this moment, what is your severity of fatigue?” with a 9-point Likert scale response. The response scale consisted of 1 = not at all, 3 = mild, 5 = moderate, 7 = severe, 9 = worst imaginable, and the number reported was used for data analysis.

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Hydration Status

To ensure the dehydrated condition, hydration status was assessed using body mass measurements and urinalysis. Body mass was measured using a Tanita BWB-800S digital medical platform scale (Tanita Inc., Brooklyn, NY). The Tanita BWB-800S digital medical platform scale consists of a digital display monitor connected via a 6-ft cord to the scale platform. The Tanita BWB-800S digital medical platform scale has a body mass capacity of 200 kg with accuracy to the nearest 0.01 kg. Nude body mass was measured following standardized instructions. Subjects stepped behind a privacy screen, disrobed, towel dried, and stepped onto the scale platform. The investigator recorded nude body mass from the digital display monitor located outside of the privacy screen.

To ensure a dehydrated condition, subjects provided urine samples for analysis. Urine concentration was determined using specific gravity and urine color. Urine specific gravity was measured using a clinical refractometer (Model 300CL, Atago Company Ltd., Tokyo, Japan). Urine concentration was determined using a color chart. Urine color has been established as a valid and reliable measure of urine concentration (1).

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Thermoregulatory Response

To monitor subjects' thermoregulatory response as a safety precaution, subjects were equipped with a rectal probe (YSI 401 series, Yellow Springs Instruments Inc., Dayton, OH) and 3 skin thermistors (YSI Model 408/708, Yellow Springs Instruments Inc.) taped on the arm, thigh, and calf. The thermistors measure a range of 0-70°C with an accuracy of ±0.1°C. The thermistors were connected via flexible coated wire to the YSI Model 4000A precision thermometer with dual-channel, dual-display (Yellow Springs Instruments Inc.). The thermometer is a self-calibrated system with independent channel operating separately for more than 1 type of temperature measurement. The thermometer has a temperature range of 0-50°C with an accuracy of ±0.10°C. Subjects' body temperature was measured throughout the heat stress trial and recovery.

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Cardiovascular Response

To monitor subjects' response to exercise as a safety precaution, cardiovascular measures were recorded at 15-minute intervals during exercise. Heart rate was measured using Polar heart rate monitor telemetry system (Polar Electro, Inc., Port Washington, NY). Resting and exercise blood pressure were measured in the standing position to avoid postural changes using a standard sphygmanometer and a stethoscope. Rating of perceived exertion was recorded using the Borg (4) scale (scale range, 6-20). The Borg scale is a reliable and valid measure of cardiovascular strain (4).

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Environmental Conditions Monitoring

Environmental conditions were monitored to standardized the heat stress trial and as a safety precaution. Ambient temperature and humidity were measured using a digital temperature humidity monitor with calibrator salts (Model PTH8709K, Linseis Inc., Princeton, NJ). Prior to each data collection session, the monitor was calibrated. The digital device has a temperature range of −10 to 50°C with an accuracy of ±1°C and a humidity range of 5-95% with an accuracy of ±5%. Wind speed and ambient conditions during exercise were measured using the Kestrel 3000 environmental meter (Richard Paul Russell Limited, New Lymington, UK), which is a combined electronic anemometer, thermometer, and hygrometer.

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Experimental Procedures

Familiarization Session

Initially, a familiarization session was conducted in which potential participants read and signed the health/injury history questionnaire and the informed consent form. Participants were also familiarized with the Wingate anaerobic tests with instruction and practice in the proper techniques used with the cycle ergometer. Participants were instructed to abstain from ingestion of alcohol, caffeine, nonprescription medication, and dehydrating behaviors (sauna, diuretics, sweat suits, etc.) for the duration of the study. At the end of the familiarization session, age, baseline body mass, resting heart rate, and blood pressure were measured and recorded.

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Euhydrated Condition Data Collection

Participants reported to the Sports Medicine Research Laboratory at 9:30 am the following day wearing an athletic supporter, mesh shorts, cotton t-shirt, sweat socks, and running shoes. Participants were asked to completely void urine, and body mass data were recorded. A euhydrated body mass was confirmed as less than ±1% (or 0.4 kg) of baseline body mass recorded the previous day. The dehydration criterion body mass loss of 3.0% of predehydration nude body mass was determined. Environmental conditions (ambient temperature and relative humidity) were recorded periodically throughout the exercise trial and recovery. A euhydrated condition Wingate anaerobic test was administered immediately followed by the heat stress trial.

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Dehydrated Condition Data Collection

Following the heat stress trial, participants voided all urine, provided a sample, and then removed all clothing and toweled dry for measurement of dehydrated nude body mass. After dehydration was confirmed, participants rested in thermoneutral environment (ambient temperature = 26.6 ± 2.6°C, relative humidity = 55.4 ± 5.8%) until core body temperature returned to baseline (52.5 ± 20.1 minute, range = 28-80 minutes). This delay was provided to allow muscle and core body temperature to return to normal and to allow the effects of heat exposure and exercise to subside before upper and lower body anaerobic power was assessed (19). Participants completed 4 administrations of a 30-second Wingate anaerobic test (2 upper body and 2 lower body in the euhydrated and dehydrated conditions), and for each test, peak power, mean power, and decrease in power output were calculated and recorded.

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Statistical Analyses

Separate 2 × 2 analysis of variance with repeated measures on both factors were used to identify differences mean power, peak power, and decrease in power output. Bonferroni corrections for multiple comparisons were used to adjust for potential inflation of alpha. Paired samples t-tests were used to analyze the euhydrated and dehydrated conditions on mood ratings and hydration status. Descriptive statistics were performed for the anthropometric, thermoregulatory response, cardiovascular response, and environmental conditions measures. Data were analyzed using the SPSS 13.0 for Windows Statistical Package (SPSS, Chicago, IL). Significance was set at P ≤ 0.05 for all statistical analyses.

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Results

Hydration Status

Hydration status descriptive statistics are presented in Table 1. As expected, our subjects became significantly dehydrated during the heat stress trial according to changes in body mass, urine color, and urine specific gravity. Compared to the euhydrated condition, the dehydrated condition body mass was significantly (P < 0.001) reduced 3.1%, urine color was significantly (P = 0.004) increased 24.6%, and urine specific gravity was significantly (P = 0.041) increased 0.196%.

Table 1

Table 1

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Mood Ratings

Subjects rated their mood including motivation and perception of fatigue prior to anaerobic power performance tests. Motivation ratings were not significantly different (P = 0.059) in the euhydrated and dehydrated conditions; however, a trend toward a 23.0% decreased motivation was found from the euhydrated condition (8.9 ± 1.5 cm) to the dehydrated condition (6.8 ± 3.2 cm). Fatigue severity was significantly (P = 0.009) increased 70% in the dehydrated (5.0 ± 2.0) compared to the euhydrated (1.5 ± 0.8) condition.

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Power

Consistent with our anticipated outcome, our mean power data (Figure 1) revealed significant (P = 0.014) differences between the euhydrated and dehydrated conditions. For the upper body, a 7.17% decrease was found in the dehydrated condition (1195.71 ± 244.14 W) compared to the euhydrated condition (1406.86 ± 260.31 W). For the lower body, a 19.20% decrease was found in the dehydrated condition (2202.00 ± 377.04 Watts) compared to the euhydrated condition (2725.14 ± 555.56 W).

Figure 1

Figure 1

Peak power data (Figure 2) data revealed significant (P = 0.013) differences between the euhydrated and dehydrated conditions. For the upper body, a 14.48% decrease was found in the dehydrated condition (1620.00 ± 258.58 W) compared to the euhydrated condition (1894.29 ± 346.16 W). For the lower body, an 18.36% decrease was found in the dehydrated condition (2888.57 ± 448.07 W) compared to the euhydrated condition (3538.29 ± 617.79 W). No significant differences between the euhydrated and dehydrated conditions (Figure 3) were found for decrease in power output (P = 0.219, power = 0.213).

Figure 2

Figure 2

Figure 3

Figure 3

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Discussion

Results from this investigation suggest that active dehydration of 3.1% via exercise in a hot, humid environment has a negative effect on anaerobic muscular power. Although previous studies (15,16,18,23,24,26,35,37,38) reported the effects of dehydration on muscular strength and power, the results are inconclusive. To our knowledge, anaerobic exercise performance has been previously evaluated in 4 studies, 2 of which employed Wingate-type cycle ergometer tests (24,37) and 2 of which employed supramaximal endurance tests (20,23). Dehydration (8% body mass loss) did not affect performance in a supramaximal (~1 minute) treadmill run (23). A comprehensive evaluation of subjects euhydrated and dehydrated by 2, 4, and 5% body mass using anaerobic exercise performance tests including a Wingate test revealed that dehydration did not affect anaerobic exercise performance or postexercise blood lactate levels (24). On the other hand, a 21% reduction in anaerobic power and a 10% reduction in anaerobic capacity in dehydrated (5% body mass loss) subjects have been demonstrated (38). Both of these studies used similar methodologies so that their disparate results are not easily explained. Compared to a euhydrated control trial, subjects performing a supramaximal (105% of maximal oxygen consumption) cycle ergometer test demonstrated an 18, 35, and 45% decrease when dehydrated (3% body mass loss) elicited via diuretics, sauna, and prior exercise, respectively (26). A significant decrease in anaerobic capacity (1984.3 ± 189.3 kg·m·s−1 to 1791.4 ± 198.0 kg·m·s−1) and anaerobic power (81.4 ± 13.3 kg·m·s−1 to 63.9 ± 9.4 kg·m·s−1) has been demonstrated in dehydrated wrestlers (37). These findings are in agreement with a report of 31% decrease in isometric endurance and 29% decrease in isotonic endurance in acute thermal dehydration rehydration (36). Further, 12 to 17% force decreases have been observed for isometric and isokinetic knee extension tests when subjects were dehydrated 2% of body mass (15). These reports are in contrast to another report (23) of hypohydration (8% body mass loss) that did not affect performance in a supramaximal (~1 minute) treadmill test. In a more recent and well-controlled study, no effects of hypohydration on isometric strength and endurance were observed (18). Greiwe et al. (18) found no significant differences on peak torque or endurance time of right knee extensors or right elbow flexors when the 2 conditions were compared (euhydrated to hypohydrated). Our findings agree with those of Nielsen et al. (26), Torranin et al. (36), Ftaiti et al. (15), and Webster et al. (38) in that we also found decreased anaerobic power in subjects dehydrated 3.1% of their body mass. We reported that upper and lower body peak power was decreased 14.5 and 18.4%, respectively. We also reported a decreased upper body (7.2%) and lower body (19.2%) mean power output. The inconclusiveness of the previous investigations may be attributed to the means of dehydration and the length of time from dehydration to actual testing of anaerobic power.

Dehydration may be achieved using several methods such as passive dehydration in which fluid and food are restricted over several days, diuretic administration, or by sauna exposure. Alternatively and practically, dehydration may be achieved by active methods such as exercise often combined with heat stress and fluid limitation or restriction. The majority of the studies previously discussed used passive dehydration methods, such as a protocol of fluid and food restriction over several days without heat stress (23). Alternatively, weight loss has been induced by intermittent passive exposure to heat (air temperature 56°C) over a period of up to 6 hours (24) or a similar protocol with weight loss induced over a 36-hour period (35). Weight loss induced by sauna exposure occurs with varying results. It is unclear why Torranin et al. (36) and Greiwe et al. (18) experienced varying results; however, Greiwe et al. (18) administered a 3.5-hour delay from the heat stress to actual performance of the exercise trials to allow the effects of the heat stress to subside. A large decrease in force production has been reported in dehydrated subjects (37); however, these findings were confounded by extraneous factors and cannot be attributed to dehydration alone. These findings (37) were confounded by using subjects who exercised an additional 1-2 hours (after a 1.5-hour wrestling practice in order to make weight) in a rubberized suit performing primarily aerobic exercise. Since the extra exercise likely lowered both muscle and liver glycogen stores and subjects reported to the laboratory 12-14 hours prior to testing, it is likely that subjects were in a greater state of glycogen depletion compared to their normal weight condition (37). Dehydration and hyperthermia induced by subjects wearing a tracksuit with an impermeable jacket and pants result from impeding sweat evaporation during running (15). On average, the prefatigue strength tests were completed 20 minutes before the treadmill run and the postfatigue strength tests started 8-12 minutes after the run to induce dehydration (15). Active and passive methods of dehydration may have led to the varied results of previous investigations.

Previous findings (15,37) cannot be solely attributed to dehydration because of the fatiguing nature of the dehydration protocol, which confounded the results. In one of the more comprehensive studies, a supramaximal exercise performance decrease resulted when subjects were dehydrated by diuretics (18% decrease in performance), sauna (35% decrease in performance), and exercise (44% decrease in performance) (27). Each procedure resulted in a relatively large reduction in plasma volume. In contrast, dehydration that develops during exercise is associated with relatively small reductions in plasma volume compared to exercising while euhydrated (30). Thus, hypohydration induced prior to exercise by diuretics or sauna exposure is not a good model for studying the effects of dehydration that develops during exercise, although it can be appropriate for studying the effects of beginning exercise in a hypohydrated state, such as in wrestling. Furthermore, passive modes of dehydration are unrelated to in situ or in vivo physical activity in which most individuals participate. Our study used an in vivo active dehydration protocol that closely simulates an actual typical physical activity environment.

Similar to a typical football practice in which dehydration commonly occurs, we sought to determine the effects of dehydration on anaerobic muscular power. The unique aspect of the current investigation was that we controlled for the confounding variables of fatigue and hyperthermia that might affect anaerobic muscular power. In the current study, subjects rested in a thermoneutral environment (approximately 1.5 hours) following the heat stress trial to reduce the fatiguing effects of exercise and hyperthermia on the anaerobic muscular performance test. Subjects were only allowed to perform the anaerobic performance trial once core body temperature returned to baseline. This normothermic condition was likely to have resulted in equilibration of the intra- and extracellular fluid compartments (10,12,28,35). Similarly, Greiwe et al. (18) reported no significant differences in isometric grip strength after resting for 120 minutes following passive heat stress. In the current study, motivation was rated prior to each administration of the Wingate anaerobic test. We found no significant difference between the euhydrated and dehydrated conditions, indicating that lack in motivation to perform the Wingate anaerobic test after the heat stress trial was not a contributing factor to the decrease in upper and lower body anaerobic power. Findings of many of the previous studies cannot be attributed entirely to dehydration since they include confounding factors such as increased muscle temperature, caloric restriction, and exercise and did not account for the confounding effects of fatigue or hyperthermia.

Muscular performance due to dehydration has been demonstrated (5,22,38) with conflicting results. While dehydration has not been demonstrated to alter anaerobic exercise performance or postexercise blood lactate values (23), a 21% decrease in anaerobic power and a 10% decrease in anaerobic capacity has been demonstrated in dehydrated (5% of body mass) subjects (38). Dehydration has been achieved by fluid restriction (5,6,23) and by a combination of exercise and heat exposure (38), with the most dramatic decreases in muscular strength a result of prolonged fluid restriction accompanied by a caloric deficit (6,23). Sjogaard (34) suggested that a loss of intracellular potassium might hyperpolarize the membrane electrochemical potential and decrease muscle contractibility. Faulkner (14) reported that high muscle temperatures may elevate hydrogen ion concentration, which would inhibit phosphofructokinase activity and therefore anaerobic performance. These studies have demonstrated that heat exposure that significantly increases muscle temperature is detrimental to muscular endurance (3,23,26,30,38). When the various methods of dehydration are compared, exercise has been shown to have the most unfavorable effect because of lowered glycogen stores, elevated core temperature, contracted plasma volume, and altered concentrations of blood components (26).

Proposed physiologic mechanisms for a decrease in anaerobic muscular power are lacking. As a limitation of the current study, electrolyte concentration was not measured; however, a reasonable mechanism is that a loss of intracellular potassium hyperpolarizes the muscle cell membrane electrochemical potential and decreases muscle contractibility by inhibiting calcium binding to troponin or by interfering with cross-bridge formation (35). Also, the decrease in pH inhibits the enzymatic activity of the cell's energy system (22). Two primary anaerobic sources, the phosphagen system and the anaerobic glycolysis system, provide energy for muscular work. These two systems work together to provide skeletal muscle with an immediate source of adenosine triphosphate (ATP) for short-duration activities or burstlike sports, such as American football and basketball. The phosphagen system is active at the start of all exercise regardless of intensity and relies on the chemical reactions of ATP and creatine phosphate as well as the enzymes myosin ATPase and creatine kinase. These reactions provide energy at a high rate; however, because ATP and creatine phosphate are stored in small amounts, the phosphagen system cannot supply energy for continuous, long-duration activities (7). Glycolysis is the breakdown of carbohydrates either glycogen, stored in the muscle, or glucose, delivered by the blood, to produce ATP that supplements the supply from the phosphagen system for high-intensity muscular activity. The process of glycolysis involves multiple enzymatically catalyzed reactions using enzymes located in the sarcoplasm of the cells. Anaerobic glycolysis produces an organic by-product, lactic acid. Muscular fatigue experienced during exercise is often associated with high tissue concentrations of lactic acid (7). The cumulative effect is a transient metabolic decrease in available energy and muscle contractile force during exercise (9,22).

Finally, enhancement of sympathetic nerve activity due to dehydration may be related to muscle glycogen utilization (10). Our findings may be attributed to an accumulation of muscle lactate, changes in plasma concentration, and a decrease in muscle blood flow, which have been observed during exercise with dehydration (16,32). The negative influence of changes in body fluid electrolytes on force production is not, however, supported by the results of Costill et al. (12) who concluded from calculated values that intracellular water and potassium changes do not impair muscle cell function even during passive dehydration of 6% body mass loss. Regardless of the method of dehydration used, based on the current state of knowledge, several reasonable physiologic mechanisms exist to support our findings of decreased anaerobic performance in actively dehydrated subjects.

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Practical Applications

Water affects athletic performance more than any other nutrient. Consuming fluids in sufficient amounts is essential for normal cellular function and of particular importance to athletes in thermal regulation. Water is the largest component of the body, representing from 45-70% of a person's body weight. Muscle tissue is approximately 75% water, whereas fat tissue is about 20% water (2). This study demonstrated a direct connection between water deficits and an athlete's ability to produce power. Ironically, during physiologic and thermal stress, humans do not adequately replace sweat losses when fluids are consumed at will; most athletes replace only about two thirds of the water that they sweat off during exercise. This phenomenon has been called voluntary dehydration (2). Strength and conditioning professionals must be aware that becoming voluntarily dehydrated can decrease anaerobic power and make athletes aware of it as well. Our study supports the need for a systematic approach to water replacement since thirst is not a reliable indicator of fluid need in athletes practicing intensely in hot environmental conditions.

For sports such as American football and basketball, in which anaerobic power is particularly important, it is imperative for athletic trainers and coaches to educate athletes on the importance of proper hydration practices to prevent a dehydration-associated decrease in power. Our findings suggest that athletes in these sports who become dehydrated 3.3% of their body mass, common during competition, have a decreased ability to generate upper and lower body anaerobic power. Coaches and athletes need to be aware that individuals who strength train after a sports practice session or a strenuous exercise bout may have decreased performance if they do not replenish lost fluids and electrolytes prior to, during, and after training. Furthermore, coaches and athletes alike must understand that sports performance requiring anaerobic strength and power can be impaired by inadequate hydration. Impairments as a result of dehydration may not only lead to reduction in performance but also increased susceptibility to musculoskeletal injury.

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Acknowledgments

The authors thank the Florida International University community and especially thank Dr. Michelle Cleary and the rest of her colleagues in the Graduate Athletic Training and Sports Medicine program.

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Keywords:

hypohydration; Wingate anaerobic test; muscular strength

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