INTRODUCTION

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THE EFFECTS THAT HYPOHYDRATION (body water deficit) has on increasing heat strain (21, 22a, 26) and cardiovascular strain (21, 24) and on reducing and/or degrading aerobic exercise performance (2, 5, 23, 30) are well documented. Less understood are the effects of hypohydration on skeletal muscle performance and metabolism. Whereas results from two studies (3, 29) indicated that hypohydration reduced muscle endurance, other investigators found no difference in fatigability during handgrip exercise (27). Similarly, anaerobic exercise performance has been reported to be decreased (30) or not altered (13, 18, 19) by hypohydration. These previous studies are somewhat confounded, however, because they did not control for prior exercise and/or heat exposure and different caloric intake before the performance tests. Research is needed that controls for these confounding variables to determine whether hypohydration has direct effects on skeletal muscle that contribute to the well-documented reductions in aerobic performance.

Hypohydration might accelerate depletion of energy stores, accumulation of metabolites (e.g., lactate, H⁺, P_i), and changes in intracellular electrolyte concentrations, and/or reduce buffering capacity (8, 13, 17, 22). Investigators examining the effects of hypohydration on muscle glycogen use have found either no effect (8) or a small increase in muscle glycogen utilization (16). Similarly, hypohydration has been reported to not alter (8) or increase muscle lactate concentration (16). The effects of hypohydration on intracellular H⁺ or P_i concentrations in skeletal muscle have not been studied. Elevated H⁺ and P_i concentrations reduce muscle force production during repeated contractions (12), and intracellular concentrations would be increased by simply reducing intracellular water. These two metabolites can be measured noninvasively and repeatedly during exhaustive exercise with the use of ³¹P-magnetic resonance spectroscopy (MRS).

The purpose of this study was to determine whether hypohydration reduces skeletal muscle performance and whether increased H⁺ and P_i concentrations might contribute to performance degradation. We hypothesized that hypohydration would reduce skeletal muscle endurance and act via increased H⁺ and P_i concentrations. To test these hypotheses, we used ³¹P-MRS to measure high-energy phosphates and pH during exhaustive single-leg, knee-extension exercise when subjects were euhydrated and hypohydrated.

	METHODS

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Subjects. Ten healthy, physically active persons (5 men and 5 women), 21-40 yr of age, participated in this study. The study was approved by the appropriate institutional review boards, and all volunteers gave their voluntary and informed consent before participation.

Experimental procedure. After several practice sessions to familiarize the volunteers with the experimental procedures and to determine the appropriate exercise intensity for the experimental trials, the volunteers reported to the laboratory on two occasions separated by a minimum of 1 wk. On arrival, at 1100-1300, an initial nude body weight was obtained to establish baseline body weight. The volunteers then entered a hot room (40°C, 20% relative humidity) to perform 2-3 h of moderate-intensity treadmill and cycling exercise. For the euhydrated trial, water was available ad libitum during the exercise. For the hypohydration trial, drinking was restricted to produce a 4% body weight loss (BWL). The exercise mode, duration, and intensity were held constant for each trial. In the event that the exercise protocol did not elicit the desired BWL, supplemental sauna exposure was added. Trial order was randomly assigned and balanced across subjects. After exercise, the volunteers were given a small standardized meal (~400 kcal; 70% carbohydrate) and 200 ml of fruit juice.

View larger version (17K): [in this window] [in a new window]   Fig. 1.   Experimental setup for 31P-magnetic resonance spectroscopy studies. NMR, nuclear magnetic resonance; RF, radio frequency.

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Statistical analysis. The data were analyzed by using one- and two-way repeated-measures analysis of variance where appropriate. For all analyses, the data obtained from each leg were treated as an independent set of measurements. During one trial, time to fatigue was not reached due to technical difficulties. Therefore, data from that leg were not included in statistical analysis. For three other trials, collected spectra were uninterpretable, and all MR data for those legs were excluded from statistical analysis. Tukey's highly significant difference procedure was used to identify differences between means when statistical significance was achieved. Statistical significance was tested at the P < 0.05 level. Data in the text are reported as means ± SE.

	RESULTS

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BWL. Before exercise was performed in the hot room, the subjects' body weights were 65.9 ± 4.1 and 66.1 ± 4.1 kg for euhydration and hypohydration, respectively. The dehydration-rehydration procedures resulted in 0.6 ± 0.2 and 4.0 ± 0.2% BWL, respectively, before the MRS tests.

Muscle endurance. Figure 2 presents the individual leg and mean endurance times to exhaustion. Hypohydration reduced endurance time 8% (coefficient of variation for time to fatigue) in 12 of 19 of the trials performed, and mean endurance was reduced (P < 0.05) from 251 ± 15 to 213 ± 12 s (15%). Four of ten subjects had reduced endurance time in both legs when hypohydrated, whereas in three others only one leg was affected. For these three subjects, the reduced exercise performance occurred in the second leg tested. For one subject, endurance time was reduced in one leg but was not tested in the other leg due to technical problems. These results were similar to our pilot work (n = 5 subjects) in which 4-5% BWL reduced endurance in 8 of 10 trials and reduced (P < 0.05) mean endurance time from 230 ± 34 to 192 ± 32 s (17%). These combined results demonstrate that hypohydration decreased mean endurance time (20 out of 29 tests) by 15-17% by using this exercise paradigm.

View larger version (24K): [in this window] [in a new window]   Fig. 2.   Individual () and group mean (solid line) results for time to fatigue for subjects euhydrated (EU) and hypohydrated (HY) by 4% of initial body weight. Inset: hypohydration less than euhydration, * P < 0.05.

Muscle strength. Figure 3 presents maximal voluntary contraction (MVC) data. Hypohydration did not alter preendurance exercise maximal isometric force. Hypohydrated persons produced a 16% higher (P < 0.05) maximal isometric force 30 s after exhaustive exercise. No other difference between trials existed during recovery from exhaustive exercise. Furthermore, there was no statistical correlation [not significant (NS)] between the increased MVC at 30 s postexercise and the reduction in endurance time when the subjects were hypohydrated, whether expressed as absolute change in MVC (r = 0.58) or percent change (r = 0.28).

View larger version (16K): [in this window] [in a new window]   Fig. 3.   Maximal isometric force before (Pre) and after exhaustive exercise when subjects were euhydrated () and hypohydrated (). Data are means ± SE for 9 subjects. MVC, maximal voluntary contraction. Hypohydration greater than euhydration, * P < 0.05.

³¹P-MRS. Figure 4 presents P_i/-ATP, pH, and ratio of P_i to PCr (P_i/PCr) data collected during all experimental trials, and these variables were not altered by hydration. P_i/-ATP values were similar at rest, averaging 1.20 ± 0.08 and 1.14 ± 0.06 during euhydration and hypohydration trials, respectively. During exhaustive exercise, the P_i/-ATP rose progressively to peak values of 5.66 ± 0.35 and 5.55 ± 0.33 during euhydration and hypohydration, respectively. Similarly, P_i/PCr rose from resting values, averaging 0.17 ± 0.01 to 3.79 ± 0.47 and 3.46 ± 0.40 at exhaustion during euhydration and hypohydration, respectively. The pH fell progressively from 7.04 ± 0.02 at rest to 6.49 ± 0.09 at exhaustion during euhydration and hypohydration. H₂PO₄ levels rose from 2.2 ± 0.2 mM at rest to similar levels (NS) at exhaustion (19.1 ± 1.9 and 20.5 ± 1.8 mM for euhydration and hypohydration trials, respectively). Similar to exercise data, hypohydration did not alter (P < 0.07) the time constant of PCr synthesis after exhaustive exercise (63 ± 6 and 72 ± 7 s for euhydration and hypohydration trials, respectively).

View larger version (12K): [in this window] [in a new window]   Fig. 4.   Muscle Pi/-ATP ratio, pH, and Pi/phosphocreatine (PCr) ratio data for all tests when subjects were euhydrated () and hypohydrated (). Data are means ± SE for 15 paired comparisons.

View larger version (12K): [in this window] [in a new window]   Fig. 5.   Muscle Pi/-ATP ratio, pH, and Pi/PCr ratio data for subgroup of tests when hypohydration shortened time to fatigue. Data are means ± SE for 10 paired comparisons. Hypohydration () greater than euhydration (), * P < 0.05.

	DISCUSSION

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To our knowledge, this study is the first to examine simultaneously the impact of hypohydration on skeletal muscle performance and muscle metabolism. The level of hypohydration studied is commonly achieved by athletes during competition and training (1). To minimize the likelihood of hypoglycemia and to replace some of the carbohydrate metabolized during the dehydration procedures, the subjects were given a small meal during the recovery period before the experimental trials. To isolate the effects of hypohydration on muscle from the potentially confounding effects of elevated body temperature (22), a minimum of 3 h of rest separated the heat exposures from experimental testing, and the MRS experiments were conducted in a cool room (~18°C).

We found that hypohydration reduced muscular endurance by 15% but had no effect on muscle strength. These findings agree with data from earlier studies, which indicated that hypohydration can impair muscle endurance (3, 29) but had no effect on muscle strength (3, 15, 23, 27, 28). They also agree with studies demonstrating that hypohydration can reduce aerobic endurance (see Ref. 25 for review). Our results extend the findings of these earlier studies by separating the effects of hypohydration from the confounding effects of elevated body temperature, cardiovascular strain, heat exposure, and differing quantities of exercise before experimental testing. The data from the present study also demonstrate that hypohydration has no effect on recovery of muscle strength after exhausting exercise.

In subjects during exercise, we employed ³¹P-MRS to assess whether hypohydration would accelerate the accumulation of H⁺ or P_i during exhaustive exercise. These two variables were chosen as both have been shown to reduce cross-bridge formation and force production and are the two variables within muscle often considered to be responsible for fatigue during high-intensity exercise (11, 12). We hypothesized that, if hypohydration had direct effects on muscle metabolism, then the hypohydration trials would likely be associated with elevated exercise H⁺ and/or P_i concentrations. The results of this study did not support this hypothesis, however, as P_i/-ATP, pH, and H₂PO₄ responses to exercise were not affected by 4% BWL. The only observation which suggested that hypohydration had an effect on muscle metabolism was the accelerated increase in P_i/PCr in the subgroup of trials with shortened time to fatigue. This would suggest that hypohydration required greater reliance on creatine kinase to sustain muscle ATP in these trials. The fact that P_i/PCr was not consistently elevated even in this subgroup or correlated with maintenance of endurance time, however, further supports the contention that moderate levels of hypohydration had little or no effect on muscle metabolism.

How hypohydration reduces muscle endurance remains an intriguing question. Approximately 50% of the water lost would be expected to come from the intracellular water compartments, and 4% BWL would be expected to reduce intramuscular water by 4-5% (7). It is unlikely that insufficient oxygen delivery was responsible for the shortened time to fatigue. The muscle mass activated during exhaustive exercise was not large enough to limit leg blood flow, and the exercise device was designed to reduce any isometric and eccentric muscle loading during the recovery phase of the contraction, thus minimizing disruptions in muscle blood flow when the muscle was not performing the knee-extension movement. Furthermore, any impairment in oxygen delivery would be expected to increase muscle glycolytic flux and formation of lactate and H⁺. We found no difference in muscle pH during exhaustive exercise regardless of whether the exercise was performed when the subjects were euhydrated or hypohydrated.

Alternative mechanisms within muscle include altered cell depolarization and changes in Ca²⁺ release and/or uptake by the sarcoplasmic reticulum. Dehydration-induced changes in the ionic status of the T-tubular lumen and intracellular compartments could contribute to the development of fatigue by negatively affecting the T-tubular charge movement (12). Similarly, longer Ca²⁺ transients might reduce Ca²⁺ flux on depolarization and reduce force production (12). The possibility that elevated intracellular Mg²⁺ plays a role appears unlikely, as Costill and Saltin (8) found no difference in intracellular Mg²⁺ concentration during exercise when the subjects were euhydrated vs. hypohydrated by 4% of initial body weight.

An alternative explanation for the detrimental effects of hypohydration on muscle endurance is that hypohydration alters central nervous system function. In the subgroup of trials in which muscle strength was measured by performance of MVC before and after exercise, muscle endurance time was reduced (P < 0.05) by 14%, yet volunteers were able to generate greater absolute force during the initial period of recovery, suggesting that the subjects were either less willing or unable to sustain voluntary concentric exercise when hypohydrated, despite having adequate muscle strength. An unwillingness or inability to generate or maintain adequate central nervous system drive to the working muscle is thought to be responsible for the debilitating fatigue that accompanies many infections and illnesses, recovery from injury, and chronic fatigue syndrome (9). Hypohydration may impair performance in a similar manner. These conditions are characterized by an increased perception of effort during physical activity, yet afflicted patients are capable of generating maximal force (9). Body water loss also increases perception of effort during physical activities (10, 21) yet has no apparent effects on maximal strength (3, 23, 25). In addition, hypohydration is known to alter neuronal firing of osmoreceptive cells located in the organum vasculosum laminae terminalis and cells near the preoptic/anterior hypothalamic areas of the brain (4). Neuronal activation mediated by hypohydration might also alter the magnitude of corollary discharge from the motor cortex.

In summary, we found that moderate hypohydration 1) decreases skeletal muscle performance by reducing endurance by 15%, 2) does not alter muscle strength or recovery of muscle strength after exhaustive exercise, and 3) does not alter pH and P_i/-ATP response to exhaustive exercise. These findings clearly identify another physiological system by which hypohydration adversely affects human exercise performance; however, the mechanisms for this action remain unclear.