Effect of Hydration State on Strength, Power, and Resistance Exercise Performance

Daniel A. Judelson; Carl M. Maresh; Mark J. Farrell; Linda M. Yamamoto; Lawrence E. Armstrong; William J. Kraemer; Jeff S. Volek; Barry A. Spiering; Douglas J. Casa; Jeffrey M. Anderson

Disclosures

Med Sci Sports Exerc. 2007;39(10):1817-1824. 

In This Article

Discussion

The primary findings of this study were that hypohydration 1) has little demonstrable effect on single, maximal-effort strength and power, but it 2) limited the ability of healthy, resistance-trained males to perform a six-set back squat protocol. These results suggest that hypohydrated individuals who complete isotonic, multiple-repetition, multiple-set, intermittent resistance exercise tasks will likely experience impaired performance.

The experimental challenges associated with isolating the effect of hydration state on exercise performance and physiological changes are significant because the dehydrating stimulus (as opposed to the actual hypohydration) frequently affects study outcomes. If the dehydration procedures used to reduce total body water are not adequately controlled, incorrectly performed, or immediately precede performance testing, confounding variables such as muscle fatigue, caloric deficit, and elevated temperature can strongly influence exercise outcome variables.[27] We believe the current research design effectively responded to the majority of these challenges. The hydration data, principally percent change in body mass (Fig. 1), indicate that subjects achieved three distinctly different hydration states before performing exercise despite completing identical dehydration bouts the night before. The overnight rest separating the exercise-heat stress from the resistance exercise helped to further minimize the effect of the dehydration process on experimental testing. Subjects also replicated dietary intake for 48 h preceding each trial and ingested a standardized high-calorie, high-carbohydrate meal approximately 12 h before resistance exercise to limit differences in energy stores. Admittedly, resting core temperatures differed between hypohydrated trials and the euhydrated trial, but 1) the temperate environmental conditions (~21°C) and 2) the minor thermal load provided by the resistance exercise (estimated to increase core temperature by approximately 0.2°C)[12] suggest the REC would only modestly increase core temperature. As isometric force production is normally maintained at core temperatures below 38°C,[26] we believe relative differences in resting core temperature among trials would not significantly affect the results, because absolute temperatures likely never achieved a performance-altering magnitude.

Exercise Performance

Although one study has documented a hypohydration-induced improvement in jumping performance,[34] the present investigation supports most previous research[19,35] and suggests that hypohydration fails to influence vertical jump height ( Table 1 ). These results describing exercise performance do not necessarily reflect the physiological capacity of muscle, however, because the decreased body mass characteristic of hypohydration might offset reduced muscular strength and/or power.[7,17] Specifically, if hypohydration fails to reduce muscle force or power, vertical jump height should increase as total body water decreases, because the jumper must move less mass. To accurately measure the effects of hydration state on muscle power, we calculated power output during jump squats through direct measures of force and time, independent of body mass. The lack of change among trials in peak concentric power during jump squats ( Table 1 ) suggests that hypohydration truly does not influence muscular power. Four other investigations[7,30,34,36] describe the isolated effect of hypohydration on power measures independent of confounding factors (e.g., body mass, endurance training, caloric restriction, increased core temperature, and/or fatigue); typical of this body of literature, two demonstrate a hypohydration-induced decrement in performance,[30,36] and two show that hypohydration failed to influence power output.[7,34] Combined with the current results, these conflicting findings (possibly attributable to exercise mode and amount of body water lost) clearly warrant further investigation into the effects of hypohydration on muscle power.

Hypohydration also failed to influence peak lower-body force ( Table 1 ), supporting some,[2,16,34] but not all,[4,5,28] previous investigations into the effect of hydration on single, maximal-effort force production. No obvious disparities in research design (muscle group, muscle action, velocity of contraction, subject population, etc.) explain these differences. Interestingly, peak force was unchanged despite possible differences in nervous stimulation of the musculature. Although the data failed to achieve statistical significance, central activation seemed to decrease as hypohydration increased, implying that alterations in the central nervous system, the peripheral nervous system, and/or excitation-contraction coupling might be related to hydration-induced decrements in endurance or resistance exercise performance. The notable effect size (ηp2 = 0.41) in a small sample (N = 6) further strengthens the chances that this relationship exists. Understanding the limits to these data, we take these inconclusive findings as a possibility that hypohydration might affect central drive, and we recommend that these results serve as a basis for future investigations examining larger subject pools. If hypohydration truly limits central drive, different patterns of muscle fiber recruitment potentially explain the equivalent peak forces produced during HY25 and HY50 isometric tests, as previous research demonstrates hypohydration potentially alters EMG output.[2,14,33]

Despite similar muscle force and power production, hypohydration significantly decreased performance of the REC. These findings support most other studies examining the effect of hydration state on high-intensity muscular endurance.[2,4,6,32] Unlike those previous investigations, the current results indicate that hypohydration significantly attenuates performance of an isotonic, multirepetition, multiset exercise bout typical of conventional resistance exercise. The magnitude of this effect seemed dose dependent: HY25 reduced performance through sets 2 and 3 but HY50 decreased performance through sets 2-5. These results demonstrate high external validity, as moderate hypohydration (as low as 2.4% body mass loss) decreased performance during three sets of resistance exercise (a very common exercise volume).

Although this reduction in muscular performance has implications for acute exercise, these findings also relate to training adaptations of those individuals who routinely complete resistance exercises in a hypohydrated condition (e.g., astronauts, the elderly, and some athletes). Because hypohydration seems to limit individual workout volume (in this case, by three to five repetitions within the first three sets of exercise), overall training volume might suffer. Quantifying the importance of three to five repetitions per exercise across an entire training program is challenging, but research suggests that small increases in daily training volume significantly benefit strength development as long as exercisers avoid overtraining.[15] Theoretically, increasing the rest interval between sets might maintain training volume despite hypohydration through enhanced recovery. Even if increased rest restored training volume, however, overall adaptations might still suffer because of the indirect relationship between rest interval and the anabolic hormonal response that crucially modulates training adaptations to resistance exercise.[18,22] Thus, although estimating the exact effects of hypohydration on long-term adaptations to resistance training is difficult, evidence reasonably suggests that hypohydration will attenuate some benefits of a resistance exercise program.

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