Purpose: Dehydration is hypothesized to cause exercise-associated muscle cramps. The theory states that dehydration contracts the interstitial space, thereby increasing the pressure on nerve terminals and cramps ensue. Research supporting this theory is often observational, and fatigue is rarely controlled. Inducing cramps with electrical stimulation minimizes many of the confounding factors associated with exercise-induced cramps (e.g., fatigue, metabolites). Thus, our goal was to minimize fatigue and determine whether hypohydration decreases the electrical stimuli required to elicit cramping (termed "threshold frequency").
Methods: Ten males cycled for 30-min bouts with their nondominant leg at 41°C and 15% relative humidity until they lost ∼3% of their body mass (∼2 h). Dominant leg flexor hallucis brevis muscle cramps were induced before and after hypohydration, and threshold frequency was recorded. Plasma osmolality (OSMp) characterized hydration status. Total sweat electrolytes (Na+, K+, Mg2+, and Ca2+) lost during exercise was calculated. Subjects repeated the protocol 1 wk later.
Results: Subjects were hypohydrated after exercise (preexercise OSMp = 282.5 ± 1 mOsm·kg−1 H2O, postexercise OSMp = 295.1 ± 1 mOsm·kg−1 H2O, P < 0.001). Subjects lost 3.0% ± 0.1% of their body mass, 144.9 ± 9.8 mmol of Na+, 11.2 ± 0.4 mmol of K+, 3.3 ± 0.3 mmol of Mg2+, and 3.1 ± 0.1 mmol of Ca2+. Mild hypohydration with minimal neuromuscular fatigue did not affect threshold frequency (euhydrated = 23.7 ± 1.5 Hz, hypohydrated = 21.3 ± 1.4 Hz; F1,9 = 2.81, P = 0.12).
Conclusions: Mild hypohydration with minimal neuromuscular fatigue does not seem to predispose individuals to cramping. Thus, cramps may be more associated with neuromuscular fatigue than dehydration/electrolyte losses. Health care professionals may have more success preventing exercise-associated muscle cramp by focusing on strategies that minimize neuromuscular fatigue rather than dehydration. However, the effect of greater fluid losses on cramp threshold frequency is unknown and merits further research.