Effects of Hydration Changes on Neuromuscular Function of Athletes

Greater muscular strength and power are relevant qualities for athletic success and decreased injury rate. It is known that dehydration impairs muscular strength and power, although the explanation for this association is not entirely clear. Besides morphological factors, strength production also depends on neural factors which in turn can be affected by dehydration. Some studies tested the effects of dehydration on neuromuscular function using electromyography (EMG) analysis. However, there is no consensus among those studies. Additionally, exercise may disturb water balance. This can further lead to dehydration if the athlete does not properly rehydrate. In this sense, the scientific evidence has identified people who are considered low drinkers that may be more susceptible to cellular shrinkage, potentially impairing health and performance. Thus, it would be expected that athletes regularly exposed to lower amounts of water intake would have beneficial effects in both performance and health if higher water ingestion was promoted, namely an improved neuromuscular function via enhanced cellular hydration. However, any potential benefit of increasing water intake on neuromuscular function is still to be determined using well-designed experimental studies and state-of-the-art methods. Lastly, there is no consensus regarding the diagnosis of dehydration in athletes. The identification of simple indices to measure dehydration in athletes is crucial as many may be inaccurately diagnosed.

Athletes are dependent on muscular strength as it is associated with a higher rate of force development and muscular power, general and specific sports skills performance, and decreased injury rates. There is scientific evidence showing that a hypohydrated state \[i.e., 2 to 3% of body mass loss (BML) attributed to water loss\] impairs muscular strength and power. However, how this reduction affects athletic performance remains in question. We know that muscular strength development is derived from a combination of morphological (muscle cross-sectional area, muscle architecture, and musculotendinous stiffness) and neural factors (motor unit recruitment, synchronization, and firing frequency). Thus, neural factors may be one possible explanation for the effects of dehydration. In fact, there is biological plausibility for this relation as dehydration may affect the electrolyte's concentration (particularly potassium and sodium) within intra- and extracellular spaces, leading to an alteration of the membrane electrochemical potential. Although some studies have tested the effects of hydration changes on neuromuscular function using electromyography (EMG) analysis, there is still no consensus among them. Some authors showed effects of dehydration on muscle endurance and EMG signal, including reduction in EMG mean power frequency (MPF) and an accelerated rate of root-mean square (RMS), possibly meaning reduced membrane excitability and an accelerated central mediated regulation of motor unit activity. While others did not find any effect of dehydration on EMG values. Thus, experimental studies using well-designed trials and state-of-the-art technology are required to better understand the effects of acute dehydration on neuromuscular function, specifically in athletes. Maintenance of a euhydrated state is crucial for the proper physiological functioning of the body, being achieved by physiological and behavioral factors. However, exercise can disturb water balance, particularly when performed in hot environments, increasing water loss. This can further lead to dehydration if the athlete does not properly rehydrate. In this sense, the scientific evidence has identified people who are considered low drinkers (i.e., people who are exposed to a low regular water intake) and high drinkers (i.e., people who are exposed to a high regular water intake). These differences in water intake lead to different physiological responses such as serum arginine vasopressin (AVP) levels and also in mood states. Although no specific total water intake guidelines have been established for athletes, when compared to the European Food Safety Authority guidelines for water intake in healthy adults, they do not meet the guidelines, specifically when higher hydration needs are considered. As mentioned before, AVP has been used to distinguish low drinkers from high drinkers, namely elevated plasma AVP in low drinkers suggesting intracellular dehydration. In fact, changes in total body water (TBW) and its compartments \[i.e., intracellular water (ICW) and the extracellular water (ECW)\] have been studied regarding their impact on sports performance. Silva and colleagues observed that judo athletes who decrease TBW, namely by decreasing ICW, were those that decreased upper-body power, regardless of changes in weight and arms' lean-soft tissue. Also, ICW was the only body water compartment whose reductions explained the higher probability of losing \>2% of forearm maximal strength, independently of changes in weight and arms' lean-soft tissue. Finally, ICW was also considered the main predictor of strength and jumping height over the season in national-level athletes. Thus, ICW and cellular hydration appear to play a relevant role in athletic power and strength, although further research is needed to link these structural fluid compartments with changes in the hydration status and its connection with neuromuscular function. Lastly, hydration testing has been considered a controversial topic and despite existing a substantial body of research, there is no clear protocol regarding the best practice for assessing hydration status in athletes. Moreover, new methods that provide hydration status safely, accurately, reliably, and feasibly are also needed. Bioelectrical Impedance Analysis (BI) is an alternative technique for this specific context. The BI method utilizes the components of impedance: resistance (R) and reactance (Xc). Phase angle (PhA) is also provided, representing a relevant indicator of cellular health and muscle functionality, but research is lacking on the usefulness of this marker for tracking strength/power in athletes exposed to short-term changes in hydration status. To summarize, there is a lack of evidence-based protocols with the state-of-the-art methodology to test the effects of modifying water intake on neuromuscular function using EMG analysis in the athletic population. Moreover, the currently available experimental designs present methodological limitations in assessing hydration status and body water compartments. Hence, to overcome the shortcomings, innovative research with cutting-edge technology is required. Thus, our primary aim is to determine the effects of hydration changes (i.e., a 4-day intervention targeting raises in water intake and acute dehydration) on the strength and power (with EMG analysis for the neuromuscular response) of athletes. Secondary aims include: i) to compare the effects of acute dehydration on neuromuscular function before and after the intervention; ii) to analyze the effects of the intervention on TBW, ECW, ICW, and fat-free mass (FFM) hydration; iii) to analyze the effects of hydration changes (I.e., a 4-day intervention targeting raises in water intake and acute dehydration) on several hydration indexes (serum, saliva, and urine osmolality) and biochemical markers (AVP and sodium concentration); iv) to test the usefulness of segmental and whole-body raw BI parameters in detecting acute dehydration using serum osmolality as the reference technique; v) to explore if PhA can be used as a marker of neuromuscular function;