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;