Corticospinal and peripheral responses to heat-induced hypo-hydration: potential physiological mechanisms and implications for neuromuscular function

The effects of exercise, heat-induced hypo-hydration and rehydration on blood-brain-barrier permeability, corticospinal and peripheral excitability

PURPOSE

The effects of low-intensity exercise, heat-induced hypo-hydration and rehydration on maximal strength and the underlying neurophysiological mechanisms are not well understood.

METHODS

To assess this, 12 participants took part in a randomised crossover study, in a prolonged (3 h) submaximal (60 W) cycling protocol under 3 conditions: (i) in 45 °C (achieving ~ 5% body mass reduction), with post-exercise rehydration in 2 h (RHY2), (ii) with rehydration across 24 h (RHY24), and (iii) a euhydrated trial in 25 °C (CON). Dependent variables included maximal voluntary contractions (MVC), maximum motor unit potential (MMAX), motor evoked potential (MEPRAW) amplitude and cortical silent period (cSP) duration. Blood-brain-barrier integrity was also assessed by serum Ubiquitin Carboxyl-terminal Hydrolase (UCH-L1) concentrations. All measures were obtained immediately pre, post, post 2 h and 24 h.

RESULTS

During both dehydration trials, MVC (RHY2: p < 0.001, RHY24: p = 0.001) and MEPRAW (RHY2: p = 0.025, RHY24: p = 0.045) decreased from pre- to post-exercise. MEPRAW returned to baseline during RHY2 and CON, but not RHY24 (p = 0.020). MEP/MMAX ratio decreased across time for all trials (p = 0.009) and returned to baseline, except RHY24 (p < 0.026). Increased cSP (p = 0.011) was observed during CON post-exercise, but not during RHY2 and RHY24. Serum UCH-L1 increased across time for all conditions (p < 0.001) but was not significantly different between conditions.

CONCLUSION

Our findings demonstrate an increase in corticospinal inhibition after exercise with fluid ingestion, but a decrease in corticospinal excitability after heat-induced hypo-hydration. In addition, low-intensity exercise increases peripheral markers of blood-brain-barrier permeability.

Passive dehydration reduces muscle thickness after resistance exercise

Dehydration-induced increased plasma osmolality (Posmo) alters whole body fluid balance which could alter resistance exercise (RE) induced intramuscular (IM) fluid shift.

PURPOSE

The purpose of the current report was to investigate the effect of dehydration on RE-induced change in whole body fluid balance in resistance trained (RT) men.

METHODS

Fourteen RT men performed two identical RE sessions, either in a hydrated (EUHY) or dehydrated (DEHY) state induced by a 24 hr fluid restriction. Total body fluid, urine osmolality (Uosmo), urine specific gravity (USG), Posmo, hematocrit (HCT), muscle thickness were measured and plasma volume (%ΔPV) was calculated using HCT.

RESULTS

A significant (p < 0.050) condition effect was observed for total body fluid and muscle thickness such that EUHY was 2.6% and 13.0% greater than DEHY, respectively. Significant time × condition effects were observed for Posmo, Uosmo, and USG. At all times, EUHY (collapsing for time: Posmo: 3.9%; Uosmo: 133.5%; USG: 1.6%) was lower than DEHY. A significant time effect was observed for %ΔPV. %ΔPV 5 min after RE was -12.5% lower than PRE.

CONCLUSION

Dehydration with RE altered whole body fluid balance indicated by greater fluid retention and efflux of IM fluid could at least partly maintain %ΔPV following DEHY in RT men.

Influence of sustained mild dehydration on thermoregulatory and cognitive functions during prolonged moderate exercise

PURPOSE

The current study investigated whether sustained mild dehydration affects thermoregulatory function and cognitive performance during prolonged exercise.

METHODS

Twelve young adults performed a test consisting of three sets of 20-min exercise with 2-min intervals under euhydrated (control, CON) and mildly dehydrated conditions (MDEH) at an ambient temperature of 30 °C and 60% relative humidity. MDEH was established by restricting water intake for 24 h, resulting in urine specific gravity of ≥ 1.020. Heart rate (HR), mean arterial blood pressure (MAP), skin blood flow (SkBF), sweat rate (SR) on the chest and forearm, and ear canal and mean skin surface temperatures (Tear and mean Tskin, respectively) were continuously recorded. For each exercise set, thermal and humid sensations and thermal discomfort were assessed using visual analog scales (VAS), and the rating of perceived exertion (RPE) was estimated. Cognitive performance on the Go/No-Go (easy) and incongruent Stroop (difficult) tasks was assessed before and after the test.

RESULTS

No differences were observed in HR, MAP, SkBF, SR, Tear, and mean Tskin between the CON and MDEH. Thermal and humidity sensations, thermal discomfort, and RPE were higher in MDEH than in CON. Moreover, response time to the Stroop task was prolonged in MDEH.

CONCLUSION

These findings suggest that sustained mild dehydration does not affect autonomic thermoregulation during exercise. Augmented thermal perception and perceived exertion, which are necessary for behavioral thermoregulation, were noted; however, cognitive function may be attenuated under MDEH.

Does acute dehydration affect the neuromuscular function in healthy adults?-a systematic review

The effects of acute dehydration on neuromuscular function have been studied. However, whether the mechanisms underpinning such function are central or peripheral is still being determined, and the results are inconsistent. This systematic review aims to elucidate the influence of acute dehydration on neuromuscular function, including a novel aspect of investigating the central and peripheral neuromuscular mechanisms. Three databases were used for the article search: PubMed, Web of Science, and Scopus. Studies were included if they had objective measurements of dehydration, muscle performance, and electromyography data or transcranial magnetic stimulation or peripheral nerve stimulation measurements with healthy individuals aged 18-65 years. Twenty-three articles met the eligibility criteria. The studies exhibited considerable heterogeneity in the methods used to induce and quantify dehydration. Despite being inconsistent, the literature shows some evidence that acute dehydration does not affect maximal strength during isometric or moderate-speed isokinetic contractions. Conversely, acute dehydration significantly reduces maximal strength during slow-speed isokinetic contractions and fatigue resistance in response to endurance tasks. The studies report that dehydration does not affect the motor cortical output or spinal circuity. The effects occur at the peripheral level within the muscle.

Observing an expert's action swapped with an observer's face increases corticospinal excitability during combined action observation and motor imagery

This study aimed to examine whether observing an expert's action swapped with an observer's face increases corticospinal excitability during combined action observation and motor imagery (AOMI). Twelve young males performed motor imagery of motor tasks with different difficulties while observing the actions of an expert performer and an expert performer with a swapped face. Motor tasks included bilateral wrist dorsiflexion (EASY) and unilateral two-ball rotating motions (DIFF). During the AOMI of EASY and DIFF, single-pulse transcranial magnetic stimulation was delivered to the left primary motor cortex, and motor-evoked potentials (MEPs) were obtained from the extensor carpi ulnaris and first dorsal interosseous muscles of the right upper limb, respectively. Visual analogue scale (VAS) assessed the subjective similarity of the expert performer with the swapped face in the EASY and DIFF to the participants themselves. The MEP amplitude in DIFF was larger in the observation of the expert performer with the swapped face than that of the expert performer (P = 0.012); however, the corresponding difference was not observed in EASY (P = 1.000). The relative change in the MEP amplitude from observing the action of the expert performer to that of the expert performer with the swapped face was positively correlated with VAS only in DIFF (r = 0.644, P = 0.024). These results indicate that observing the action of an expert performer with the observer's face enhances corticospinal excitability during AOMI, depending on the task difficulty and subjective similarity between the expert performer being observed and the observer.