Effect of Hypohydration on Peripheral and Corticospinal Excitability and Voluntary Activation

Abstract

We investigated whether altered peripheral and/or corticospinal excitatory output and voluntary activation are implicated in hypohydration-induced reductions in muscle isometric and isokinetic (90°.s−1) strength. Nine male athletes completed two trials (hypohydrated, euhydrated) comprising 90 min cycling at 40°C, with body weight losses replaced in euhydrated trial. Peripheral nerve and transcranial magnetic stimulations were applied during voluntary contractions pre- and 40 min post-exercise to quantify voluntary activation and peripheral (M-wave) and corticospinal (motor evoked potential) evoked responses in m. vastus medialis. Both maximum isometric (−15.3±3.1 vs −5.4±3.5%) and isokinetic eccentric (−24.8±4.6 vs −7.3±7.2%) torque decreased to a greater extent in hypohydrated than euhydrated trials (p<0.05). Half relaxation time of the twitch evoked by peripheral nerve stimulation during maximal contractions increased after exercise in the hypohydrated (21.8±9.3%) but stayed constant in the euhydrated (1.6±10.7%; p = 0.017) condition. M-wave amplitude during maximum voluntary contraction increased after exercise in the heat in hypohydrated (10.7±18.0%) but decreased in euhydrated condition (−17.4±16.9%; p = 0.067). Neither peripheral nor cortical voluntary activation were significantly different between conditions. Motor evoked potential amplitude increased similarly in both conditions (hypohydrated: 25.7±28.5%; euhydrated: 52.9±33.5%) and was accompanied by lengthening of the cortical silent period in euhydrated but not hypohydrated condition (p = 0.019). Different neural strategies seem to be adopted to regulate neural drive in the two conditions, with increases in inhibitory input of either intracortical or corticospinal origin during the euhydrated trial. Such changes were absent in the hypohydrated condition, yet voluntary activation was similar to the euhydrated condition, perhaps due to smaller increases in excitatory drive rather than increased inhibition. Despite this maximal isometric and eccentric strength were impaired in the hypohydrated condition. The increase in peripheral muscle excitability evident in the hypohydrated condition was not sufficient to preserve performance in the face of reduced muscle contractility or impaired excitation-contraction coupling.

Effect of Hypohydration on Muscle Endurance, Strength, Anaerobic Power and Capacity and Vertical Jumping Ability: A Meta-Analysis

“…One likely explanation for these findings is that core temperature may not have been sufficiently different between hydration conditions at the time of testing to render hyperthermia a significant performance-altering factor. Another is that hyperthermia was not considered a confounder if C1 h separated the end of the dehydration protocol from the testing procedures, as suggested by previous studies [13,17,18]. However, it cannot be ruled out that, in certain studies, physiologically significant differences in core temperature between hydration conditions[1 h after dehydration may have been present, which would have confounded our model.…”

“…The latter criteria was based on evidence suggesting that *1 h is needed for core temperature to return to baseline after active [13,18] or passive [17] dehydration undertaken in the heat.…”

“…The magnitude of inconsistency across the findings of studies was quantified using the I 2 statistic, with values \25 % indicating low, 25-50 % moderate, and [50 % high heterogeneity [18]. The 95 % CI for the I 2 statistic was computed according to Higgins and Thompson [28].…”

“…Of the 28 articles retained for analysis, hypohydration was induced via heat exposure in ten [8, 17, 33, 35-38, 40, 42, 44], via exercise plus heat exposure in ten [13,14,18,19,24,25,30,39,41,43], via exercise in a sweat suit in two [34,45], and via fluid restriction in six [7,11,12,20,31,32]. Although Webster et al [12] used exercise in a sweat suit and Judelson et al [11] used exercise plus heat exposure to dehydrate participants, the dehydration mode was All studies except that by Bosco et al [31] provided the percent change in BW (i.e., hypohydration level).…”

Nota técnica: el uso de una camiseta adicional aumenta significativamente el grado de deshidratación durante la práctica del fútbol

“…La relevancia de estos datos consiste en que en el pasado se ha demostrado que los incrementos de la temperatura de la piel afectan negativamente el rendimiento físico aeróbico y este efecto se ve exacerbado por porcentajes de deshidratación cercanos al 2 %, como los aquí reportados (Sawka et al, 2012). Otros efectos negativos de la deshidratación incluyen el deterioro agudo de ciertas funciones cognitivas y alteraciones en el estado de ánimo, cuyos mecanismos fisiológicos se empiezan a dilucidar (Masento, Golightly, Field, Butler, & van Reekum, 2014), además de disminuciones en la capacidad contráctil del músculo esquelético (Bowtell, Avenell, Hunter, & Mileva, 2013). A partir de lo anterior, no se recomienda desde el punto de vista de la termorregulación el uso de una camiseta adicional durante los juegos de fútbol practicados en condiciones ambientales similares a las de esta nota técnica.…”