The Effects of Passive Dehydration on Neuromuscular Function

Abstract

Purpose: The purpose of this study was to compare the effects of passive dehydration on neuromuscular function, specifically analyzing maximal voluntary contraction(MVC) peak torque, mean firing rates (MFR) vs. recruitment threshold (RT), motor unit action potential amplitudes (MUAPAMP) vs RT, recruitment ranges, muscle cross sectional area (mCSA), subcutaneous fat and echo intensity of the vastus lateralis (VL). Methods: Nine recreationally trained males (age= 26.89 ± 3.72 yr, weight= 95.52 ± 17.07 kg, height= 180.60 ± 3.76 cm) completed a familiarization and 3 consecutive experimental visits. Participants were euhydrated on visit 1, hypohydrated by 5% of their body mass for visit 2 and rehydrated back to baseline body mass for visit 3. Hypohydration was achieved through fluid restriction and fasting and passive sweat loss in a hot, humid environment room. Each experimental visit consisted of participants completing two isometric MVCs measured by an isokinetic dynamometer, with strong verbal encouragement. The highest peak torque was used for a 70% MVC isometric muscle action. Surface electromyography signals were collected from the VL during the 70% MVC isometric muscle action and each MU was analyzed for MUAPAMP and MFR during the steady force and force at recruitment. Ultrasonography was used in B mode to measure mCSA, subcutaneous fat, and echo intensity of the VL. Urine and blood samples were collected for hydration measurements. Heart rate was measured at the beginning, during and at the end of each visit. RESULTS: Participants experienced significant dehydration from visit 1 to visit 2, verified by Usg (visit 1 = 1.014 ± 0.007 vs. visit 2 = 1.034 ± 0.004, p < 0.001) and body mass loss (visit 1 = 95.5 ± 17.1 kg vs. visit 2 = 90.9 ± 16.4 kg, p < 0.001). Participants reached baseline body mass by visit 3 (95.6 ± 17.2 kg), showing no significant differences between body mass between hydration visit 1 or visit 3 (p= 1.00). Usg indicated participants were euhydrated (1.019 ± 0.006), and plasma volume significantly increased from visit 2 to 3 (50.3 ± 3.4 to 56.0 ± 4.7, p = 0.021). There was no significant difference in Usg between visit 1 and 3 (p = 0.675). HR was significantly higher visit 2 (92 ± 11 bpm) compared to visit 1 (66 ± 6 bpm, p< 0.001) and visit 3 (71 ± 12 bpm, p= 0.024). HR was also significantly higher post- strength testing visit 2 than visit 1 (94 ± 15 bpm and 76 ± 10 bpm, p= 0.023). There was no significant difference in HR visit 1 to visit 3 pre- (p= 0.597) or post- (p= 1.00) strength testing, or post- strength testing visit 2 to visit 3 (p= 0.083). MVC peak torque significantly decreased from visit 1 (263.6 ± 59.6 Nm) to visit 2 (241.5 ± 59.1 Nm), p = 0.047. Visit 2 mCSA was significantly smaller (31.4 ± 7.4 cm2) compared to visit 1 (34.0 ± 7.2 cm2, p = 0.003) and visit 3 (35.2 ± 7.6 cm2, p = 0.003). There were no significant differences in MFR vs RT or MUAPAMP vs RT relationships, recruitment ranges, subcutaneous fat, or echo intensity. HR) immediately after becoming hypohydrated = 128.9 ± 20 bpm. Conclusion: Extreme hypohydration resulted in significantly decreased MVC peak torque and mCSA and increased cardiovascular strain; however, there were no significant changes in MU behavior, subcutaneous fat or echo intensity. It is important for individuals to attenuate the effects hypodration has on the body such as decreased peak torque, decreased muscle size and increased cardiovascular strain especially during exercise or performance. Through maintaining euhydration and rehydrating after acute bouts of dehydration it will help prevent other serious complications such as exertional heath illness.