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American College of Sports Medicine position stand. Exercise and Fluid Replacement

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American College of Sports Medicine position stand. Exercise and Fluid Replacement

Abstract

This Position Stand provides guidance on fluid replacement to sustain appropriate hydration of individuals performing physical activity. The goal of prehydrating is to start the activity euhydrated and with normal plasma electrolyte levels. Prehydrating with beverages, in addition to normal meals and fluid intake, should be initiated when needed at least several hours before the activity to enable fluid absorption and allow urine output to return to normal levels. The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. Because there is considerable variability in sweating rates and sweat electrolyte content between individuals, customized fluid replacement programs are recommended. Individual sweat rates can be estimated by measuring body weight before and after exercise. During exercise, consuming beverages containing electrolytes and carbohydrates can provide benefits over water alone under certain circumstances. After exercise, the goal is to replace any fluid electrolyte deficit. The speed with which rehydration is needed and the magnitude of fluid electrolyte deficits will determine if an aggressive replacement program is merited.
... These biomarkers, in isolation, have considerable limitations (Armstrong et al., 2013). However, when used together in the same context, they can provide very valuable information (American College of Sports Medicine et al., 2007). Urine specific gravity and osmolarity are quantifiable, while colour and volume (Opliger and Bartok, 2002) offer more subjective information (American College or Sports Medicine et al., 2007). ...
... However, when used together in the same context, they can provide very valuable information (American College of Sports Medicine et al., 2007). Urine specific gravity and osmolarity are quantifiable, while colour and volume (Opliger and Bartok, 2002) offer more subjective information (American College or Sports Medicine et al., 2007). Another method is to measure body weight. ...
... Another method is to measure body weight. This method is simple, non-invasive, and is valid for estimating hydration changes in team sports by calculating the difference in pre-and post-activity weight (American College of Sports Medicine et al., 2007;García-Jiménez et al., 2015). Owen et al. (2013) and Sawka et al. (2015) reported dehydration to be a loss of fluid of more than 2% of body weight. ...
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An optimal state of hydration is essential to maintaining health. The objective of this cross-sectional study was to evaluate the water intake of adolescents aged 12 to 16 years and their hydration level during an official soccer match. Three hundred and six players participated in the study (N = 306). Their water intake was recorded and the level of hydration was evaluated using the density of urine as an indicator. Weight measurements were made before and after the match. Water intake control, urine collection and analysis, and the recording of minutes played were carried out after the match. The average weight loss was 746.2 g (SD: 474.07; p < 0.001), with 36.5% with less than 1% loss and 23.3% with more than 2% loss. The mean volume of water ingested was 229.35 ml (SD: 211.11) and a significant correlation was observed between minutes of activity (ρ-value = 0.206; p < 0.001), environmental humidity (ρ-value = - 0.281; p < 0.001), and temperature (ρ-value = 0.200; p < 0.001). The sweat rate was 0.69 l/h (SD: 0.56) and it was significantly associated with playing time (ρ-value = -0.276; p < 0.001). The mean urine density was 1.019 (SD: 0.007), with 64.9% of youth athletes showing dehydration (≥ 1.020). An association was observed between dehydration and activity time (U- value = 4.124; p < 0.001). Approximately 10% of the participants stated that they had not drunk any water during the match. In conclusion, it is necessary to establish individual hydration guidelines based on personal, environmental and activity-related factors, as well as establish a minimum volume of fluids to consume.
... Upon arrival at the laboratory, participants provided a urine sample to determine hydration status (urine specific gravity [ATAGO Uricon-Ne 2722, Fisher Scientific, UK]) (Sawka et al. 2007). Stature and body mass (837 digital scales & 213 portable stadiometer, Seca, Seca Ltd, UK) were then recorded to the nearest 0.01 m and 0.01 kg respectively, prior to body composition being assessed using the bioelectrical impedance method (Tanita BC -418MA, Tanita EU, Netherlands). ...
... Eltek, Cambridge, UK). To ensure euhydration, 150 mL of water was provided to participants every 15 minutes (Sawka et al. 2007). All measures taken throughout the protocol are detailed in Table 1. ...
... All measures taken throughout the protocol are detailed in Table 1. Speed (km·h -1 ) 0 5.1 5.1 5.1 5.1 6.5 6.5 6.5 6.5 6.5 6.5 6.5 6.5 FM After completion of the treadmill protocol, clothed, and nude body mass were measured to determine sweat loss, euhydration was deemed to have been maintained if <1% reduction in nude body mass had occurred (Sawka et al. 2007). Additionally, a post-exercise urine sample was collected and assessed for urine specific gravity. ...
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Treadmill-based load carriage protocols typically use a single fixed speed; however, these are not representative of occupational load carriage tasks. This study aimed to quantify the metabolic, cardiovascular, thermal, neuromuscular, and perceptual responses to a treadmill-based, military-specific, fast load carriage protocol (FLCP). This protocol comprised of carrying 25 kg, for 20 minutes at 5.1 km·h-1; 40 minutes, at 6.5 km·h-1 ; and 8 x 9 s shuttles, at 11 km·h-1 with 11 s recovery. Twelve men (age, 27 ± 6 y; stature, 1.83 ± 0.05 m; body mass, 80.6 ± 8.0 kg; maximal oxygen uptake, 52.7 ± 5.5 mL·kg-1·min-1 ), completed a FLCP during which oxygen consumption (V̇O2), heart rate, core body temperature, and perceptual ratings were recorded. Performance assessments (weighted counter-movement jump [wCMJ], maximal isometric voluntary contraction [MIVC] of the quadriceps, seated medicine ball throw [SMBT]) were completed pre-FLCP, immediately post and, 30, 60, 120 minutes’ post. V̇O2 was similar for 5.1 km·h-1, but increased by 7.4% during the 40 minutes at 6.5 km·h-1 (p = 0.013). Core temperature increased by 0.92 ± 0.22 ºC in response to the FLCP. Post-FLCP, SMBT was not dissimilar across measurement points, (p = 0.315), however, MIVC peak force reduced by 12.6 ± 10.9% 60 minutes postFLCP (p = 0.031), and wCMJ height decreased by 8.7 ± 5.9% 120 minutes post-FLCP (p = 0.011). The completion of the FLCP does not affect upper body power (SMBT), but appears to modestly decrease lower body explosiveness (wCMJ and MIVC) up to two hours’ post. Future investigations can use the FLCP protocol to investigate occupationally relevant scenarios, such as the interaction between physical and cognitive performance during load carriage, or the implications of multiple repeated load carriage bouts.
... 123 Thus, most athletes should include a solution with 0.5-0.7 g/L of sodium in their hydration plan when exercising >1 hour. 111 124 125 Sodium supplementation may be increased to 1.5 g/L for athletes prone to exerciseassociated muscle cramping in the heat, 126 or rather 1.5 g/hour as organoleptic properties of beverages are affected if the sodium concentration is >1 g/L. As for postexercise recovery, this can be achieved through a combination of fluids and solid foods. ...
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This document presents the recommendations developed by the IOC Medical and Scientific Commission and several international federations (IF) on the protection of athletes competing in the heat. It is based on a working group, meetings, field experience and a Delphi process. The first section presents recommendations for event organisers to monitor environmental conditions before and during an event; to provide sufficient ice, shading and cooling; and to work with the IF to remove regulatory and logistical limitations. The second section summarises recommendations that are directly associated with athletes’ behaviours, which include the role and methods for heat acclimation; the management of hydration; and adaptation to the warm-up and clothing. The third section explains the specific medical management of exertional heat stroke (EHS) from the field of play triage to the prehospital management in a dedicated heat deck, complementing the usual medical services. The fourth section provides an example for developing an environmental heat risk analysis for sport competitions across all IFs. In summary, while EHS is one of the leading life-threatening conditions for athletes, it is preventable and treatable with the proper risk mitigation and medical response. The protection of athletes competing in the heat involves the close cooperation of the local organising committee, the national and international federations, the athletes and their entourages and the medical team.
... They were then sent a photocopy of the food log which was used to mimic their diets prior to their second experimental trial. Baseline hydration status was estimated from a spot urine collection immediately prior to exercise (USG ≤ 1.020) using a handheld refractometer (Cole-Parmer, RSA-BR90A, Vernon Hills, IL) (American College of Sports Medicine et al. 2007). The participants were then asked to obtain a nude body weight, insert a rectal thermistor (Level 1 esophageal/rectal temperature probe, Smiths Medical, Minneapolis, MN, USA), and were fitted with a heart rate monitor (Polar H1, Kempele, Finland). ...
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Purpose The purpose of this study was to determine the effect of prolonged high-intensity interval (INT) and moderate-intensity continuous (CONT) treadmill exercise in the heat on markers of enterocyte injury and bacterial endotoxin translocation. Methods Nine males completed 2 h of work-matched exercise in the heat (40 °C and 15% RH) as either INT (2 min at 80% VO2max and 3 min at 30% VO2max) or CONT (~ 50% of VO2max). Blood samples collected pre- and post-exercise were assayed for intestinal fatty acid-binding protein (I-FABP), claudin-3 (CLDN-3), and lipopolysaccharide-binding protein (LBP). Results I-FABP was significantly increased from pre- to post-exercise in CONT (913.96 ± 625.13 to 1477.26 ± 760.99 pg•mL⁻¹; p = 0.014, d = 0.766) and INT (714.59 ± 470.27 to 1547.93 ± 760.99 pg•mL⁻¹; p = 0.001, d = 1.160). Pre- to post-exercise changes in I-FABP were not different between CONT and INT (p = 0.088, d = 0.414). LBP was significantly increased from pre- to post-exercise in INT (15.94 ± 2.90 to 17.35 ± 3.26 μg•mL⁻¹; p = 0.028, d = 0.459) but not CONT (18.11 ± 5.35 to 16.93 ± 5.39 μg•mL⁻¹; p = 0.070, d = 0.226), and pre- to post-exercise changes in LBP were higher in the INT compared to CONT (p < 0.001, d = 1.160). No significant changes were detected from pre- to post-exercise for CLDN-3 in CONT (14.90 ± 2.21 to 15.30 ± 3.07 μg•mL⁻¹) or INT (15.55 ± 1.63 to 16.41 ± 2.11 μg•mL⁻¹) (p > 0.05). Conclusions We conclude that prolonged exercise in the heat induces enterocyte injury, but interval (or intermittent) exercise may cause greater bacterial endotoxin translocation which may increase the risk for local and systemic inflammation.
... Maintenance of body fluid balance is important to mitigate elevations of body core temperature and cardiovascular strain during exercise in the heat (Hamilton et al. 1991;Shirreffs 2005). To maintain fluid balance, consumption of sports drinks containing carbohydrate and electrolytes is recommended to reduce dehydration caused by excessive sweating during an exercise-heat stress (Sawka et al. 2007;Coombes and Hamilton 2000). Carbohydrate-electrolyte beverages typically contain a mix of carbohydrates including glucose, fructose, sucrose, and maltodextrin, all of which are absorbed quickly into the blood stream, maintaining substrate availability to preserve exercise performance. ...
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Purpose Isomaltulose is a low glycemic and insulinaemic carbohydrate increasingly used as an alternative sweetener in commercial beverages. While isomaltulose beverages can improve hydration status compared to sucrose-based beverages, it remains unclear if ingestion of an isomaltulose beverage prior to exercise in the heat may improve plasma volume (PV) and thermoregulatory responses. Methods Twelve endurance-trained men consumed a 1L carbohydrate beverage containing either 6.5%-sucrose (SUC) or 6.5%-isomaltulose (ISO) 60 min prior to 5 successive, 15-min bouts of moderate-intensity (60% of their pre-determined maximum oxygen uptake) in the heat (32 °C, 50% relative humidity), each separated by a 5 min rest. A 6th bout was performed, wherein the participant adjusted running speed to maximize distance covered within the 15-min period. The change (Δ) in PV, heart rate (HR), body core (rectal and gastrointestinal) and skin temperatures, and whole-body sweat loss were assessed during each exercise bout. Results Ingestion of ISO induced a higher ΔPV at 4th bout only (P < 0.001) and lower HR (P = 0.032, main effect of beverage) during exercise compared to those of SUC. Body core and skin temperatures and whole-body sweat loss did not differ between conditions (all P ≥ 0.192, interaction effect). Running distance covered in final exercise bout tended to increase (~ 5%) in ISO versus SUC (P = 0.057, d = 0.64). Conclusions Relative to a sucrose-based beverage, ISO ingestion prior to exercise in the heat reduced cardiovascular strain by preserving PV and attenuating HR, albeit with no corresponding benefit on thermoregulatory function. The former response may facilitate improvements in exercise performance.
... One participant passed the 2% BML threshold at a projected 8 hr. Further, if the NIOSH recommendations on water consumption pattern of drinking at 15 min intervals were followed instead of drinking every 20 min as in the present study, this participant would likely have gained even more mass and increased the risk of hyponatremia (Sawka et al. 2007), considering that electrolytes were not replaced during the 2 hr of work. This further highlights the need for an individualized hydration strategy. ...
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The National Institute for Occupational Safety and Health recommendations for work in the heat suggest workers consume 237 mL of water every 15-20 min and allow for continuous work at heavy intensities in hot environments up to 34 °C and 30% relative humidity. The goal was to determine whether the National Institute for Occupational Safety and Health recommendations prevented core temperature from exceeding 38.0 °C and greater than 2% body mass loss during heavy-intensity work in the heat. Eight males consumed 237 mL of water every 20 min during 2 hours of continuous heavy-intensity walking (6.4 kph, 1% grade) in a 34 °C/30% relative humidity environment, in accordance with the National Institute for Occupational Safety and Health recommendations. Projected core temperature and percent body mass loss were calculated for 4 and 8 hr of continuous work. Core temperature rose from baseline (36.8 ± 0.3 °C) to completion of 2 hr of work (38.1 ± 0.6 °C, p < 0.01), with two participants reaching the 38.0 °C threshold. Projected core temperatures remained elevated from baseline (p < 0.01), did not change from 2 to 4 hr (38.1 ± 0.7 °C, p > 0.99) and 4 to 8 hr (38.1 ± 0.8 °C, p > 0.99), respectively, and one participant exceeded 38.0 °C at 4 to 8 hr. There was no change in body mass loss over time (p > 0.99). During two hours of continuous heavy-intensity work in the heat, 75% of participants did not reach 38 °C core temperature and 88% did not reach 2% body mass loss when working in accordance with National Institute for Occupational Safety and Health recommendations.
... From 1996 to 2006, the American College of Sports Medicine recommended that "During exercise, athletes should consume fluids at a rate sufficient to replace all the water lost through sweating or consume the maximal amount that can be tolerated (Convertino et al., 2006). In 2007, they reported that aerobic exercise performance could be impaired when dehydration exceeds >2% of body weight, and that exercise in warm environments could lead to more water loss (Sawka et al., 2007). Generally, sudden changes of >2% of body weight result in a loss of body fluids, which in turn impairs endurance. ...
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Competitions in combat sports are divided into certain weight classes in order to compete fairly among athletes of similar body weight. However, athletes lose weight compulsorily or tactically (to encounter weaker opponents, to be between two weights, to be faster and more agile, etc.). In many scientific studies, it is seen that the athletes lose their weight close to the competition scale, and generally they lose a lot of weight on the last day. In addition, it is stated that the rate of weight loss varies between 2-10% of the body weight of the athletes. Many methods are used to set weight before the competition scale. These; Reducing energy intake, reducing fluid consumption, not consuming anything especially the day before the competition scale, increasing the duration and intensity of exercise, loss of fluid with intense sweating, use of sauna, use of diuretic drugs. As the use of these harmful methods and the rate of weight loss increase, their negative effects on physical, physiological, psychological and performance increase at the same rate. In this study, studies in the literature on the effects of weight loss on strength, endurance, anaerobic performance, reaction and balance performance will be examined and the causes of possible effects will be determined.
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Whether at school, work, or running errands, it is common to see people sipping on their water bottles. This fashionable trend is a healthy one because water is essential for life. But while most people know that drinking water is good for them, many misconceptions exist about water and hydration. Learn the truth behind some of the most common hydration myths. This 4-page fact sheet was written by Lauren Caruso, Karla P. Shelnutt, and Gail Kauwell, and published by the UF Department of Family, Youth and Community Sciences, August 2014.
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New findings: In pennate muscle, changes in myofiber cross-sectional area (fCSA), fascicle length (Lf ), and pennation angle (PA) with exercise training likely interact to alter whole-muscle cross-sectional area (mCSA). Herein, we are the first to use multiple regression to show that changes in vastus lateralis (VL) mean fCSA, Lf and PA following a period of resistance training did not collectively predict changes in mCSA. Thus, the n-size is limited herein, it remains difficult to generalize the morphological adaptations that predominantly drive tissue-level VL muscle hypertrophy. We also present compelling evidence suggesting the mode of hypertrophy differs between individuals, which requires further interrogation. Abstract: Myofiber This article is protected by copyright. All rights reserved.
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Background Sports science research in elite female Gaelic team sports has increased in recent years, but still a large disparity exists between the volume of studies involving male and female players. As a consequence of this, it is difficult for practitioners to develop an evidence-based approach when working with female players. Main body In this review, we discuss the current research available in elite female Gaelic team sports with focus on seven specific areas including physical and physiological demands, anthropometric and performance characteristics, injury risk, nutritional considerations, and female physiology. There appears to be unique physical demands data in match play across positions in Camogie, however, there is currently no comparative data available in ladies Gaelic football. Similarly, there is no research available on the physiological demands of both elite female Gaelic team sports. According to existing literature, performance characteristics such as speed and power are lower in this population compared to other elite female team sports. Although data is limited, the anthropometric characteristics of elite female Gaelic team sport players appear homogenous with some positional differences observed at a sub-elite level. Previous research has demonstrated a high prevalence of lower limb injuries in female elite Gaelic team sports and the provision of quality, evidence-based strength & conditioning could help mitigate these injury risks. Female Gaelic team sport players have been shown to have poor nutrition knowledge and inadequate intakes of micronutrients. Finally, although menstrual cycle phase and oral contraceptives have been shown to influence performance in other female intermittent sports, to date there has not been any research carried out in elite female Gaelic team sport players. Conclusions It is evident that limited research has been carried out on elite female Gaelic sport players. More up-to-date, high-quality investigations are needed to address the research gaps, which in turn should enable practitioners in the field to apply sound, evidence-based practice/theory when working with this population.
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It is the position of the American College of Sports Medicine that adequate fluid replacement helps maintain hydration and, therefore, promotes the health, safety, and optimal physical performance of individuals participating in regular physical activity. This position statement is based on a comprehensive review and interpretation of scientific literature concerning the influence of fluid replacement on exercise performance and the risk of thermal injury associated with dehydration and hyperthermia. Based on available evidence, the American College of Sports Medicine makes the following general recommendations on the amount and composition of fluid that should be ingested in preparation for, during, and after exercise or athletic competition: 1. It is recommended that individuals consume a nutritionally balanced diet and drink adequate fluids during the 24-h period before an event, especially during the period that includes the meal prior to exercise, to promote proper hydration before exercise or competition. 2. It is recommended that individuals drink about 500 ml (about 17 ounces) of fluid about 2 h before exercise to promote adequate hydration and allow time for excretion of excess ingested water. 3. During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated. 4. It is recommended that ingested fluids be cooler than ambient temperature[between 15° and 22°C (59° and 72°F)] and flavored to enhance palatability and promote fluid replacement. Fluids should be readily available and served in containers that allow adequate volumes to be ingested with ease and with minimal interruption of exercise. 5. Addition of proper amounts of carbohydrates and/or electrolytes to a fluid replacement solution is recommended for exercise events of duration greater than 1 h since it does not significantly impair water delivery to the body and may enhance performance. During exercise lasting less than 1 h, there is little evidence of physiological or physical performance differences between consuming a carbohydrate-electrolyte drink and plain water. 6. During intense exercise lasting longer than 1 h, it is recommended that carbohydrates be ingested at a rate of 30-60 g · h-1 to maintain oxidation of carbohydrates and delay fatigue. This rate of carbohydrate intake can be achieved without compromising fluid delivery by drinking 600-1200 ml· h-1 of solutions containing 4%-8% carbohydrates (g · 100 ml-1). The carbohydrates can be sugars (glucose or sucrose) or starch (e.g., maltodextrin). 7. Inclusion of sodium (0.5-0.7 g · 1-1 of water) in the rehydration solution ingested during exercise lasting longer than 1 h is recommended since it may be advantageous in enhancing palatability, promoting fluid retention, and possibly preventing hyponatremia in certain individuals who drink excessive quantities of fluid. There is little physiological basis for the presence of sodium in an oral rehydration solution for enhancing intestinal water absorption as long as sodium is sufficiently available from the previous meal.
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Objective: To record weight changes, fluid intake and changes in serum sodium concentration in ultradistance triathletes. Design: Descriptive research. Setting: Ironman triathlon (3.8 km swim, 180 km cycle, 42.2 km run). Air temperature at 1200 h was 21°C, (relative humidity 91%). Water temperature was 20.7°C. Participants: 18 triathletes. Interventions: None. Main Outcome Measures: Subjects were weighed and had blood drawn for serum sodium concentration [Na], hemoglobin, and hematocrit, pre-race, post-race, and at 0800 h on the morning following the race (recovery); subjects were also weighed at transitions. Fluid intake during the race was estimated by athlete recall. Results: Median weight change during the race = -2.5 kg (p < 0.0006). Subjects lost weight during recovery (median = -1.0 kg) (p < 0.03). Median hourly fluid intake = 716 ml/h (range 421-970). Fluid intakes were higher on the bike than on the run (median 889 versus 632 ml/h, p = 0.03). Median calculated fluid losses cycling were 808 ml/h and running were 1,021 ml/h. No significant difference existed between pre-race and post-race [Na] (median 140 versus 138 mmol/L) or between post-race and recovery [Na] (median 138 versus 137 mmol/L). Plasma volume increased during the race, median + 10.8% (p = 0.0005). There was an inverse relationship between change in [Na] pre-race to post-race and relative weight change (r = -0.68, p = 0.0029). Five subjects developed hyponatremia ([Na] 128-133 mmol/L). Conclusions: Athletes lose 2.5 kg of weight during an ultra-distance triathlon, most likely from sources other than fluid loss. Fluid intakes during this event are more modest than that recommended for shorter duration exercise. Plasma volume increases during the ultradistance triathlon. Subjects who developed hyponatremia had evidence of fluid overload despite modest fluid intakes.
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While exercise heat stress and hydration status are known to independently influence heart rate variability (HRV), the combined effect of these physiological stressors is unknown. Thus, heat-acclimated subjects (n=5) performed exercise heat trials (40 °C, 20% relative humidity) in the euhydrated and hypohydrated state (3.9±0.7% body weight loss). During each trial, cardiac cycle R–R interval data were collected for 45 min at rest (pre-) and after (post-) completing 90 min of cycle ergometer exercise. Pre- and post-exercise RRI data were analyzed by Fast Fourier Power Spectral analysis to determine the high-frequency (HF), low-frequency (LF), very low-frequency (VLF), and total power (TP) components of HRV. Overall HRV was decreased by both hypohydration and exercise heat stress. Hypohydration reduced TP, LF, VLF, and LF:HF ratio (P<0.05) while HF was significantly higher. The change in both LF and HF power (pre- vs. post-exercise) were blunted during hypohydration compared to euhydration. These data suggest that dehydration alone positively influences the parasympathetic (HF) control of HRV, but the reduction in overall HRV and the blunted oscillations in LF and HF power following exercise heat stress support an overall deleterious effect of dehydration on autonomic cardiac stability.
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Dehydration by means of exercise, heat, diuretics, semistarvation, or a combination of these is common practice among competitors in weight class sports. Many studies have demonstrated a reduced aerobic work capacity following each of these forms of dehydration. The effects of these practices on performance that requires energy derived primarily from anaerobic sources is not well documented. The purpose of this study was to examine the effects of progressive, acute, thermal dehydration on performance of an anaerobic criterion task. Eleven collegiate wrestlers performed the Wingate Anaerobic Test (WAnT) prior to and after each of the following mean weight losses: 2%, 4%, and 5%. Weight loss was induced by passive thermal dehydration (56°C, 15% RH). Approximately 2 h were required in the environmental chamber to lose the required weight at each stage. There was no significant change (P > 0.05) in the ability to perform the WAnT or its various indices at any stage of dehydration, nor were blood lactate concentrations post WAnT significantly different from predehydration levels. This suggests that anaerobic performance may not be impaired to the extent that aerobic performance is by passive, thermal dehydration to a 5% body weight loss. However, deleterious physiologic effects may result from dehydration practices even though performance levels are maintained.
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To determine the differences in sweat composition between sweat induced by thermal stress alone and that induced by physical exercise, seven young healthy men first sat in a hot room and then performed running exercise. A 20-minute stay in a climate chamber at 40°C resulted in a 5% reduction in body weight. The same body weight loss was induced by running exercise. Both sodium and chloride concentrations were much lower in the sweat induced by thermal exposure than that induced by the running exercise (p<0.01), while urea nitrogen and creatinine concentrations were significantly higher after thermal exposure than after the running exercise (p<0.01). Potassium concentrations did not differ significantly with either procedure. These findings suggest that sweat composition varies with the kind of induction and that more salt seems to be lost through exercise-induced sweating than by just sitting in a hot environment.
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The aim of this study was to examine if the pattern of fluid intake with a carbohydrate-electrolyte solution during 4 h recovery from prolonged, submaximal running would influence the subsequent endurance capacity. Seven well-trained athletes aged 19.8 +/- 0.3 years (mean +/- s(mean)) took part in the study, which had university ethical committee approval. They ran at 70% VO2 max on a level treadmill for 90 min (T1), or until volitional fatigue, whichever came first, on two occasions, at least 7-10 days apart. Four hours later, the subjects ran at the same speed for as long as possible (T2), as a measure of their endurance capacity. During the 4 h rehydration recovery period, the runners were allowed to drink a carbohydrate-electrolyte solution (6.9% Lucozade-Sport; sodium, 24 mmol l(-1); potassium, 2.6 mmol l(-1); calcium, 1.2 mmol l(-1); osmolality, 300 mOsm kg(-1)) ad libitum on one occasion. On the other occasion, the volume of the same fluid was prescribed from calculations of the body mass loss during T1 (2.6% of pre-exercise body mass). All subjects completed the 90 min run during T1 on both trials. However, during T2, in the prescribed intake trial, the exercise time to exhaustion was 16% longer (P< 0.05) than during T2 in the ad libitum trial (69.9 +/- 9.1 vs 60.2 +/- 10.2 min). Although there was no difference between conditions in the total volume ingested (1499 +/- 155 vs 1405 +/- 215 ml), the volume of carbohydrate-electrolyte solution ingested in the fourth hour of the rehydration recovery period was greater in the prescribed intake trial than in the ad libitum trial (258 +/- 52 vs 78 +/- 34 ml; P< 0.05). The amount of glucose ingested in this period during the prescribed intake trial was also greater than during the ad libitum trial (17.8 +/- 3.6 vs 5.4 +/- 2.4 g; P< 0.05). There was a higher blood lactate concentration at the start of T2 in the prescribed intake trial than in the ad libitum trial (1.12 +/- 0.20 vs 0.94 +/- 0.09 mmol l(-1); P< 0.05), but there were no differences in blood glucose, plasma insulin, free fatty acid concentrations or urine volume between trials. The results of this study suggest that drinking a prescribed volume of a carbohydrate-electrolyte solution after prolonged exercise, calculated to replace the body fluid losses, restores endurance capacity to a greater extent than ad libitum rehydration during 4 h of recovery, even though the total volumes ingested were the same between trials.
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To examine the effects of rapid dehydration on isometric muscular strength and endurance, seven men were tested at baseline (control) and after a dehydration (dHST) and a euhydration (eHST) heat stress trial. The dHST consisted of intermittent sauna exposure until 4% of body mass was lost, whereas the eHST consisted of intermittent sauna exposure (same duration as dHST) with water replacement. Peak torque was determined for the knee extensors and elbow flexors during three isometric maximal voluntary contractions. Time to fatigue was determined by holding a maximal voluntary contraction until torque dropped below 50% peak torque for 5 s. Strength and endurance were assessed 3.5 h after the HSTs (no food or water intake). Body mass was decreased 3.8+/-0.4% post dHST and 0.4+/-0.3% post eHST. Plasma volume was decreased 7.5+/-4.6% and 5.7+/-4.4%, 60 and 120 min post dHST, respectively. A small (1.6 mEq x L[-1]) but significant increase was found for serum Na+ concentration 60 min post dHST but had returned to predehydration level 120 min post dHST. Serum K+ and myoglobin concentrations were not affected by HSTs. Peak torque was not different (P > 0.05) among control, dHST, and eHST for the knee extensors (Mean (Nm)+/-SD, 285+/-79, 311+/-113, and 297+/-79) and elbow flexors (79+/-12, 83+/-15, and 80+/-12). Time to fatigue was not different (P > 0.05) among control, dHST and eHST for the knee extensors (Mean (s)+/-SD. 42.4+/-11.5, 45.3+/-7.6, and 41.8+/-6.0) and elbow flexors (48.2+/-8.9, 44.0+/-9.4, and 46.0+/-6.4). These results provide evidence that isometric strength and endurance are unaffected 3.5 h after dehydration of approximately 4% body mass.
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