Voluntary dehydration and electrolyte losses during prolonged exercise in the heat

The effects of water temperature (6 degrees, 22 degrees, 46 degrees C) and chlorination on voluntary dehydration (D), sweat electrolyte losses (SEL), and total body electrolyte losses (BEL) were studied in 12 healthy males during 6 h of intermittent treadmill exercise (1.34 m X s-1, 5% grade) in a climatic chamber (40.6 degrees C DB, 25.5 degrees C WB). Body weight (BW), rectal temperature (Tre), mean weighted skin temperature (Tsk), heart rate (HR), plasma osmolality (PO), sweat rate (SR), sweat sodium (Na+), chloride (Cl-), potassium (K+), and magnesium (MG++), urine volume, Na+, and K+ were measured. No differences were found between chlorinated and non-chlorinated treatments except SEL of Mg++. Subjects (Ss) who drank 46 degrees C (-2.1% BW) consumed approximately 50% less water (p less than 0.001), and had D which was 1.050 kg larger (p less than 0.001) than subjects who consumed 6 degrees C (-0.5 %BW). There were no significant between-group PO differences, but Tre and Tsk differed between 46 degrees and 6 degrees C (p less than 0.01), and the HR of 22 degrees and 46 degrees C were both different from 6 degrees C (p less than 0.05). SR of all groups were essentially equal, although differences in total sweat Na+ (p less than 0.02) and Cl- (p less than 0.04) losses were observed between 46 degrees and 6 degrees C. SEL of sweat K+ and Mg++ were not affected by the experimental design. Based on 24 h projections of BEL, it was concluded that K+ depletion is more likely than Na+ depletion because food is often supplemented with sodium chloride.(ABSTRACT TRUNCATED AT 250 WORDS)     

RPE drift during cycling in 18 degrees C vs 30 degrees C wet bulb globe temperature

AIM: The potential influence of a hotter vs cooler environment on ratings of perceived exertion (RPE) estimations during longer duration exercise is not well-understood. This study compared overall and differentiated RPEs during cycling in 18 degrees C vs 30 degrees C wet bulb globe temperature (WBGT). METHODS: Male volunteers (n=16) completed a maximal cycling trial (60 rev . min(-1), 25 Watts . min(-1)) to determine VO(2) max and ventilatory threshold (VT) before completing 2 (counterbalanced) longer duration cycling trials. At 30 degrees C WBGT (30C) and 18 degrees C WBGT (18C), subjects cycled 60 min (60 rev . min(-1), 90% individualized VT). Heart rate (HR, b . min(-1)) and rectal temperature (Tre, degrees C) were recorded every 5 min with corresponding RPE-overall (RPE-O), RPE-legs (RPE-L) and RPE-chest (RPE-C) estimations. RESULTS: HR was not significantly different at 5 min but was greater (P<0.05) for 30C at all other time points. During 30C, Tre was significantly greater (25, 30, 35, 40, 45, 50, 55 and 60 min), RPE-O was significantly greater (5, 40, 45, 50, 55 and 60 min), RPE-L was significantly greater (55 and 60 min) and RPE-C was significantly greater (35, 40, 45, 50, 55 and 60 min). CONCLUSIONS: Greater cardiovascular (HR) and thermal (Tre) strain partially explain greater perceptual ratings during 30C. Discernible RPE differences resulted mid-way through 60 min cycling with minimal differences initially. Results suggest RPEs are magnified in a 30 degrees C (vs 18 degrees C) environment beyond 30 min duration. Additionally, a 30 degrees C environment resulted in a less pronounced impact on RPE-L (vs RPE-C and RPE-O).     

Neutrophil degranulation response to 2 hours of exercise in a 30 degrees C environment

INTRODUCTION: Evidence supports an interaction between neuro-endocrine responses to exercise and immune responses to exercise. We hypothesized that prolonged exercise in the heat would evoke a greater stress hormone response and a greater decrease in neutrophil degranulation [lipopolysaccharide (LPS)-stimulated elastase release] than when the same exercise was performed in thermoneutral conditions. METHODS: In counterbalanced order and separated by 7 d, 13 male cyclists cycled for 2 h at 62 +/- 3% VO2max (mean +/- SEM), with ad libitum water intake, on one occasion with heat (HOT: 30.3 degrees C, 76% RH) and on another occasion without (CONTROL: 20.4 degrees C, 60% RH). Venous blood samples were collected at pre-, post-, and 2 h post-exercise. RESULTS: Exercising HR, rating of perceived exertion, rectal temperature, corrected body mass loss, and plasma cortisol at post- and 2 h post-exercise were greater during HOT. A marked neutrophilia was evident at post- and 2 h post-exercise with no difference between trials. LPS-stimulated elastase release per neutrophil decreased post-exercise with no difference between trials (pre-exercise: HOT 189 +/- 20 and CONTROL 210 +/- 32; post-exercise: HOT 127 +/- 18 and CONTROL 136 +/- 29 fg x cell(-1)). There was no effect of exercise or trial on neutrophil CD11b expression (neutrophil activation index) or band cell percentage (neutrophil maturity index). CONCLUSIONS: Prolonged exercise results in a decrease in neutrophil degranulation that is unaffected by performing the exercise in hot conditions despite the increase in physiological stress. Additionally, these data suggest that the decrease in neutrophil degranulation after prolonged exercise is not associated with a change in neutrophil activation or maturity as previously suggested.     

The effect of training on the norepinephrine response at rest and during exercise in 5 degrees and 20 degrees C environments

Eleven males were examined at rest and during submaximal exercise in 5 degrees C and 20 degrees C environments to determine if the norepinephrine (NE) and other physiological responses in the cold would be altered by eight weeks of training. Blood samples were obtained at the end of 15 minutes of rest and submaximal exercise, and were assayed for NE. Pretraining resting NE levels in the 5 degrees C condition were significantly higher than those found in the 20 degrees C environment (684 +/- 89 vs 491 +/- 48 pg/ml). A significant training effect reduced resting NE levels in the 5 degrees C (502 +/- 77 pg/ml) but not the 20 degrees C (392 +/- 45 pg/ml) condition. Pre and posttraining exercise NE levels were elevated above resting in both the 5 degrees C (1477 +/- 136 vs 1559 +/- 208) and the 20 degrees C environments (1623 +/- 176 pg/ml vs 1444 +/- 224 pg/ml), but were not significantly different between conditions. Skin temperatures were significantly lower, and resting blood pressure was significantly higher in the 5 degrees C condition. These data suggest that both cold and exercise act as stimulators of NE release, but an additive effect on NE of cold and exercise does not occur. The resting NE levels pre and posttraining in the 5 degrees C condition suggest that a cross tolerance to cold stress was present.     

Cardiovascular responses during prolonged exercise at ventilatory threshold in boys and men

PURPOSE: The purpose of this study was to examine the cardiovascular responses during prolonged exercise in boys and men at an intensity set relative to ventilatory threshold (VT). METHODS: Eight boys (10-13 yr) and 10 men (18-25 yr) completed an orientation trial, a maximal exercise test, and a 40-min submaximal exercise bout at an intensity equal to the VO2 at VT (approximately 64.5% VO2max). RESULTS: Heart rate (HR) was higher and stroke volume (SV) was lower in the boys compared with the men (P < or = 0.05). From 10 to 40 min, HR significantly increased 9.5% and 13.6% and SV significantly decreased 8.8% and 11.6% in the boys and men, respectively. Despite the tendency for the changes in HR and SV to be greater in the men, the group-by-time interaction was not significant. Cardiac output was greater in the men (P < or = 0.05) but remained constant over time (P > 0.05). In men, mean arterial blood pressure was higher (P < or = 0.05) and decreased 4.2% over time. In boys, mean arterial blood pressure remained constant, which resulted in a significant group-by-time interaction. Total peripheral resistance (TPR) was significantly higher in the boys and remained constant over time (P > 0.05). From 0 to 40 min, the decrease in plasma volume was significantly greater in the men (-10.2%) than the boys (-5.7%) but was unrelated to the changes in SV in either group (P > 0.05). CONCLUSION: In conclusion, the cardiovascular responses during prolonged exercise are similar in boys and men, although there is a tendency for the magnitude of cardiovascular drift to be greater in the men.     

Cardiovascular fitness and thermoregulation during prolonged exercise in man.

Nine healthy male subjects differing in their training status (VO2 max 54 +/- 7 ml.min-1.kg-1, mean +/- SD; 43-64 ml.min-1 kg-1, range) exercised on two occasions separated by one week. On each occasion, having fasted overnight, subjects exercised for 1 h on an electrically braked cycle ergometer at a workload equivalent to 70 per cent VO2 max (test A) or at a fixed workload of 140 W (test B). Each test was assigned in a randomized manner and was performed at an ambient temperature of 22.5 +/- 0.0 degrees C and a relative humidity of 85 +/- 0 per cent. Absolute exercise workload was the most successful predictor of sweat loss during test A (r = 0.82, p less than 0.01). Sweat loss was also related to VO2 max tests A (r = 0.67, p less than 0.05) and B (r = 0.67, p less than 0.05). There was no relationship between resting pre-exercise core temperature and VO2 max. However, core temperature recorded during the final min of exercise in test B was inversely related to VO2 max (r = -0.86, p less than 0.01). As a consequence, core temperature during the final minute of exercise was also related to the relative exercise intensity (% VO2 max) performed (r = 0.82, p less than 0.01). The heart rate response during test B was inversely related to VO2 max (r = -0.71, p less than 0.05) and was positively related to the relative exercise intensity performed (r = 0.68, p less than 0.05). No relationship was found between weighted mean skin temperature during the final minute of exercise and the relative (r = 0.26) or absolute (r = 0.03) workloads performed during exercise. The results of the present experiment suggest that cardiovascular fitness (as indicated by VO2 max) will have a significant influence upon the thermoregulatory responses of Man during exercise.     

Oxidation of carbohydrate ingested during prolonged endurance exercise.

Classic studies conducted in the 1920s and 1930s established that the consumption of a high carbohydrate (CHO) diet before exercise and the ingestion of glucose during exercise delayed the onset of fatigue, in part by preventing the development of hypoglycaemia. For the next 30 to 40 years, however, interest in CHO ingestion during exercise waned. Indeed, it was not until the reintroduction of the muscle biopsy technique into exercise physiology in the 1960s that a series of studies on CHO utilisation during exercise appeared. Investigations by Scandinavian physiologists showed that muscle glycogen depletion during prolonged exercise coincided with the development of fatigue. Despite this finding, attempts to delay fatigue during prolonged exercise focused principally on techniques that would increase muscle glycogen storage before exercise. The possibility that CHO ingestion during exercise might also delay the development of muscle glycogen depletion and hence, at least potentially, fatigue, was not extensively investigated. This, in part, can be explained by the popular belief that water replacement to prevent dehydration and hyperthermia was of greater importance than CHO replacement during prolonged exercise. This position was strengthened by studies in the early 1970s which showed that the ingestion of CHO solutions delayed gastric emptying compared with water, and might therefore exacerbate dehydration. As a result, athletes were actively discouraged from ingesting even mildly concentrated (greater than 5 g/100ml) CHO solutions during exercise. Only in the early 1980s, when commercial interest in the sale of CHO products to athletes was aroused, did exercise physiologists again begin to study the effects of CHO ingestion during exercise. These studies soon established that CHO ingestion during prolonged exercise could delay fatigue; this finding added urgency to the search for the optimum CHO type for ingestion during exercise. Whereas in the earlier studies, estimates of CHO oxidation were made using respiratory gas exchange measurements, investigations since the early 1970s have employed stable 13C and radioactive 14C isotope techniques to determine the amount of ingested CHO that is oxidised during exercise. Most of the early interest was in glucose ingestion during exercise. These studies showed that significant quantities of ingested glucose can be oxidised during exercise. Peak rates of glucose oxidation occur approximately 75 to 90 minutes after ingestion and are unaffected by the time of glucose ingestion during exercise. Rates of oxidation also appear not to be influenced to a major extent by the use of different feeding schedules.(ABSTRACT TRUNCATED AT 400 WORDS)     

Serum leptin during recovery following maximal incremental and prolonged exercise.

This study investigated the delayed circulating leptin response to maximal and prolonged treadmill exercise. Six healthy untrained males performed three sessions after an overnight fast: control, maximal exercise, and prolonged exercise at 50% of maximal oxygen consumption. Blood samples were collected prior to exercise, at the end of exercise, and at 60, 120, 180, and 240 min following exercise and control sessions. Blood samples were analyzed for serum leptin, insulin, glucose, free fatty acids, and glycerol. Hemoglobin and hematocrit were measured to correct for plasma volume changes. Resting energy expenditure (REE) and body fat (BF) were also assessed. Immediately at the end of maximal and prolonged exercise, and during the 4 hours of recovery, serum leptin levels did not change significantly compared to their respective baseline values. At 240 min of recovery serum leptin decreased 7% and 9% (p>0.05) from the baseline in the maximal and prolonged sessions, respectively. In the control experiment serum leptin decreased 27% from the baseline at 240 min of the recovery (p < 0.05). No significant differences were found in leptin values between the control and exercise sessions. Control serum leptin was positively correlated (p < 0.05) to BF (r = 0.88) and glucose (r=0.96), and negatively correlated to REE (r= -0.81). In conclusion, maximal or prolonged exercise do not appear to have an influence on circulating serum leptin in the delayed (4 hr) post exercise recovery period.     

Gastric emptying during prolonged cycling exercise in the heat

Eight trained male cyclists (age 20-33 yr) completed four 3-h bouts of cycling at 60% peak VO2 in the heat (33 degrees C) drinking either water (W), 5% glucose (G), 5% glucose polymer (GP), or 3.2% glucose polymer + 1.8% fructose (GP/F) at a rate of 350 ml every 20 min (3.15 l total volume). Similar changes in heart rate, sweat rate, rectal and mean skin temperatures, and plasma [Na+], [K+], and osmolality were observed during all trials. Mean changes in plasma volume, although not significantly different between trials, were lowest for the GP/F drink (-2.6%) and greatest for the G (-8.1%) drink. Plasma volume decreased (P less than 0.05) below pre-exercise control values during the W, G, and GP trails but was maintained at control values during the GP/F trials. In contrast to water ingestion, G, GP, and GP/F ingestion maintained plasma glucose and respiratory exchange ratios throughout the 3-h exercise bouts. Gastric residual volume (GRV) obtained at the end of exercise was similar for the W, GP, and GP/F trials. The G trials yielded greater (P less than 0.05) GRV than W trials. For all drinks ingested, over 90% of the 3.15 l consumed was emptied from the stomach during the 3-h exercise bouts. At a mean sweat rate of 1.2 l.h-1, cyclists replaced 73% of fluid lost and experienced only a 1.6% loss in body weight. This study demonstrates that, during prolonged (3-h) cycling exercise in the heat, large volumes of W and 5% carbohydrate can be emptied from the stomach to help minimize the effects of dehydration.     

Muscle metabolism, temperature, and function during prolonged, intermittent, high-intensity running in air temperatures of 33 degrees and 17 degrees C

Nine unacclimatized university sportsmen performed a prolonged, intermittent, high-intensity shuttle running test in hot (HT) (33 degrees C, dry bulb temperature, approximately 28 %, relative humidity) and moderate (MT) (17 degrees C, 63 %) environmental conditions. Subjects performed 60 m of walking, a 15-m sprint, 60 m of cruising ( approximately 85 % V.O (2max)), and 60 m of jogging ( approximately 45 %V.O (2max)) for 14.8 +/- 0.1 min followed by a 3-min rest, repeated until volitional exhaustion. The hot trial was performed first followed, 14 days later, by the moderate trial. During exercise subjects drank water ad libitum. Subjects ran almost twice as far in the moderate as in the hot trial (HT 11216 +/- 1411, MT 21644 +/- 1629, m, p < 0.01), and the decline in average 15-m sprint performance was greater in the heat (HT, 0.17 +/- 0.05, MT, 0.09 +/- 0.03, s, p < 0.05). Average heart rates, blood lactate and glucose, and plasma adrenaline and noradrenaline concentrations were greater in the HT (main effect trial, p < 0.01), as were serum cortisol concentration (main effect trial p < 0.05, n = 5) and muscle temperature (HT exhaustion vs. same time point in MT, 40.2 +/- 0.3 vs. 39.3 +/- 0.2, degrees C, p < 0.01). Peak torque during knee flexion and extension was not different pre-and post-exercise in the HT. Muscle glycogen utilization tended to be greater in the heat (HT 193.2 +/- 19.5, MT 143.8 +/- 23.9, mmol . kg dry wt (-1), p = 0.055, n = 8). In 7 out of the 8 subjects the increase in utilization was between 19 and just over 200 % greater in the HT. Glycogen remaining in the muscle at exhaustion was greater in the hot than moderate trial (HT 207.4 +/- 34.3, MT 126.5 +/- 46.8, mmol . kg dry wt (-1), p < 0.01, n = 8). Rectal temperature (T (rec)) was higher in the HT at exhaustion than at the same point in time in the moderate trial (HT, 39.60 +/- 0.15 vs. MT 38.75 +/- 0.10, degrees C, interaction trial-time, p < 0.01). There was a very strong negative relationship between rate of rise in T (rec) and distance completed in the HT (HT r = - 0.90, p < 0.01, MT r = - 0.76, p < 0.05). Thus, the earlier onset of exhaustion during prolonged intermittent shuttle running in the heat is associated with hyperthermia. However, while muscle glycogen utilization may be elevated by heat stress, low whole muscle glycogen concentrations would not seem to be the cause of this earlier exhaustion.     

Sex difference in fluid balance responses during prolonged exercise

Maintaining a proper fluid balance is important during exercise as athletes are prone to develop dehydration during exercise. Although several factors may regulate the fluid balance, little is known about the role of sex during prolonged moderate‐intensity exercise. Therefore, we compared body mass changes and fluid balance parameters in men vs women in a large heterogeneous group of participants during prolonged exercise. Ninety‐eight volunteers walked 30–50 km at a self‐selected pace. Exercise duration (8 h, 32 min) and intensity (69% HR_max) were comparable between groups. Men demonstrated a significantly larger change in body mass than women (?1.6% vs ?0.9%, respectively, P < 0.001) and a higher incidence of dehydration (defined as ≥2% body mass loss) compared with women (34% vs 12%, respectively, odds ratio = 4.2, 95% CI = 1.1–16.7). Changes in blood sodium levels were significantly different between men (+1.5 mmol/L) and women (?0.4 mmol/L), while 27% of the men vs 0% of the women showed postexercise hypernatremia (sodium levels ≥ 145 mmol/L). Moreover, men demonstrated a significantly lower fluid intake (2.9 mL/kg/h) and higher fluid loss (5.0 mL/kg/h) compared with women (3.7 and 4.8 mL/kg/h, respectively). Taken together, our data suggest that men and women demonstrate different changes in fluid balance in response to a similar bout of exercise.     

The incidence of hyponatremia during prolonged ultraendurance exercise

Recent studies have shown that potentially fatal hyponatremia can develop during prolonged exercise. To determine the incidence of hyponatremia in athletes competing in ultradistance events, we measured serum sodium levels in 315 of 626 (50%) runners who were treated for collapse after two 90 km ultramarathon footraces (total starters 20,335; total finishers 18,031) and in 101 of 147 (69%) finishers in a 186 km ultratriathlon. In both races the athletes drank fluids with low sodium chloride content (less than 6.8 mmol.l-1). Hyponatremia (serum sodium level less than 130 mmol.l-1) was identified in 27 of 315 (9%) collapsed runners in the 90 km races and in none of the triathletes. In response to diuretic therapy, the runner with the most severe hyponatremia (serum sodium level = 112 mmol.l-1) excreted in excess of 7.5 l dilute urine during the first 17 h of hospitalization. These data suggest that, although symptomatic hyponatremia occurs in less than 0.3% of competitors during prolonged exercise even when they ingest little sodium chloride, it is found in a significant proportion (9%) of collapsed runners. A regulated contraction of the extracellular fluid volume would explain why the majority of athletes maintain normal serum sodium levels even though they develop a significant sodium chloride deficit during prolonged exercise. Alternatively, sodium chloride losses during prolonged exercise may be substantially less than are currently believed. Physicians treating collapsed ultradistance athletes need to be aware that as many as 10% or more of such patients may be hyponatremic.     

Cardiovascular drift during prolonged exercise: new perspectives

We propose that cardiovascular drift, characterized by a progressive decline in stroke volume after 10-20 min of exercise, is primarily due to increased heart rate rather tahn a progressive increase in cutaneous blood flow as body temperature rises.     

Electrolyte diffusion of cesium bromide in water at 25 degrees C.

Electrolyte diffusion coefficients of CsBr in water have been measured using 137Cs isotope with an open ended capillary technique without stirring. The measurements were done at 25 degrees C over the concentration range of 10(-6) -0.25 M and the measured values are compared with the theoretical values of diffusion coefficients estimated on the basis of Onsager-Fuoss theory. At low concentrations, there is a good agreement between the experimental and theoretical values of diffusion coefficients. The deviations at higher concentrations are explained on the basis of the occurrence of ion-ion interactions.     

Effectiveness of carbohydrate feeding in delaying fatigue during prolonged exercise

Prolonged exercise in the fasted state frequently results in a lowering of blood glucose concentration, and when the intensity is moderate (i.e. 60-80% of VO2 max), muscle often becomes depleted of glycogen. The extent to which carbohydrate feedings contribute to energy production, and their effectiveness for improving endurance during prolonged exercise, are reviewed in this article. Prolonged exercise (i.e. greater than 2 hours) results in a failure of hepatic glucose output to keep pace with muscle glucose uptake. As a result, blood glucose concentration frequently declines below 2.5 mmol/L. Despite this hypoglycaemia, fewer than 25% of subjects display symptoms suggestive of central nervous system dysfunction. Since fatigue rarely results from hypoglycaemia alone, the effectiveness of carbohydrate feeding should be judged by its potential for muscle glycogen sparing. Carbohydrate feeding during moderate intensity exercise postpones the development of fatigue by approximately 15 to 30 minutes, yet it does not prevent fatigue. This observation agrees with data suggesting that carbohydrate supplementation reduces muscle glycogen depletion. It is not certain whether carbohydrate feeding increases muscle glucose uptake throughout moderate exercise or if glucose uptake is higher only during the latter stages of exercise. In contrast to moderate intensity exercise, carbohydrate feeding during low intensity exercise (i.e. less than 45% of VO2 max) results in hyperinsulinaemia. Consequently, muscle glucose uptake and total carbohydrate oxidation are increased by approximately the same amount. The amount of ingested glucose which is oxidised is greater than the increase in total carbohydrate oxidation and therefore endogenous carbohydrate is spared. The majority of sparing appears to occur in the liver, which is reasonable since muscle glycogen is not utilised to a large extent during mild exercise. Although carbohydrate feedings prevent hypoglycaemia and are readily used for energy during mild exercise, there is little data indicating that feedings improve endurance during low intensity exercise. When the reliance on carbohydrate for fuel is greater, as during moderate intensity exercise, carbohydrate feedings delay fatigue by apparently slowing the depletion of muscle glycogen.     

Protein and energy metabolism during prolonged exercise in trained athletes

Eight elite triathlon athletes participated in a laboratory study of the effects of endurance exercise on protein and energy metabolism. The study consisted of 3 h of cycling and 5 h of treadmill running; 3.5 h before beginning the exercise, a primed constant infusion of 1-13C leucine and 6,6(-2)H glucose was begun. Serial blood samples were collected during the rest and exercise periods for isotopic analysis. Respiratory gas exchange was determined every half hour. Results: the subjects exercised at an average of 53% +/- 3% of peak VO2. During the 8-h exercise period there was a decline in glucose utilization and an increase in lipid oxidation. For the first part of the exercise, most of the glucose oxidized was of muscle origin. Hepatic glucose production increased with exercise from 20 g/h to a maximum of about 60 g/h after 4 h of exercise and then decreased toward the pre-exercise rate. The plasma urea concentration was unchanged during the study. The leucine flux decreased during the first 4 h of exercise and then attained a new plateau about 20% lower than the pre-exercise value indicating an adaptive reduction in protein turnover.     

Changes of white blood cell count during prolonged exercise

Leukocytosis was postulated to accompany short- and medium-length exercise; in this report, we have studied the changes in leukocyte count during and following prolonged exercise. White blood cell (WBC) counts were obtained in 15 endurance-trained subjects before, during, and at a recovery period after an ultralong exercise (120 km march), lasting 24 h. WBC counts increased after 16 h march from a baseline value of 8.5 +/- 0.3 10(9) l-1 to 11.3 +/- 0.8 10(9) l-1 (P less than 0.05) and then declined to 7.1 +/- 0.9 10(9) l-1 after 24 h march with no further significant changes during 64 h of recovery. These observations were supported by previous findings in three separate marches performed by a second group (40, 70, and 120 km). A parallel increase in plasma creatine phosphokinase activity from 127 +/- 4.4 ul-1 to 539 +/- 106.3 ul-1 was observed after 16 h march (P less than 0.01), indicating muscle cell damage. Our findings suggest that in extremely long marches, WBC counts return to baseline values before exercise is terminated. This phenomenon may reflect WBC infiltration to damaged muscle tissue.