Core temperature "null zone"

1991 ◽  
Vol 71 (4) ◽  
pp. 1289-1295 ◽  
Author(s):  
I. B. Mekjavic ◽  
C. J. Sundberg ◽  
D. Linnarsson
Keyword(s):  
Core Temperature ◽  
Recovery Period ◽  
Rest Period ◽  
Cycle Ergometer ◽  
Work Rate ◽  
The Core ◽  
Maximum Work ◽  
Male Subjects ◽  
Shivering Thermogenesis ◽  
Peak Value

An experimental protocol was designed to investigate whether human core temperature is regulated at a “set point” or whether there is a neutral zone between the core thresholds for shivering thermogenesis and sweating. Nine male subjects exercised on an underwater cycle ergometer at a work rate equivalent to 50% of their maximum work rate. Throughout an initial 2-min rest period, the 20-min exercise protocol, and the 100-min recovery period, subjects remained immersed to the chin in water maintained at 28 degrees C. On completion of the exercise, the rate of forehead sweating (Esw) decayed from a mean peak value of 7.7 +/- 4.2 (SD) to 0.6 +/- 0.3 g.m-2.min-1, which corresponds to the rate of passive transpiration, at core temperatures of 37.42 +/- 0.29 and 37.39 +/- 0.48 degrees C, as measured in the esophagus (Tes) and rectum (Tre), respectively. Oxygen uptake (VO2) decreased rapidly from an exercising level of 2.11 +/- 0.25 to 0.46 +/- 0.09 l/min within 4 min of the recovery period. Thereafter, VO2 remained stable for approximately 20 min, eventually increased with progressive cooling of the core region, and was elevated above the median resting values determined between 15 and 20 min at Tes = 36.84 +/- 0.38 degrees C and Tre = 36.80 +/- 0.39 degrees C. These results indicate that the core temperatures at which sweating ceases and shivering commences are significantly different (P less than 0.001) regardless of whether core temperature is measured within the esophagus or rectum.(ABSTRACT TRUNCATED AT 250 WORDS)

Human temperature regulation during narcosis induced by inhalation of 30% nitrous oxide

1992 ◽  
Vol 73 (6) ◽  
pp. 2246-2254 ◽  
Author(s):  
I. B. Mekjavic ◽  
C. J. Sundberg
Keyword(s):  
Nitrous Oxide ◽  
Temperature Regulation ◽  
Recovery Period ◽  
Cycle Ergometer ◽  
Work Rate ◽  
O2 Uptake ◽  
The Core ◽  
Maximum Work ◽  
Male Subjects ◽  
Shivering Thermogenesis

The study investigated the effect of inhalation of 30% nitrous oxide (N2O) on temperature regulation in humans. Seven male subjects were immersed to the neck in 28 degrees C water on two separate occasions. They exercised at a rate equivalent to 50% of their maximum work rate on an underwater cycle ergometer for 20 min and remained immersed for an additional 100 min after the exercise. In one trial (AIR) the subjects inspired compressed air, and in the other trial (N2O) they inspired a gas mixture containing N2O (20.93% O2–30% N2O-49.07% N2). Sweating, measured at the forehead, and shivering thermogenesis, as reflected by O2 uptake, were monitored throughout the 100-min recovery period. The threshold core temperatures at which sweating was extinguished and shivering was initiated were established relative to resting preexercise levels. Neither the magnitude of the sweating response nor the core threshold at which it was extinguished was significantly affected by the inhalation of N2O. In contrast, shivering thermogenesis was both significantly reduced during the N2O condition and initiated at significantly lower core temperatures [change in esophageal temperature (delta T(es)) = -0.98 +/- 0.33 degrees C and change in rectal temperature (delta T(re)) = -1.26 degrees C] during the N2O than during the AIR condition (delta T(es) = -0.36 +/- 0.31 degrees C and delta T(re) = -0.44 +/- 0.22 degrees C).(ABSTRACT TRUNCATED AT 250 WORDS)

scholarly journals Sprint training reduces urinary purine loss following intense exercise in humans

2006 ◽  
Vol 31 (6) ◽  
pp. 702-708 ◽  
Author(s):  
Christos G. Stathis ◽  
Michael F. Carey ◽  
Alan Hayes ◽  
Andrew P. Garnham ◽  
Rodney J. Snow
Keyword(s):  
Uric Acid ◽  
Performance Test ◽  
Recovery Period ◽  
Rest Period ◽  
Cycle Ergometer ◽  
Sprint Training ◽  
Intense Exercise ◽  
Before And After ◽  
Exercise Induced ◽  
Male Subjects

The influence of sprint training on endogenous urinary purine loss was examined in 7 active male subjects (age, 23.1 ± 1.8 y; body mass, 76.1 ± 3.1 kg; VO2 peak, 56.3 ± 4.0 mL·kg–1·min–1). Each subject performed a 30 s sprint performance test (PT), before and after 7 d of sprint training. Training consisted of 15 sprints, each lasting 10 s, on an air-braked cycle ergometer performed twice each day. A rest period of 50 s separated each sprint during training. Sprint training resulted in a 20% higher muscle ATP immediately after PT, a lower IMP (57% and 89%, immediately after and 10 min after PT, respectively), and inosine accumulation (53% and 56%, immediately after and 10 min after the PT, respectively). Sprint training also attenuated the exercise-induced increases in plasma inosine, hypoxanthine (Hx), and uric acid during the first 120 min of recovery and reduced the total urinary excretion of purines (inosine + Hx + uric acid) in the 24 h recovery period following intense exercise. These results show that intermittent sprint training reduces the total urinary purine excretion after a 30 s sprint bout.

A second postcooling afterdrop: more evidence for a convective mechanism

1992 ◽  
Vol 73 (4) ◽  
pp. 1253-1258 ◽  
Author(s):  
G. G. Giesbrecht ◽  
G. K. Bristow
Keyword(s):  
Core Temperature ◽  
Initial Rate ◽  
Treadmill Exercise ◽  
Cold Water ◽  
Water Immersion ◽  
Cold Water Immersion ◽  
Probable Mechanism ◽  
Warm Bath ◽  
Male Subjects ◽  
Shivering Thermogenesis

An attempt was made to demonstrate the importance of increased perfusion of cold tissue in core temperature afterdrop. Five male subjects were cooled twice in water (8 degrees C) for 53–80 min. They were then rewarmed by one of two methods (shivering thermogenesis or treadmill exercise) for another 40–65 min, after which they entered a warm bath (40 degrees C). Esophageal temperature (Tes) as well as thigh and calf muscle temperatures at three depths (1.5, 3.0, and 4.5 cm) were measured. Cold water immersion was terminated at Tes varying between 33.0 and 34.5 degrees C. For each subject this temperature was similar in both trials. The initial core temperature afterdrop was 58% greater during exercise (mean +/- SE, 0.65 +/- 0.10 degrees C) than shivering (0.41 +/- 0.06 degrees C) (P < 0.005). Within the first 5 min after subjects entered the warm bath the initial rate of rewarming (previously established during shivering or exercise, approximately 0.07 degrees C/min) decreased. The attenuation was 0.088 +/- 0.03 degrees C/min (P < 0.025) after shivering and 0.062 +/- 0.022 degrees C/min (P < 0.025) after exercise. In 4 of 10 trials (2 after shivering and 2 after exercise) a second afterdrop occurred during this period. We suggest that increased perfusion of cold tissue is one probable mechanism responsible for attenuation or reversal of the initial rewarming rate. These results have important implications for treatment of hypothermia victims, even when treatment commences long after removal from cold water.

The Effect of Propranolol or Metoprolol on Thermoregulation during Insulin-Induced Hypoglycaemia in Man

1982 ◽  
Vol 63 (3) ◽  
pp. 301-310 ◽  
Author(s):  
I. A. Macdonald ◽  
T. Bennett ◽  
E. A. M. Gale ◽  
J. Hilary Green ◽  
S. Walford
Keyword(s):  
Core Temperature ◽  
Heat Production ◽  
Insulin Infusion ◽  
Recovery Period ◽  
Cardiovascular Responses ◽  
Plasma Adrenaline ◽  
Selective Antagonist ◽  
Male Subjects ◽  
Production Response ◽  
Very High

1. The effects of metoprolol and propranolol on heat production and body temperature have been studied in six male subjects during insulin-induced hypoglycaemia in a thermoneutral environment. Hypoglycaemia was induced by insulin infusion on three occasions in each subject, accompanied by the infusion of sodium chloride solution (154 mmol/l) (control), metoprolol (β1-selective antagonist) or propranolol (non-selective antagonist). 2. During the period of hypoglycaemia in the control experiments mean heat production (calculated from respiratory gas exchange) increased by 1·07 ± sem 0·13 kJ/min and remained elevated for 30−40 min. This heat production response was reduced by metoprolol and abolished by propranolol. During the recovery period, heat production was significantly reduced in the presence of propranolol. 3. Skin and core temperatures fell during the period of hypoglycaemia in all three experiments. The fall in skin temperature was significantly greater in the presence of propranolol (−2·51 ± 0·47°C). The reductions in core temperature recorded during the three experiments were similar (control −0·73 ± 0·17, metoprolol −0·99 ± 0·21, propranolol −0·88 ± 0·22°C), but core temperature was still falling at the end of the propranolol experiment. 4. The cardiovascular responses to hypoglycaemia were similar in the control and metoprolol experiments but were substantially modified by propranolol. During the period of hypoglycaemia in the control experiments, plasma adrenaline levels rose to 7·78 ± 1·79 nmol/l; significantly higher levels were measured in the metoprolol (10·11 ± 1·64) and propranolol (22·76 ± 7·02) experiments. The very high adrenaline levels may have been responsible for the modified cardiovascular responses to hypoglycaemia observed in the propranolol experiment.

Muscular efficiency during steady-rate exercise: effects of speed and work rate

1975 ◽  
Vol 38 (6) ◽  
pp. 1132-1139 ◽  
Author(s):  
G. A. Gaesser ◽  
G. A. Brooks
Keyword(s):  
Coupling Efficiency ◽  
Cycle Ergometer ◽  
Work Rate ◽  
Base Line ◽  
Exponential Relationship ◽  
Steady Rate ◽  
Resting Metabolism ◽  
Traditional Mode ◽  
Delta G ◽  
Male Subjects

In a comparison of traditional and theoretical exercise efficiency calculations male subjects were studied during steady-rate cycle ergometer exercises of “0,” 200, 400, 600, and 800 kgm/min while pedaling at 40, 60, 80, and 100 rpm. Gross (no base-line correction), net (resting metabolism as base-line correction), work (unloading cycling as base-line correction), and delta (measurable work rate as base-line correction) efficiencies were computed. The result that gross (range 7.5–20.4%) and net (9.8–24.1%) efficiencies increased with increments in work rate was considered to be an artifact of calculation. A LINEAR OR SLIGHTLY EXPONENTIAL RELATIONSHIP BETWEEN CALORIC OUTPUT AND WORK RATE DICTATES EITHER CONSTANT OR DECREASING EFFICIENCY WITH INCREMENTS IN WORK. The delta efficiency (24.4–34.0%) definition produced this result. Due to the difficulty in obtaining 0 work equivalents, the work efficiency definition proved difficult to apply. All definitions yielded the result of decreasing efficiency with increments in speed. Since the theoretical-thermodynamic computation (assuming mitochondrial P/O = 3.0 and delta G = -11.0 kcal/mol for ATP) holds only for CHO, the traditional mode of computation (based upon VO2 and R) was judged to be superior since R less than 1.0. Assuming a constant phosphorylative-coupling efficiency of 60%, the mechanical contraction-coupling efficiency appears to vary between 41 and 57%.

Exercise hyperpnea and hyperthermia in humans

1996 ◽  
Vol 81 (3) ◽  
pp. 1249-1254 ◽  
Author(s):  
M. D. White ◽  
M. Cabanac
Keyword(s):  
Work Capacity ◽  
Cycle Ergometer ◽  
Work Rate ◽  
Brain Cooling ◽  
Supportive Evidence ◽  
Climatic Chamber ◽  
Selective Brain Cooling ◽  
Temperature Thresholds ◽  
Exercise Hyperpnea ◽  
Male Subjects

The problem of the relative hyperpnea occurring at high levels of exercise remains unresolved. This study examined whether the hyperpnea observed in humans during exercise at approximately 70% of maximal work capacity was related to cranial (tympanic) and thoracic (esophageal) temperatures. Six trained male subjects pedaled at approximately 60 revolutions/min on an electrically braked cycle ergometer in a climatic chamber at 25 degrees C and approximately 35% relative humidity in two sessions. The subjects pedaled until exhaustion in two sessions. In one session work rate was increased by 40 W every 2 min and in the other session by 20 W every 2 min. In both exercise sessions, core temperature thresholds for ventilation were evident and subsequently tympanic and esophageal temperatures diverged. This suggested that the hyperpnea in humans observed after approximately 70% of an individual's maximal work rate was determined, in part, by core temperatures and revealed supportive evidence for selective brain cooling in humans.

Enhancement of fat metabolism by repeated bouts of moderate endurance exercise

2007 ◽  
Vol 102 (6) ◽  
pp. 2158-2164 ◽  
Author(s):  
Kazushige Goto ◽  
Naokata Ishii ◽  
Ayuko Mizuno ◽  
Kaoru Takamatsu
Keyword(s):  
Prolonged Exercise ◽  
Recovery Period ◽  
Fat Metabolism ◽  
Rest Period ◽  
Exercise Bout ◽  
Exercise Duration ◽  
Cycle Ergometer ◽  
Single Trial ◽  
Repeated Trial ◽  
Single Bout

This study compared the fat metabolism between “a single bout of prolonged exercise” and “repeated bouts of exercise” of equivalent exercise intensity and total exercise duration. Seven men performed three trials: 1) a single bout of 60-min exercise (Single); 2) two bouts of 30-min exercise, separated by a 20-min rest between exercise bouts (Repeated); and 3) rest. Each exercise was performed with a cycle ergometer at 60% of maximal oxygen uptake. In the Single and Repeated trials, serum glycerol, growth hormone, plasma epinephrine, and norepinephrine concentrations increased significantly ( P < 0.05) during the first 30-min exercise bout. In the Repeated trial, serum free fatty acids (FFA), acetoacetate, and 3-hydroxybutyrate concentrations showed rapid increases ( P < 0.05) during a subsequent 20-min rest period. During the second 30-min exercise bout, FFA and epinephrine responses were significantly greater in the Repeated trial than in the Single trial ( P < 0.05). Moreover, the Repeated trial showed significantly lower values of insulin and glucose than the Single trial. During the 60-min recovery period after the exercise, FFA, glycerol, and 3-hydroxybutyrate concentrations were significantly higher in the Repeated trial than in the Single trial ( P < 0.05). The relative contribution of fat oxidation to the energy expenditure showed significantly higher values ( P < 0.05) in the Repeated trial than in the Single trial during the recovery period. These results indicate that repeated bouts of exercise cause enhanced fat metabolism compared with a single bout of prolonged exercise of equivalent total exercise duration.

scholarly journals Diurnal Variation in the Core Interthreshold Zone in Women and its Sex Difference

2018 ◽  
pp. 26-37 ◽  
Author(s):  
Naoshi Kakitsuba ◽  
Igor B. Mekjavic
Keyword(s):  
Oxygen Uptake ◽  
Diurnal Variation ◽  
Sex Difference ◽  
Core Temperature ◽  
Diurnal Changes ◽  
Sweating Rate ◽  
Cutaneous Sensation ◽  
The Core ◽  
Male Subjects ◽  
Female Subjects

Background: The core interthreshold zone (CIZ) is defined as the range between core temperature (Tc) at the onset of shivering and the Tc at the onset of sweating under consistent mean skin temperatures of 28°C to 30°C. A previous study demonstrated a diurnal change in the CIZ for male subjects and its relationship to the cutaneous sensation threshold zone (CSZ). In the present study, diurnal changes in the CIZ and the CSZ for young Japanese female subjects were investigated using the same experimental protocol from the study of male subjects and the sex differences in these responses were then examined. Methods: The CIZ and the CSZ were measured in 10 female subjects who participated in three experiments in a single day during the morning, afternoon, and evening in the summer of 2014 (single-day experiment), and six female subjects who participated in the same experiments on the morning of day 1, the afternoon of day 2, and the evening of day 3 during the summer of 2016 (multiple-day experiment). Air temperature was controlled at 25°C. Each subject wore a suit perfused with 25°C water at a rate of 600 cc/min, and exercised at 50% of their maximum work rate on an ergometer for 10–15 min until their sweating rate increased. They then remained seated, without exercising, until their oxygen uptake increased. Rectal temperature, skin temperature at seven sites, the forehead-sweating rate, and oxygen uptake were continuously monitored throughout the experiment. Cutaneous warm and cold sensation thresholds were measured at three sites using 1-cm2 and 2-cm2 probes. Results: The results from the single-day experiment demonstrated that the CIZ was proportional to core temperature prior to exercise (Tc-init) whereas the results from the multiple-day experiment demonstrated that the CIZ increased continuously from morning to evening despite almost a constant Tc-init. The CIZ appeared to be proportional to the CSZ measured with the 2-cm2 probe. When compared with the results from the previous study of men, no significant sex difference was observed between the CIZ of 0.25±0.07°C for female subjects and 0.21±0.05°C for male subjects. Conclusion: No significant sex difference or diurnal variation in the CIZ was confirmed. Continuous increase in the CIZ from morning until evening is expected in both men and women under a normal Tc circadian rhythm.

Eucapnic hypoxia lowers human cold thermoregulatory response thresholds and accelerates core cooling

1996 ◽  
Vol 80 (2) ◽  
pp. 422-429 ◽  
Author(s):  
C. E. Johnston ◽  
M. D. White ◽  
M. Wu ◽  
G. K. Bristow ◽  
G. G. Giesbrecht
Keyword(s):  
Cold Stress ◽  
Cooling Rate ◽  
Cycle Ergometer ◽  
The Core ◽  
Thermoregulatory Responses ◽  
Respiratory Heat Loss ◽  
Response Thresholds ◽  
Core Cooling ◽  
Sweating Threshold ◽  
Shivering Thermogenesis

Hypoxia lowers the basic thermoregulatory responses of animals and humans. In cold-exposed animals, hypoxia increases core temperature (Tco) cooling rate and suppresses shivering thermogenesis. In humans, the experimental effects of hypoxia on thermoregulation are equivocal. Also, the effect of hypoxia has not been separated from that of hypocapnia consequent to hypoxic hyperventilation. To determine the isolated effects of hypoxia on warm and cold thermoregulatory responses and core cooling during mild cold stress, we examined the Tco thresholds for sweating, vasoconstriction, and shivering as well as the core cooling rates of eight subjects immersed in 28 degrees C water under eucapnic conditions. On 2 separate days, subjects exercised on an underwater cycle ergometer to elevate Tco above the sweating threshold. They then rested and cooled until they shivered vigorously. Subjects inspired humidified room air during the control trial. For the eucapnic hypoxia trial, they inspired 12% O2-balance N2 with CO2 added to maintain eucapnia. Eucapnic hypoxia lowered the Tco thresholds for vasoconstriction and shivering by 0.14 and 0.19 degrees C, respectively, and increased core cooling rate by 33% (1.83 vs. 1.38 degrees C/h). These results demonstrate that eucapnic hypoxia enhances the core cooling rate in humans during mild cold stress. This may be attributed in part to a delay in the onset of vasoconstriction and shivering as well as increased respiratory heat loss during hypoxic hyperventilation.

Excess postexercise oxygen consumption and recovery rate in trained and untrained subjects

1997 ◽  
Vol 83 (1) ◽  
pp. 153-159 ◽  
Author(s):  
Kevin R. Short ◽  
Darlene A. Sedlock
Keyword(s):  
Oxygen Consumption ◽  
Recovery Rate ◽  
Recovery Period ◽  
Rest Period ◽  
Recovery Phase ◽  
Cycle Ergometer ◽  
Peak Oxygen Consumption ◽  
Fitness Level ◽  
Relative Decline ◽  
Excess Postexercise Oxygen Consumption

Short, Kevin R., and Darlene A. Sedlock. Excess postexercise oxygen consumption and recovery rate in trained and untrained subjects. J. Appl. Physiol.83(1): 153–159, 1997.—The purpose of this study was to determine whether aerobic fitness level would influence measurements of excess postexercise oxygen consumption (EPOC) and initial rate of recovery. Twelve trained [Tr; peak oxygen consumption (V˙o 2 peak) = 53.3 ± 6.4 ml ⋅ kg−1 ⋅ min−1] and ten untrained (UT;V˙o 2 peak = 37.4 ± 3.2 ml ⋅ kg−1 ⋅ min−1) subjects completed two 30-min cycle ergometer tests on separate days in the morning, after a 12-h fast and an abstinence from vigorous activity of 24 h. Baseline metabolic rate was established during the last 10 min of a 30-min seated preexercise rest period. Exercise workloads were manipulated so that they elicited the same relative, 70%V˙o 2 peak (W70%), or the same absolute, 1.5 l/min oxygen uptake (V˙o 2) (W1.5), intensity for all subjects, respectively. RecoveryV˙o 2, heart rate (HR), and respiratory exchange ratio (RER) were monitored in a seated position until baseline V˙o 2 was reestablished. Under both exercise conditions, Tr had shorter EPOC duration (W70% = 40 ± 15 min, W1.5 = 21 ± 9 min) than UT (W70% = 50 ± 14 min; W1.5 = 39 ± 14 min), but EPOC magnitude (Tr: W70% = 3.2 ± 1.0 liters O2, W1.5 = 1.5 ± 0.6 liters O2; UT: W70% = 3.5 ± 0.9 liters O2, W1.5 = 2.4 ± 0.6 liters O2) was not different between groups. The similarity of Tr and UT EPOC accumulation in the W70% trial is attributed to the parallel decline in absolute V˙o 2 during most of the initial recovery period. Tr subjects had faster relative decline during the fast-recovery phase, however, when a correction for their higher exerciseV˙o 2 was taken. Postexercise V˙o 2 was lower for Tr group for nearly all of the W1.5 trial and particularly during the fast phase. Recovery HR kinetics were remarkably similar for both groups in W70%, but recovery was faster for Tr during W1.5. RER values were at or below baseline throughout much of the recovery period in both groups, with UT experiencing larger changes than Tr in both trials. These findings indicate that Tr individuals have faster regulation of postexercise metabolism when exercising at either the same relative or same absolute work rate.

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