Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man

Local tissue temperature effects on peak torque and muscular endurance during isometric knee extension

The aim of the study was to investigate the relationship between local tissue temperature, peak torque and time to fatigue during isometric knee extensions. Nine males performed maximum voluntary contractions (MVCs) and isometric knee extensions at 70% MVC to exhaustion after 30 min of hot [H, 47.7 (1.3) degrees C; mean (SD)], warm [W, 34.6 (0.4) degrees C], temperate [T, 24.5 (1.3) degrees C], and cold [C, -11.9 (1.8) degrees C] localized temperature applications. Isometric peak torque was not significantly affected by temperature. Time to fatigue was strongly and negatively correlated ( r=-0.98) to temperature, with endurance after H [46.99 (4.98) s] and W [54.36 (9.18) s] significantly shorter than after C [73.27 (13.43) s]. We conclude that local tissue temperature does not impair peak force production but may change muscular endurance through local factors.

Responses to exercise in the heat related to measures of hypothalamic serotonergic and dopaminergic function

We have studied 12 recreationally active men to measure their responses to exercise in the heat and relate these to measures of hypothalamic function explored with a buspirone [5-hydroxytryptamine 1A (5-HT(1A)) agonist, dopaminergic D(2) antagonist] neuroendocrine challenge, with and without pretreatment with pindolol (5-HT(1A) antagonist). Pindolol treatment allowed the serotonergic and non-serotonergic components of prolactin release to be distinguished. Subjects exercised at 73 (5)% maximal rate of oxygen uptake (VO(2max)) until volitional fatigue at 35 degrees C (relative humidity, 30%). On another two occasions they underwent a buspirone challenge [0.5 mg (kg body mass)(-1)], once with, and once without, pindolol [0.5 mg (kg body mass)(-1)] pretreatment and the circulating plasma concentrations of prolactin were measured for the next 2.5 h. Rectal temperature increased throughout exercise, whilst mean skin temperature remained constant. There was a wide inter-subject variation in prolactin response to the neuroendocrine challenges. The proportion of the prolactin response to buspirone attributable to a non-serotonergic component (most likely dopaminergic) correlated both with exercise duration (r=0.657, P=0.028), rectal temperature at fatigue (r=0.623, P=0.041) and the rate of temperature rise (r=-0.669, P=0.024). Our results suggest that high activity of the dopaminergic pathways in the hypothalamus is a predictor of exercise tolerance in the heat.

Effect of fourteen days of acclimatization on athletic performance in tropical climate

In order to study the acclimatization process over 14 days of exposure to tropical climate, 9 triathletes performed 4 outdoor indirect continuous multistage tests in both thermoneutral and tropical conditions. The thermoneutral test (TN, 14 degree C, 45% rh) was performed before traveling to the tropical area (Martinique, FWI). The tropical tests were performed 2, 8, and 14 days after arrival (32.9 degree C, 78% rh). During each trial, we measured tympanic temperature, sweat rate, body mass loss, heart rate (HR), and performance. The results showed that 1). the mean tympanic temperature was greater in T2 (P <.001), T8 (P <.01) and T14 (P <.01) than in TN and significantly lower in T14 than in T2 (P <.05); 2). the mean sweat rate was significantly greater (P <.001) in T2, T8 and T14 than in TN and significantly greater (P <.05) in T8 and T14 than in T2; 3). the body mass loss after trials was significantly greater (P <.001) in T2, T8 and T14 than in TN and significantly greater (P <.05) in T8 and T14 than in T2; 4). the mean HR and HR at rest were significantly higher (P <.005) in T2 than in TN, T8, T14 and the mean HR was significantly lower (P <.05) in T14 than in the other trials; and 5). the performance time was significantly lower in T2 (P < 0.02), T8 (P < 0.03) and T14 (P < 0.05) than in TN. We concluded that 14 days of exposure to tropical climate led to changes in physiological parameters but were still insufficient to ensure complete acclimatization in well-trained athletes. The hot/wet climate induced impairment of physiological responses and performance that were still evident on the 14th day.

Effect of body temperature during exercise on skeletal muscle cytochrome c oxidase content

This study determined the role of body temperature during exercise on cytochrome-c oxidase (CytOx) activity, a marker of mitochondrial content, and mitochondrial heat shock protein 70 (mtHSP70), which is required for import of nuclear-coded preproteins. Male, 10-wk-old, Sprague-Dawley rats exercised identically for 9 wk in ambient temperatures of 23 degrees C (n = 10), 8 degrees C with wetted fur (n = 8), and 4 degrees C with wetted fur and fan (n = 7). These conditions maintained exercising core temperature (T(c)) at 40.4, 39.2, or 38.0 degrees C (resting temperature), respectively. During weeks 3-9, exercisers ran 5 days/wk up a 6% grade at 20 m/min for 60 min. Animals were housed at 23 degrees C. Gastrocnemius CytOx activity in T(c)=38.0 degrees C (83.5 +/- 5.5 microatoms O x min(-1) x g wet wt(-1)) was greater than all other groups (P < 0.05), exceeding sedentary (n = 7) by 73.2%. T(c) of 40.4 and 39.2 degrees C also were higher than sedentary by 22.4 and 37.4%, respectively (P < 0.05). Quantification of CytOx content verified that the increased activity was due to an increase in protein content. In extensor digitorum longus, a nonactive muscle, CytOx was not elevated in T(c) = 38.0 degrees C. mtHSP70 was significantly elevated in gastrocnemius of T(c) = 38.0 degrees C compared with sedentary (P < 0.05) but was not elevated in extensor digitorum longus (P > 0.05). The data indicate that decreasing exercise T(c) may enhance mitochondrial biogenesis and that mtHSP70 expression is not dependent on temperature.

A reproducible and variable intensity cycling performance protocol for warm conditions

This study was undertaken to assess the reproducibility of a variable intensitycycling protocol using subjects of varying abilities, under warm humid conditions. Eleven subjects (Age 21.4+/-2.6 years; VO2peak 3.30+/-0.9 l x min(-1); peak power 322.8+/-86.3 W; mean+/-SD) performed a 60 min cycling trial punctuated with six one-min "all-out" sprints at 10-min intervals on three occasions 5-14 days apart. Ambient temperature and relative humidity were set at 33+/-0.7 degrees C and 63+/-2.0%, respectively. Subjects used their own bicycle mounted to an electromagnetic trainer and were only permitted to monitor elapsed time and heart rate. Repeatability was assessed using the limits of agreement which were best between trials 2 and 3 where the distance cycled was -0.54 km below and 1.34 km above the distance cycled for trial 2. The co-efficient of variation (CV) for distance for three trials was 3.58%. For trials 1 and 2 the CV was 3.54% (r = 0.97, p< 0.001) decreasing to 1.34% (r = 0.99, p< 0.001) for trials 2 and 3. The intra-class correlation for three trials was 0.93. Distance for trial 1 (26.3+/-5.0 km; p< 0.05) was less than trials 2 (27.7+/-5.7 km) and 3 (28.1+/-5.6 km). It was concluded that repeatability for this performance protocol with cyclists of varying abilities In warm humid conditions was acceptable given at least one familiarisation trial. However, it is not yet known whether other protocols designed for moderate environments are applicable to less favorable conditions. Further studies are needed before results of treatment effects under differing ambient conditions can be fully understood and assigned appropriate significance.

Effects of reduced ambient temperature on fat utilization during submaximal exercise

PURPOSE

The influence of cold air exposure on fuel utilization during prolonged cycle exercise was investigated.

METHODS

Nine male subjects cycled for 90 min in ambient temperatures of -10 degrees C, 0 degrees C, 10 degrees C, and 20 degrees C. External work performed between conditions was constant. Mean oxygen consumption (VO2) over the 90 min in the 20 degrees C trial corresponded to 64 +/- 5.8% VO2peak.

RESULTS

Although mean skin temperature was different between trials (P < 0.05), rectal temperatures were not different. At -10 degrees C and 0 degrees C, the respiratory exchange ratio was higher compared with 10 degrees C and 20 degrees C (0.98 +/- 0.01 and 0.97 +/- 0.01 vs 0.92 +/- 0.01 and 0.91 +/- 0.01; P < 0.05). The associated rates of fat oxidation were lower at -10 degrees C and 0 degrees C compared with 10 degrees C and 20 degrees C (0.15 +/- 0.06 and 0.17 +/- 0.06 vs 0.35 +/- 0.06 and 0.40 +/- 0.04 g.min-1; P < 0.05). Blood glycerol was lower at -10 degrees C and 0 degrees C compared with 20 degrees C (P < 0.05); mean values were 0.13 +/- 0.0, 0.13 +/- 0.0, and 0.18 +/- 0.0 mmol.L-1 for the -10 degrees C, 0 degrees C, and 20 degrees C trials, respectively. Mean VO2 was lower in the -10 degrees C trial than the 20 degrees C trial (2.53 +/- 0.06 vs 2.77 +/- 0.09. L.min-1; P < 0.05). Mean blood glucose concentrations were lower at -10 degrees C than 20 degrees C (4.9 +/- 0.2 vs 5.3 +/- 0.1 mmol.L-1; P < 0.05). Although plasma epinephrine concentrations were greater during the 20 degrees C trial compared with all other trials (P < 0.05), plasma norepinephrine did not differ between trials.

CONCLUSION

The diminished fat oxidation at colder temperatures potentially reflects a reduction in lipolysis and/or mobilization of FFA or impairment in the oxidative capacity of the muscle.

Food and fluid intake during exercise

The intake of fluid and CHO offers benefits to the performance of a number of sports events and exercise activities. The effects of dehydration on performance are now well known, with the penalties ranging from subtle, but often important, decrements in performance at low levels of fluid deficit to the severe health risks associated with substantial fluid losses during exercise in the heat. Although evidence of the beneficial effects of CHO intake during exercise have existed for over 70 years, sports scientists are still to discover all the situations in which benefits occur and to explain the mechanisms involved. Optimal strategies for CHO and fluid intake during exercise are yet to be fine-tuned, and ultimately will be determined by practical issues such as the opportunity to eat or drink during an event, and gastrointestinal comfort.

Thermoregulation and marathon running: biological and environmental influences

The extreme physical endurance demands and varied environmental settings of marathon footraces have provided a unique opportunity to study the limits of human thermoregulation for more than a century. High post-race rectal temperatures (Tre) are commonly and consistently documented in marathon runners, yet a clear divergence of thought surrounds the cause for this observation. A close examination of the literature reveals that this phenomenon is commonly attributed to either biological (dehydration, metabolic rate, gender) or environmental factors. Marathon climatic conditions vary as much as their course topography and can change considerably from year to year and even from start to finish in the same race. The fact that climate can significantly limit temperature regulation and performance is evident from the direct relationship between heat casualties and Wet Bulb Globe Temperature (WBGT), as well as the inverse relationship between record setting race performances and ambient temperatures. However, the usual range of compensable racing environments actually appears to play more of an indirect role in predicting Tre by acting to modulate heat loss and fluid balance. The importance of fluid balance in thermoregulation is well established. Dehydration-mediated perturbations in blood volume and blood flow can compromise exercise heat loss and increase thermal strain. Although progressive dehydration reduces heat dissipation and increases Tre during exercise, the loss of plasma volume contributing to this effect is not always observed for prolonged running and may therefore complicate the predictive influence of dehydration on Tre for marathon running. Metabolic heat production consequent to muscle contraction creates an internal heat load proportional to exercise intensity. The correlation between running speed and Tre, especially over the final stages of a marathon event, is often significant but fails to reliably explain more than a fraction of the variability in post-marathon Tre. Additionally, the submaximal exercise intensities observed throughout 42 km races suggest the need for other synergistic factors or circumstances in explaining this occurrence. There is a paucity of research on women marathon runners. Some biological determinants of exercise thermoregulation, including body mass, surface area-to-mass ratio, sweat rate, and menstrual cycle phase are gender-discrete variables with the potential to alter the exercise-thermoregulatory response to different environments, fluid intake, and exercise metabolism. However, these gender differences appear to be more quantitative than qualitative for most marathon road racing environments.

Hyperthermia and central fatigue during prolonged exercise in humans

The present study investigated the effects of hyperthermia on the contributions of central and peripheral factors to the development of neuromuscular fatigue. Fourteen men exercised at 60% maximal oxygen consumption on a cycle ergometer in hot (40 degrees C; hyperthermia) and thermoneutral (18 degrees C; control) environments. In hyperthermia, the core temperature increased throughout the exercise period and reached a peak value of 40.0 +/- 0.1 degrees C (mean +/- SE) at exhaustion after 50 +/- 3 min of exercise. In control, core temperature stabilized at approximately 38.0 +/- 0.1 degrees C, and exercise was maintained for 1 h without exhausting the subjects. Immediately after the cycle trials, subjects performed 2 min of sustained maximal voluntary contraction (MVC) either with the exercised legs (knee extension) or with a "nonexercised" muscle group (handgrip). The degree of voluntary activation during sustained maximal knee extensions was assessed by superimposing electrical stimulation (EL) to nervus femoralis. Voluntary knee extensor force was similar during the first 5 s of contraction in hyperthermia and control. Thereafter, force declined in both trials, but the reduction in maximal voluntary force was more pronounced in the hyperthermic trial, and, from 30 to 120 s, the force was significantly lower in hyperthermia compared with control. Calculation of the voluntary activation percentage (MVC/MVC + EL) revealed that the degree of central activation was significantly lower in hyperthermia (54 +/- 7%) compared with control (82 +/- 6%). In contrast, total force of the knee extensors (MVC + force from EL) was not different in the two trials. Force development during handgrip contraction followed the same pattern of response as was observed for the knee extensors. In conclusion, these data demonstrate that the ability to generate force during a prolonged MVC is attenuated with hyperthermia, and the impaired performance is associated with a reduction in the voluntary activation percentage.

Metabolic responses to prolonged work during treadmill and water immersion running

The primary aim of this study was to compare the physiological responses to prolonged treadmill (TM) and water immersion to the neck (WI) running at threshold intensity. Ten endurance runners performed TM and WI running VO2max tests. Subjects completed submaximal performance tests at ventilatory threshold (Tvent) intensities under TM and WI conditions and responses at 15 and 42 minutes examined. VO2 was lower in WI (p<0.05) at maximal effort and Tvent. The Tvent VO2 intensities interpolated from the TM and WI VO2max tests were performed in both TM (i.e., TM@TM(tvent),TM@WI(tvent), corresponding to 77.6 and 71.3% respectively of TM VO2max) and WI conditions (i.e., WI@TM(tvent), WI@WI(tvent), corresponding to 85.5% and 78.2% respectively of WI VO2max). Each of the dependent variables was analyzed using a 3-way repeated measures ANOVA (2 conditions X 2 exercise intensities X 7 time points during exercise). VO2max values were significantly lower in the WI (52.4(5.1) ml.kg(-1) min(-1)) versus TM (59.7(6.5) ml.kg(-1) min(-1)) condition. VO2 during submaximal tests were similar during the TM and WI conditions. HR and [BLa] responses to exercise at and above WI(tvent) were similar during short-term exercise, but values tended to be lower during prolonged exercise in the WI condition. There were no statistical differences in VE responses in the 2 conditions, however as with HR and [BLa] an upward trend was noted with TM exercise over the 42 minute duration of the tests. RPE at WI(tvent) was similar for TM and WI exercise sessions, however, RPE at TM(tvent) was higher during WI compared to TM running. Cardiovascular drift was observed during prolonged TM but not WI running. Results suggest differences in metabolic responses to prolonged submaximal exercise in WI, however it can be used effectively for cross training.

Evidence of neuromuscular fatigue after prolonged cycling exercise

PURPOSE

The purpose of this study was to analyze the effects of prolonged cycling exercise on metabolic, neuromuscular, and biomechanical parameters.

METHODS

Eight well-trained male cyclists or triathletes performed a 2-h cycling exercise at a power output corresponding to 65% of their maximal aerobic power. Maximal concentric (CON; 60, 120, 240 degrees x s(-1)), isometric (ISO; 0 degrees s(-1)), and eccentric (ECC; -120, -60 degrees x s(-1)) contractions, electromyographic (EMG) activity of vastus lateralis (VL) and vastus medialis (VM) muscles were recorded before and after the exercise. Neural (M-wave) and contractile (isometric muscular twitch) parameters of quadriceps muscle were also analyzed using electrical stimulation techniques.

RESULTS

Oxygen uptake (VO2), minute ventilation (VE), and heart rate (HR) significantly increased (P < 0.01) during the 2-h by, respectively, 9.6%, 17.7%, and 12.7%, whereas pedaling rate significantly decreased (P < 0.01) by 21% (from 87 to 69 rpm). Reductions in muscular peak torque were quite similar during CON, ISO, and ECC contractions, ranging from 11 to 15%. M-wave duration significantly increased (P < 0.05) postexercise in both VL and VM, whereas maximal amplitude and total area decreased (VM: P < 0.05, VL: NS). Significant decreases in maximal twitch tension (P < 0.01), total area of mechanical response (P < 0.01), and maximal rate of twitch tension development (P < 0.05) were found postexercise.

CONCLUSIONS

A reduction in leg muscular capacity after prolonged cycling exercise resulted from both reduced neural input to the muscles and a failure of peripheral contractile mechanisms. Several hypothesis are proposed to explain a decrease in pedaling rate during the 2-h cycling with a constancy of power output and an increase in energy cost.

Effects of heat stress on physiological responses and exercise performance in elite cyclists

This study examined the effect of heat stress on physiological responses and exercise performance in elite road cyclists. Eleven members of the Australian National Road Cycling Squad completed two 30 min cycling time-trials in an environmental chamber set at either 32 degrees C, (HT) or 23 degrees C (NT) with a relative humidity of 60% in each circumstance. The trials were separated by two days, with six subjects performing HT first. Power output was 6.5% lower (P<0.05) during HT compared with NT. Mean skin temperature and sweat rate were higher (P<0.05) in HT compared with NT. In contrast, rectal temperature was remarkably similar throughout each trial. During the first 10 min of exercise in HT when power output was not different between trials, blood lactate was higher (P<0.05), and blood pH lower (P<0.05). In contrast, during the last 10 min of exercise when power output was reduced (P<0.05), blood lactate was lower (P<0.05), and pH higher (P<0.05), in HT. These data indicate that heat stress is associated with a reduced power output during self-paced exercise in highly trained men. This decrease in performance appears to be associated with factors associated with body temperature rather than metabolic capacity.

A lifetime of fitness. Exercise in the perimenopausal and postmenopausal woman

The peri- and postmenopausal woman experiences physiologic changes of aging that include alterations in hormone levels. Research has shown that the perimenopausal and postmenopausal woman can benefit significantly from exercise, whether endurance or strength training. Exercise can improve the quality of life and attenuate some of the physiologic changes associated with aging. Additionally, exercise can ameliorate the decline in fitness and bone, prevent chronic disease, and promote functional independence. Women who exercise regularly throughout life are physiologically 20 to 30 years younger than their sedentary counterparts. Fitness is a lifetime endeavor that has many positive benefits. Weightbearing activities are especially important as bone loss increases in the perimenopausal phase of life. Women should perform aerobic exercise 3 to 7 days per week for 15 to 60 minutes at 65% to 70% HRreserve. Strengthening exercises should be done 2 to 3 days per week at 40% to 80% 1RM with appropriately selected exercises. An exercise program should be functional and enjoyable. There is no better motivation to exercise than having a partner to work out with and keep the motivation alive. Most important, age itself is not a deterrent to exercise.

Increased fat availability enhances the capacity of trained individuals to perform prolonged exercise

METHODS

After a familiarization period, six well-trained males participated in a diet and exercise regimen lasting 9 d and comprising three cycling tests to exhaustion. A work rate was selected during the familiarization period that would result in fatigue after approximately 90-100 min at an ambient temperature of 10 degrees C (i.e., approximately 75% of VO2max). The first exercise test was a depletion trial and was preceded by a period during which the subjects' normal diet was consumed. A prescribed 70% carbohydrate (CHO) diet was then consumed for 3.5 d. After this diet, a second exercise test was performed; one of two isoenergetic experimental meals was consumed 4 h before this test (70% CHO meal, CHO trial; or 90% fat meal, fat trial). The second exercise test was followed by a further 3.5-d period on the high CHO diet. Four hours before the third test, subjects consumed the other meal. Heparin was administered intravenously 30 min (1000 U), 15 min (500 U), and 0 min (500 U) before exercise on the fat trial. Subjects were assigned to the two meals in randomized order.

RESULTS

Time to exhaustion increased from 118.2 (12.4) min on the CHO trial to 127.9 (12.1) min on the fat trial (P = 0.001). Although no difference in VO2, RER, HR or RPE was found between trials, there was an earlier reduction in RER and an earlier rise in RPE on the fat trial. No difference in total CHO oxidation was found between trials (383 +/- 70 g on the CHO trial and 362 +/- 59 g on the fat trial).

CONCLUSIONS

These results suggest that increasing fat availability immediately before exercise by acute fat feeding and heparin infusion can improve endurance exercise in a cool environment in well-trained individuals. This study was not intended to have immediate application to the sports performance field but rather to contribute to our understanding of the factors that may limit endurance performance. Heparin injection to elevate plasma fatty acid concentration would not represent sound medical practice.

The effects of exercise and diet manipulation on the capacity to perform prolonged exercise in the heat and in the cold in trained humans

1. This study examined the effects of exercise and diet manipulation intended to alter initial muscle glycogen levels on the capacity to perform prolonged exercise at two ambient temperatures. 2. Six well-trained cyclists participated in randomized order in two diet and exercise regimens each lasting 8 days and comprising four cycle tests to exhaustion at 70 % of maximum oxygen uptake. On days 1 and 5, subjects exercised to exhaustion to deplete muscle glycogen. Three days after each depletion trial a diet providing 10 % (low carbohydrate (CHO)) or 80 % (high CHO) of energy as CHO was consumed, and each diet was followed by a performance trial at the same ambient temperature, either 10 or 30 C (days 4 and 8). This schedule was repeated after a week, but performance trials were carried out at the other ambient temperature. 3. In the cold, cycling time increased (median (range)) from 89.2 min (78.0-129.5 min) on the low CHO trial to 158.2 min (116.9-165.6 min) on the high CHO trial (P < 0.01). In the heat, cycling time increased from 44.0 min (31.8-51.4 min) on the low CHO trial to 53.2 min (50.2-82.2 min) on the high CHO trial (P = 0.02). Total CHO oxidized during exercise in the cold after the low CHO diet was higher than in the heat after either diet suggesting that exercise in the heat was terminated before all available CHO stores had been emptied.

Influence of body temperature on the development of fatigue during prolonged exercise in the heat

We investigated whether fatigue during prolonged exercise in uncompensable hot environments occurred at the same critical level of hyperthermia when the initial value and the rate of increase in body temperature are altered. To examine the effect of initial body temperature [esophageal temperature (Tes) = 35.9 +/- 0.2, 37.4 +/- 0. 1, or 38.2 +/- 0.1 (SE) degrees C induced by 30 min of water immersion], seven cyclists (maximal O2 uptake = 5.1 +/- 0.1 l/min) performed three randomly assigned bouts of cycle ergometer exercise (60% maximal O2 uptake) in the heat (40 degrees C) until volitional exhaustion. To determine the influence of rate of heat storage (0.10 vs. 0.05 degrees C/min induced by a water-perfused jacket), four cyclists performed two additional exercise bouts, starting with Tes of 37.0 degrees C. Despite different initial temperatures, all subjects fatigued at an identical level of hyperthermia (Tes = 40. 1-40.2 degrees C, muscle temperature = 40.7-40.9 degrees C, skin temperature = 37.0-37.2 degrees C) and cardiovascular strain (heart rate = 196-198 beats/min, cardiac output = 19.9-20.8 l/min). Time to exhaustion was inversely related to the initial body temperature: 63 +/- 3, 46 +/- 3, and 28 +/- 2 min with initial Tes of approximately 36, 37, and 38 degrees C, respectively (all P < 0.05). Similarly, with different rates of heat storage, all subjects reached exhaustion at similar Tes and muscle temperature (40.1-40.3 and 40. 7-40.9 degrees C, respectively), but with significantly different skin temperature (38.4 +/- 0.4 vs. 35.6 +/- 0.2 degrees C during high vs. low rate of heat storage, respectively, P < 0.05). Time to exhaustion was significantly shorter at the high than at the lower rate of heat storage (31 +/- 4 vs. 56 +/- 11 min, respectively, P < 0.05). Increases in heart rate and reductions in stroke volume paralleled the rise in core temperature (36-40 degrees C), with skin blood flow plateauing at Tes of approximately 38 degrees C. These results demonstrate that high internal body temperature per se causes fatigue in trained subjects during prolonged exercise in uncompensable hot environments. Furthermore, time to exhaustion in hot environments is inversely related to the initial temperature and directly related to the rate of heat storage.

Effect of ambient temperature on human skeletal muscle metabolism during fatiguing submaximal exercise

To examine the effect of ambient temperature on metabolism during fatiguing submaximal exercise, eight men cycled to exhaustion at a workload requiring 70% peak pulmonary oxygen uptake on three separate occasions, at least 1 wk apart. These trials were conducted in ambient temperatures of 3 degrees C (CT), 20 degrees C (NT), and 40 degrees C (HT). Although no differences in muscle or rectal temperature were observed before exercise, both muscle and rectal temperature were higher (P < 0.05) at fatigue in HT compared with CT and NT. Exercise time was longer in CT compared with NT, which, in turn, was longer compared with HT (85 +/- 8 vs. 60 +/- 11 vs. 30 +/- 3 min, respectively; P < 0.05). Plasma epinephrine concentration was not different at rest or at the point of fatigue when the three trials were compared, but concentrations of this hormone were higher (P < 0.05) when HT was compared with NT, which in turn was higher (P < 0.05) compared with CT after 20 min of exercise. Muscle glycogen concentration was not different at rest when the three trials were compared but was higher at fatigue in HT compared with NT and CT, which were not different (299 +/- 33 vs. 153 +/- 27 and 116 +/- 28 mmol/kg dry wt, respectively; P < 0.01). Intramuscular lactate concentration was not different at rest when the three trials were compared but was higher (P < 0.05) at fatigue in HT compared with CT. No differences in the concentration of the total intramuscular adenine nucleotide pool (ATP + ADP + AMP), phosphocreatine, or creatine were observed before or after exercise when the trials were compared. Although intramuscular IMP concentrations were not statistically different before or after exercise when the three trials were compared, there was an exercise-induced increase (P < 0.01) in IMP. These results demonstrate that fatigue during prolonged exercise in hot conditions is not related to carbohydrate availability. Furthermore, the increased endurance in CT compared with NT is probably due to a reduced glycogenolytic rate.