Human temperature regulation during cycling with moderate leg ischaemia

The effect of graded ischaemia in the legs on the regulation of body temperature during steady-state exercise was investigated in seven healthy males. It was hypothesised that graded ischaemia in the working muscles increases heat storage within the muscles, which in turn potentiates sweat secretion during exercise. Blood perfusion in the working muscles was reduced by applying a supra-atmospheric pressure (+6.6 kPa) around the legs, which reduced maximal working capacity by 29%. Each subject conducted three separate test trials comprising 30 min of steady-state cycling in a supine position. Exercise with unrestricted blood flow (Control trial) was compared to ischaemic exercise conducted at an identical relative work rate (Relative trial), as well as at an identical absolute work rate (Absolute trial); the latter corresponding to a 20% increase in relative workload. The average (SD) increases in both the rectal and oesophageal temperatures during steady-state cycling was 0.3 (0.2)°C and did not significantly differ between the three trials. The increase in muscle temperature was similar in the Control (2.7 (0.3)°C) and Absolute (2.4 (0.7)°C) trials, but was substantially lower (P<0.01) in the Relative trial (1.4 (0.8)°C). Ischaemia potentiated (P<0.01) sweating on the forehead in the Absolute trial (24.2 (7.3) g m^–2 min^–1) compared to the Control trial (13.4 (6.2) g m^–2 min^–1), concomitant with an attenuated (P<0.05) vasodilatation in the skin during exercise. It is concluded that graded ischaemia in working muscles potentiates the exercise sweating response and attenuates vasodilatation in the skin initiated by increased core temperature, effects which may be attributed to an augmented muscle metaboreflex.     

Effect of ambient temperature on learning

After a three week exposure at 34, 25 or 10°C, three strains of mice (C57BL/6, BALB/c and A/J) underwent a conventional active two-way avoidance task. As compared to 25°C of ambient temperature, acquisition on the first day was severely impaired in the three strains at 34°C and in the C57BL/6, and A/J mice at 10°C. Learning scores on the second day were diminished in the C57BL/6 strain at 34°C and 10°C and in the BALB/c strain at 34°C. The mechanisms of such an impairment in learning at high or low temperature are discussed.     

The effect of exercise-induced muscle damage on perceived exertion and cycling endurance performance

This study evaluated the effects of exercise-induced muscle damage (EIMD) on fixed-load cycling and 5-min time-trial performance. Seven recreational athletes performed two submaximal fixed-load exercise bouts followed by a 5-min time-trial before, 48 and 168 h following 100 counter-movement jumps. Measurements of heart rate, RER and blood lactate concentration remained unchanged during the fixed-load bouts following jumping exercise. However, and increased (P < 0.05) at 48 h. RPE values were higher at 48 h as were the ratio of RPE:HR and RPE: (P < 0.05). In the time-trial, mean peak power output, mean power output, distance covered and post exercise blood lactate were lower at 48 h (P < 0.05). RPE remained unchanged between trials. These findings indicate that the ventilatory equivalent for oxygen and perceived exertion at submaximal work rates are increased 48 h following eccentric exercise. Furthermore, EIMD increases perceived exertion and impairs performance during a 5-min all-out effort.     

Effects of leg activity and ambient barometric pressure on foot swelling and lower-limb skin temperature during 8 h of sitting

Summary Prolonged immobilization in an upright position often leads to discomfort and oedema in the feet of otherwise healthy subjects. To determine the significance of leg activity and ambient pressure on oedema formation, skin temperature (T_sk) and discomfort, 6 volunteers sat for 8 h with one leg immobilized and the other spontaneously active; one day at sea level (750 mmHg) and one day at reduced barometric pressure (540 mmHg). Foot swelling was measured by water plethysmography. Leg movements were continuously monitored by a Vitalog computer, and foot discomfort was estimated by analog-visual scales. The 8 hour swelling averaged 5.7% in the inactive foot, and 2.7% in the active foot (p<0.001). T_sk of the inactive foot levelled off towards ambient temperature (21 ° C) within 4 h. For the active foot this fall was reduced by 2–3 ° C (p<0.025). The increase in foot discomfort during the day was lowest in the active foot (p<0.005). High foot T_sk was associated with a high foot swelling rate. Reduced ambient barometric pressure had no effects on foot swelling or T_sk. It is concluded that modest leg activity during 8 h of sitting has several effects on the circulation in the feet: some effects promote and some prevent oedema formation. However, the net result is a reduction in foot swelling.     

The effects of pre-warming on the metabolic and thermoregulatory responses to prolonged submaximal exercise in moderate ambient temperatures

To determine the effects of pre-warming on the human metabolic and thermoregulatory responses to prolonged steady-rate exercise in moderate ambient temperatures and relative humidities [means (SD) 21.7 (2.1)° C and 36.7 (5.4)%, respectively], six healthy men each ran at a steady-rate (70% maximal oxygen uptake) on a treadmill until exhausted after being actively pre-warmed (AH), passively pre-warmed (PH), and rested (Cont). Exercise time to exhaustion was significantly reduced following both AH and PH compared to Cont [AH 47.8 (14.0) min, PH 39.6 (16.0) min, Cont 62.0 (8.8) min; P<0.05]. During exercise there were no significant differences in oxygen uptake, total sweat loss, mean skin temperature (T_sk) and the thermal gradient (T _re–T_sk, where T _re is rectal temperature) following the three conditions. Serum prolactin, plasma catecholamine and plasma free fatty acid concentrations were also similar between all three trials. In contrast, T _re, mean body temperature, heart rate and ratings of perceived exertion were significantly greater during the initial 25 min of exercise following both AH and PH, compared with Cont (P<0.05). At exhaustion, there were no significant differences in the metabolic and thermoregulatory responses to exercise between the trials. The current findings demonstrate that AH and PH promote a reduction in prolonged submaximal endurance performance under moderate environmental temperatures compared with pre-exercise rest. Such observations appear likely to have been mediated through mechanisms associated with the earlier development of high internal body temperature which resulted in changes in the capacity for heat storage. Electronic Publication     

Separation of water and ambient temperature effects on polydipsia

The separate contributions to schedule-induced polydipsia in rats of water temperature and ambient temperature were examined by varying each while the other was held constant. Water intake varied directly with water temperature between 5° and 30°C, and decreased at 35°C and 40°C relative to 30°C. Ambient temperature did not influence water intake when water temperature was held constant. These observations favor an orolingual locus for the effect of temperature on water intake, and rule out the participation of skin temperature receptors.     

The effect of cycling followed by running on respiratory muscle performance in elite and competition triathletes

This study investigated the possibility of there being differences in respiratory muscle strength and endurance in elite and competition triathletes who have similar maximal oxygen uptakes (VO_2max) and ventilatory thresholds (Th_vent). Five internationally-ranked elite, [mean (SD) age 23.8 (1.4) years] and six nationally- and regionally-ranked competition [age 21.1 (1.1) years] male triathletes performed two successive trials: first an incremental cycle test to assess VO_2max and Th_vent and second 20 min of cycling followed by 20 min of running (C-R) at intensities higher than 85% VO_2max. Cardioventilatory data were collected every minute during the two trials, using an automated breath-by-breath system. Maximal expiratory and inspiratory (P _Imax) strength were assessed before and 10 min after C-R from the functional residual capacity. Respiratory muscle endurance was assessed 1 day before and 30 min after C-R by measuring the time limit (t _lim). The results showed firstly that during C-R, the competition triathletes had significantly (P<0.05) higher minute ventilation [mean (SEM) 107.4 (3.1) compared to 99.8 (3.7) l·min^–1], breathing frequency [44.4 (2.0) compared to 40.2 (3.4) ·min^–1] and heart rate [166 (3) compared to 159 (4) beats·min^–1] and secondly that after C-R, they had significantly lower P _Imax [127.1 (4.2) compared to 130.7 (3.0) cmH₂O] and t_lim [2:35 (0:29) compared to 4:12 (0:20) min] than the elite triathletes. We conclude that, despite similar VO_2max and Th_vent, the competition triathletes showed less extensive adaptive mechanisms, including those in the respiratory muscles, than did the elite triathletes. This led to higher ventilation, which appeared to be the cause of the faster development of fatigue in the inspiratory muscles in this group. Electronic Publication     

A thermographic study of the effect of body composition and ambient temperature on the accuracy of mean skin temperature calculations

Summary The problem associated with using measurements from a small number of sites to determine mean skin temperature was investigated by studying variations in distributions of skin temperatures of the bare torsos of humans exposed to ambient temperatures of 18, 23, and 28° C. Following a 60 minute equilibration period the temperatures of four regions (chest, abdomen, upper back, and lower back) were measured using both thermistors and an infra-red thermographic system. Regions of the torso usually represented by a single temperature exhibited significant point-to-point temperature variations especially in chilled subjects. Also an earlier finding was confirmed: in that larger variations in skin temperature distributions occur as body fat content increases. Caution must therefore be used in applying the concept of a mean skin temperature derived from a few select sites, especially with nude subjects who are chilled or have a high body fat content.     

环境温度对大鼠血液流变特性的影响

目的研究在不同环境温度对大鼠的血液流变学指标的影响。方法将7组大鼠(每组10只)分别在温度为7、12、16、22、28、32、35℃环境下饲养7 d后,用心脏穿刺法采集大鼠血液5 mL,肝素抗凝,血液采集完毕后2 h内检测血液流变学各项指标。结果环境温度在7、12、32、35℃时,血液流变学指标异常,红细胞可变性降低,毛细血管阻力增高,红细胞易聚集,从而引起机体血液循环障碍,使局部血流缓慢,造成缺血缺氧,影响组织的代谢和功能。结论血液流变性指标值在16～28℃的温度范围内最佳,血流的阻力小,红细胞的流变性好。机体保持良好微循环的最低环境温度7℃,最高环境温度32℃。     

The metabolic response to skin temperature

Experiments were done to assess that fraction of the metabolic response to external cold exposure, which is attributable to skin temperature. In 5 conscious and closely clipped goats the metabolic rate was determined at various stable levels of skin temperature in the range from 13 to 41°C, while core temperature was kept constant at 38.8°C. Skin temperature was manipulated by a rapidly circulating shower bath, while core temperature was controlled by means of heat exchangers acting on arterial blood temperature in a chronic arteriovenous shunt. The metabolic response to skin temperature fell into two clearly discernible sections: a first zone with skin temperatures above 25–30°C, within which the metabolic rate rose at a rate of –0.34±0.07 W/kg·°C with decreasing skin temperature, and a second zone with skin temperatures below 25–30°C, within which the metabolic rate either plateaued or even grew smaller with further decreasing skin temperature. It is concluded that the relationship between skin temperature and metabolic rate does not directly reproduce the temperature-response curve of cutaneous cold receptors but also reflects a complex interaction of several factors, including an unspecific temperature effect on muscle metabolism.     

Effects of metabolic rate and ambient vapour pressure on heat strain in protective clothing

Studies have shown that variations in ambient water vapour pressure from 1.7 to 3.7 kPa have little effect on heat tolerance time at a metabolic rate above 450 W while wearing protective clothing. With lighter exercise, where tolerance times exceed 60 min, variations in vapour pressure have a significant impact on evaporative heat loss and, therefore, heat tolerance. The present study has examined whether these findings extend to conditions with more extreme variations in vapour pressure. Twelve males performed light (L, 350 W) and heavy (H, 500 W) exercise at 40°C in a dry (D, 1.1 kPa) and humid (H, 4.8 kPa) environment while wearing a semi-permeable nuclear, biological and chemical protective clothing ensemble (0.29 m²×°C⁻¹·W⁻¹ or 1.88 clo; Woodcock vapour permeability coefficient,i _m=0.33). Partitional calorimetry was used to determine the rate of heat storage ( ) with evaporative heat loss from the skin ( ) calculated from changes in dressed mass or the physical properties of the clothing and the vapour pressure gradient between the skin and the environment. Skin vapour pressure was predicted from measurements of water vapour pressure above the skin surface and in the clothing with humidity sensors coupled with thermistors. Final mean skin temperature ( _sk) was higher for the humid trials and averaged 37.4 (0.3)°C, 38.9 (0.4)°C, 37.6 (0.5)°C and 38.5 (0.4)°C for LD, LH, HD and HH, respectively. Final rectal temperature (T _re ) was higher for D with respective values for LD, LH, HD and HH of 39.0 (0.4)°C, 38.7 (0.4)°C, 38.8 (0.4)°C and 38.5 (0.4)°C. Tolerance time was significantly different among the trials and averaged 120.3 (19.3) min, 54.8 (7.3) min, 63.5 (6.9) min and 36.8 (3.1) min for LD, LH, HD and HH, respectively. was overestimated and, therefore, was underestimated when the changes in dressed mass were used to determine evaporative heat loss. When skin vapour pressure determined from the humidity sensor data was used to calculate , heat storage was significantly different among the trials and averaged 15.0 (3.0), 13.0 (1.8), 14.2 (2.6) and 12.2 (1.9) kJ·kg⁻¹ for LD, LH, HD and HH, respectively. It was concluded that while wearing the protective clothing all indices of heat strain, including tolerance time, were significantly affected by the change in ambient water vapour pressure from 1.1 to 4.8 kPa during both light and heavy exercise.     

The effect of ambient temperature and exercise intensity on post-exercise thermal homeostasis

We have previously demonstrated a prolonged (65?min or longer) elevated plateau of esophageal temperature (T _es ) (0.5–0.6°C above pre-exercise values) in humans following heavy dynamic exercise (70% maximal oxygen consumption, V˙O_2max) at a thermoneutral temperature (T _a) of 29°C. The elevated T _es value was equal to the threshold T _es at which active skin vasodilation was initiated during exercise (Th_dil). A subsequent observation, i.e., that successive exercise/recovery cycles (performed at progressively increasing pre-exercise T _es levels) produced parallel increases of Th_dil and the post-exercise T _es, further supports a physiological relationship between these two variables. However, since all of these tests have been conducted at the same T _a (29°C) and exercise intensity (70% V˙O_2max) it is possible that the relationship is limited to a narrow range of T _a/exercise intensity conditions. Therefore, five male subjects completed 18?min of treadmill exercise followed by 20?min of recovery in the following T _a/exercise intensity conditions: (1) cool with light exercise, T _a?=?20°C, 45% V˙O_2max (CL); (2) temperature with heavy exercise, T _a?=?24°C, 75% O_{2 max} (TH); (3) warm with heavy exercise, T _a?=?29°C, 75% V˙O_2max (WH); and (4) hot with light exercise, T _a?=?40°C, 45% V˙O_2max (HL). An abrupt decrease in the forearm-to-finger temperature gradient (T _fa??T _fi) was used to identify the Th_dil during exercise. Mean pre-exercise T _es values were 36.80, 36.60, 36.72, and 37.20°C for CL, TH, WH, and HL conditions respectively. T _es increased during exercise, and end post-exercise fell to stable values of 37.13, 37.19, 37.29, and 37.55°C for CL, TH, WH, and HL trials respectively. Each plateau value was significantly higher than pre-exercise values (P?dil values (i.e., 37.20, 37.23, 37.37, and 37.48°C for CL, TH, WH, and HL) were comparable to the post-exercise T _es values for each condition. The relationship between Th_dil and post-exercise T _es remained intact in all T _a/exercise intensity conditions, providing further evidence that the relationship between these two variables is physiological and not coincidental.     

Water ingestion does not improve 1-h cycling performance in moderate ambient temperatures

Eight endurance-trained cyclists rode as far as possible in 1 h on a stationary cycle simulator in a moderate environment (20°C, 60% relative humidity, 3 m·s^–1 wind speed) while randomly receiving either no fluid (NF) or attempting to replace their approximate 1.71 sweat loss measured in a previous 1-h familiarisation performance ride at approximately 85% of peak oxygen uptake with artificially sweetened, coloured water (F). During F, the cyclists drank mean 1.49 (SEM 0.14)1 of which mean 0.27 (SEM 0.08)1 remained in the stomach at the end of exercise and mean 0.20 (SEM 0.05) 1 was urinated after the trial. Thus, only mean 1.02 (SEM 0.12)1 of the ingested fluid was available to replace sweat losses during the 1-h performance ride. That fluid decreased the mean average heart rate from 166 (SEM 3) to 157 (SEM 5) beats·min^–1 (P < 0.0001) and reduced the final mean serum [Na^–] and osmolalities from 143 (SEM 0.6) to 139 (SEM 0.6) matom·1^–1(P < 0.005) and from 294 (SEM 1.7) to 290 (SEM 1.9) mosmol·1^–1 (P = 0.05), respectively. Fluid ingestion did not significantly attenuate rises in plasma anti-diuretic hormone and angiotensin concentrations, or decrease the approximate-15% falls in estimated plasma volume in the F and NF trials. Nor did fluid ingestion significantly affect the approximate 1.71 · h^–1 sweat rates, the rises in rectal temperature (from 36.6° to 38.3°C) or the ratings of perceived exertion in the two trials. Ingestion of approximately 1.51 of fluid produced an uncomfortable feeling of stomach fullness and reduced the mean distance covered in 1 h from 43.1 (SEM 0.7) to 42.3 (SEM 0.6) km (P < 0.05). Thus, trying to replace more than 1.01·h^–1 sweat losses during high-intensity, short duration exercise in a moderate environment would not appear to induce beneficial physiological effects, and may impair exercise performance.     

The effect of irrigation temperature on bone healing

Objective: Osteotomies, performed by rotational instruments, can cause temperature rise on the bone and elevated temperature can disrupt the bone healing. When the osteotomies are performed for the insertion of miniscrews, the bone healing disruption may cause stability loosening or failures. Saline irrigation is mostly used for the prevention of the heat generation during osteotomy.Purpose: The purpose of this study was to evaluate the effect of the saline irrigation temperature on bone healing.Material and Method: Standardized drilling and miniscrew placement was performed in the tibias of 18 Sprague Dawley rats with rotating bur uncooled, cooled with 25°C and 4°C saline irrigations. After the 21 days, the difference in healing was observed between the uncooled and cooled groups.Results: Although there was no statistically significant difference between the group irrigated with 25°C and 4°C saline for newly bone formation, osteoblasts were seen more active and bone marrow was more dynamic in group 4°C than group 25°C. There is no disadvantage to use 25°C, but it may be better to use 4°C for rapid healing.     

Influence of ambient temperature on plasma ammonia and lactate accumulation during prolonged submaximal and self-paced running

This study examined the effects of heat stress on the accumulation of plasma ammonia, lactate, and urate during prolonged running. Nine highly trained endurance runners completed two running trials in a counterbalanced fashion in cool (15°C) and in hot (35°C) humid (60% relative humidity) conditions. Subjects ran on a motorised treadmill at 70% of peak treadmill running speed for 30 min (submaximal) followed by a self-paced 8-km performance run. Blood was drawn at pre-exercise, end-submaximal and end-performance run and analysed for plasma ammonia, lactate, and urate. Four subjects failed to complete the performance run in the heat and the performance times for the rest of the subjects was increased from 27.3 (0.6) min in cool conditions to 31.3 (1.2) min in hot conditions (P<0.05). The end-performance rectal temperature was 38.6 (0.1) and 39.2 (0.1)°C (P<0.05) in cool and hot conditions, respectively. Differences in plasma lactate at the end of submaximal running were not significant. However, at the end of performance runs lactate was 6.0 (0.9) mmol·l^–1 in cool and 3.1 (0.5) mmol·l^–1 in hot conditions, values that were significantly different (P<0.05). Plasma ammonia increased from pre-exercise to ≅59 μmol·l^–1 at the end-submaximal runs for both coditions and further at the end of performance runs to 108.5 (11) μmol·l^–1 (P<0.05) in hot but not in cool conditions. Plasma urate increased from pre-exercise to 311.2 (25.9) μmol·l^–1 at end-submaximal runs and to ≅320.4 μmol·l^–1 at end-performance runs in hot and cool environments. The findings that plasma urate accumulation was similar at the completion of running in both conditions, while ammonia was significantly augmented in hot conditions compared with cool, suggest that ammonia accumulation during heat stress exercise might be derived from sources other than purine catabolism. Electronic Publication     

Combined effect of heat stress, dehydration and exercise on neuromuscular function in humans

This study examined the combined effect of exercise induced hyperthermia and dehydration on neuromuscular function in human subjects. Six trained male runners ran for 40 min on a treadmill at 65% of their maximal aerobic velocity while wearing a tracksuit covered with an impermeable jacket and pants to impair the evaporation of sweat. These stressful experimental running conditions led the runners to a physiological status close to exhaustion. On average, the 40 min run ended at a heart rate of 196 (SD 8) beats · min⁻¹, a tympanic temperature of 40 (SD 0.3) °C and with a loss of body mass of 2 (SD 0.5)%. Pre- and post-running strength tests included measurements of maximal knee extension and flexion torques in both isometric and isokinetic (at 60 and 240° · s⁻¹) conditions. A 20 s endurance test at 240° · s⁻¹ was also performed. Surface electromyographic (EMG) activity was recorded from six knee extensor and flexor muscles during the entire protocol. The treadmill run led to clear decrements in maximal extension torque and EMG activity both in isometric and at the slowest isokinetic velocity (60° · s⁻¹). However, no differences in these parameters were observed at 240° · s⁻¹. Furthermore, the EMG patterns of the major knee extensor and flexor muscles remained remarkably stable during the treadmill run. These results demonstrate that the exercise-induced hyperthermia and dehydration in the present experiments had only minor effects on the neuromuscular performance. However, it is also suggested that high internal body temperature per se could limit the production of high force levels. Accepted: 26 September 2000     

Revisiting the effect of posture on high-intensity constant-load cycling performance in men and women

It was recently observed that inclining the body from a supine to upright position improved the performance of high-intensity, constant-load cycling to a larger extent in men than women (Egaña et al. in Eur J Appl Physiol 96:1–9, 2006), although this gender-related effect was based on a small number of men (n  =  5) and women (n  =  5). To explore this effect further, we studied the effect of body tilt on cycling performance in a larger and different group of men (n  =  8) and women (n  =  18). Peak power, peak \({{\dot{V}}\hbox{O}_{2}}\) and the ventilatory threshold (VT) were determined during an upright maximal graded cycle test, and a high-intensity test (80% peak power) was performed to failure in both the upright and supine positions. Performance was significantly longer in the upright compared with supine position in men (17.4 ± 7.7 vs. 7.6 ± 3.4 min) and women (14.1 ± 6.0 vs. 6.0 ± 3.7 min). The magnitude of this postural effect was not significantly different between men and women; whereas it was significantly correlated with the relative intensity of exercise expressed as a function of VT (r  =  ?0.39). These data demonstrate that the postural effect on high-intensity cycling performance is not influenced by gender; but that it is related to the intensity of exercise relative to the ventilatory threshold.     

Effects of temperature on electromyogram and muscle function

Summary The effects of 30 min of cooling (15°C water) and warming (40°C water) on arm muscle function were measured. A reference condition (24°C air) was included. Of nine young male subjects the maximal grip force (F _max), the time to reach 66% ofF _max (rate of force buildup) and the maximal rhythmic grip frequency were determined, together with surface electromyographic activity (EMG) of a forearm muscle (flexor digitorum superficialis). The results showed that in contrast to warming, cooling resulted in a significant decrease of 20% in the FmaX and a significant 50% decrease in force build-up time and the maximal rhythmic grip frequency. The relationship between the root mean square value (rms) of the EMG and the static grip force did not change due to temperature changes. The median power frequency (MPF) in the power spectrum of the EMG signal decreased by 50% due to cooling but remained unchanged with heating. During a sustained contraction at 15% ofF _max (F _max depending on the temperature) the increase in the rms value with contraction time was 90% larger in the warm condition and 80016 smaller in the cold condition compared to the increase in the reference condition. The MPF value remained constant during the warm and reference conditions but in the cold it started at a 50% lower value and increased with contraction time. Since the endurance time was not affected in the cold but 60% reduced in the warm, it was concluded that neither the rms nor the MPF reflected unambiguously the temperature related changes in functional performance of muscle strain with an equal relative load.     

No effect of caffeine on exercise performance in high ambient temperature

Caffeine, an adenosine receptor antagonist, has shown to improve performance in normal ambient temperature, presumably via an effect on dopaminergic neurotransmission through the antagonism of adenosine receptors. However, there is very limited evidence from studies that administered caffeine and examined its effects on exercise in the heat. Therefore, we wanted to study the effects of caffeine on performance and thermoregulation in high ambient temperature. Eight healthy trained male cyclists completed two experimental trials (in 30°C) in a double-blind-randomized crossover design. Subjects ingested either placebo (6 mg/kg) or caffeine (6 mg/kg) 1 h prior to exercise. Subjects cycled for 60 min at 55% W (max), immediately followed by a time trial to measure performance. The significance level was set at p < 0.05. Caffeine did not change performance (p = 0.462). Rectal temperature was significantly elevated after caffeine administration (p < 0.036). Caffeine significantly increased B-endorphin plasma concentrations at the end of the time trial (p = 0.032). The present study showed no ergogenic effect of caffeine when administered 1 h before exercise in 30°C. This confirms results from a previous study that examined the effects of caffeine administration on a short (15 min) time trial in 40°C. However, caffeine increased core temperature during exercise. Presumably, the rate of increase in core temperature may have counteracted the ergogenic effects of caffeine. However, other factors such as interindividual differences in response to caffeine and changes in neurotransmitter concentrations might also be responsible for the lack of performance improvement of caffeine in high ambient temperature.