Thermoregulation, pacing and fluid balance during mass participation distance running in a warm and humid environment

Deep body temperature (T _c), pacing strategy and fluid balance were investigated during a 21-km road race in a warm and humid environment. Thirty-one males (age 25.3 ± 3.2 years; maximal oxygen uptake 59.1 ± 4.2 ml kg⁻¹ min⁻¹) volunteered for this study. Continuous T _c responses were obtained in 25 runners. Research stations at approximately 3-km intervals permitted accurate assessment of split times and fluid intake. Environmental conditions averaged 26.4°C dry bulb temperature and 81% relative humidity. Peak T _c was 39.8 ± 0.5 (38.5–40.7) °C with 24 runners achieving T _c > 39.0°C, 17 runners ≥39.5°C, and 10 runners ≥40.0°C. In 12 runners attaining peak T _c ≥ 39.8°C, running speed did not differ significantly when T _c was below or above this threshold (208 ± 15 cf. 205 ± 24 m min⁻¹; P = 0.532). Running velocity was the main significant predictor variable of ∆T _c at 21 km (R ² = 0.42, P < 0.001) and was the main discriminating variable between hyperthermic (T _c ≥ 39.8°C) and normothermic runners (T _c < 39.8°C) up to 11.8 km. A reverse J-shaped pacing profile characterised by a marked reduction in running speed after 6.9 km and evidence of an end-spurt in 16 runners was observed. Variables relating to fluid balance were not associated with any T _c parameters or pacing. We conclude that hyperthermia, defined by a deep body temperature greater than 39.5°C, is common in trained individuals undertaking outdoor distance running in environmental heat, without evidence of fatigue or heat illness.     

Advantages of a smaller bodymass in humans when distance-running in warm, humid conditions

Using a 65-kg athlete running a 2?h 10?min marathon as an example, we estimated that imbalances between approximately 1400?W of heat production and dissipation would occur in ambient temperatures of 17°C at 90% relative humidity (rh) to 37°C at 50% rh. Because heat production during running depends on body mass and heat loss depends on surface area, intercepts between predicted heat production and maximal heat loss with increasing speeds depend on an athlete's body mass. At 35°C and 60% rh, a 45-kg athlete could maintain thermal balance by running a 2?h 13?min marathon at 19.1?km?·?h^?1 but a 75-kg athlete would only be able run a 3?h 28?min marathon at 12.2?km?·?h^?1. In both cases, the production of 970–1020?W of heat would necessitate the evaporation of at least 1.5–1.6?l of sweat per hour. A lower metabolic heat production in lighter runners at any given speed may be one reason why smallness of stature is an asset in distance running.     

Influence of individual energy cost on running capacity in warm, humid environments

Purpose

Challenging environmental conditions including heat and humidity are associated with particular risks to the health of runners and triathletes during prolonged events. The heat production of a runner is the product of its energy cost of running (C _r) by its velocity. Since C _r varies greatly among humans, those individuals with high C _r are more exposed to heat stress in warm and humid conditions. Although risk factor awareness is crucial to the prevention of heat stroke and potential fatalities associated therewith, how C _r affects the highest sustainable velocity (V) at which maximal heat loss matches heat production has not been quantified to date.

Methods

Here, we computed in virtual runners weighting 45–75 kg, the influence of C _r variability from 3.8 to 4.4 J·m^?1·kg^?1 on V. Heat loss by radiation, convection, and conduction was assessed from known equations including body dimensions, running velocity (3.4–6.2 m·s^?1), air temperature (T _a, 10–35 °C) and relative humidity (r _h, 50, 70 and 90 %).

Results

We demonstrated a marked and almost linear influence of C _r on V in hot and humid conditions: +0.1 J·kg^?1·m^?1 in C _r corresponded to ?4 % in V. For instance, in conditions 25 °C r _h 70 %, 65-kg runners with low C _r could sustain a running speed of 5.7 m·s^?1 as compared to only 4.3 m·s^?1 in runners with high C _r, which is huge.

Conclusion

We conclude that prior knowledge of individual C _r in athletes exposed to somewhat warm and humid environments during prolonged running is one obvious recommendation for minimizing heat illness risk.     

Evidence for neuromuscular fatigue during high-intensity cycling in warm, humid conditions

The purpose of this study was to examine and describe the neuromuscular changes associated with fatigue using a self-paced cycling protocol of 60-min duration, under warm, humid conditions. Eleven subjects [mean (SE) age 21.8 (0.8) years; height 174.9 (3.0) cm; body mass 74.8 (2.7) kg; maximum oxygen consumption 50.3 (1.8) ml · kg · min⁻¹] performed one 60-min self-paced cycling time trial punctuated with six 1-min “all out” sprints at 10-min intervals, while 4 subjects repeated the trial for the purpose of determining reproducibility. Power output, integrated electromyographic signal (IEMG), and mean percentile frequency shifts (MPFS) were recorded at the mid-point of each sprint. There were no differences between trials for EMG variables, distance cycled, mean heart rate, and subjective rating of perceived exertion for the subjects who repeated the trial (n=4). The results from the repeated trials suggest that neuromuscular responses to self-paced cycling are reproducible between trials. The mean heart rate for the 11 subjects was 163.6 (0.71) beats · min⁻¹. Values for power output and IEMG expressed as a percentage of that recorded for the initial sprint decreased during sprints 2–5, with normalised values being 94%, 91%, 87% and 87%, respectively, and 71%, 71%, 73%, and 77%, respectively. However, during the final sprint normalised power output and IEMG increased to 94% and 90% of initial values, respectively. MPFS displayed an increase with time; however, this was not significant (P=0.06). The main finding of this investigation is the ability of subjects to return power output to near initial values during the final of six maximal effort sprints that were included as part of a self-paced cycling protocol. This appears to be due to a combination of changes in neuromuscular recruitment, central or peripheral control systems, or the EMG signal itself. Further investigations in which changes in multiple physiological systems are assessed systematically are required so that the underlying mechanisms related to the development of fatigue during normal dynamic movements such as cycling can be more clearly delineated. Accepted: 25 September 2000     

The influence of menthol on thermoregulation and perception during exercise in warm, humid conditions

Menthol has recently been added to various cooling products that claim to enhance athletic performance. This study assessed the effect of two such solutions during exercise in warm, humid conditions. Twelve participants (22 ± 2.9 years; [(V)\dot]\textO_2\textpeak \dot{V}{\text{O}}_{{2{\text{peak}}}} 47.4 ± 6.2 mL kg⁻¹ min⁻¹) completed a peak power (PO_peak) test and three separate exercise bouts in 30°C and 70% relative humidity after being sprayed with 100 mL of water containing either 0.05 or 0.2% l-menthol, or a control spray. During each trial, participants underwent 15 min of rest, spraying, 15 min of rest and 45 min of exercise at 45% of PO_peak. The following variables were measured: rectal temperature (T _re), sweat rate (SR), skin blood flow (SBF), heart rate (HR), thermal comfort (TC) and sensation (TS) votes, irritation (IRR) and rating of perceived exertion (RPE). Mean skin (MST) and body temperatures ( [`(T)]<sub>\textbody</sub> \bar{T}_{\text{body}} ) were calculated. There was no significant difference in MST, [`(T)]_\textbody \bar{T}_{\text{body}} SR, SBF, HR, TC or RPE between conditions. Spraying with 0.2% menthol significantly (P < 0.05) elevated T _re by 0.2°C compared to the other conditions. Both menthol sprays caused participants to feel significantly cooler than control spraying (P = 0.001), but 0.2% spraying induced significantly cooler sensations (P = 0.01) than 0.05% spraying. Both menthol sprays induced greater irritation (P < 0.001) than control spraying. These findings suggest that 0.05% menthol spraying induced cooler upper body sensations without measurable thermoregulatory impairment. T _re was significantly elevated with 0.2% spraying. Irritation persisted with both menthol sprays while TC remained unchanged, suggesting a causal relationship. The use in sport of a spray similar to those tested here remains equivocal.     

Mean skin temperature in warm humid climates

Summary Mean skin temperature was measured in 24 subjects during experiments in a climatic chamber. Three conditions of ambient temperature (T _a=25.6°, 28.9° and 32.2° C), and three of humidity (relative humidity = 50%, 70% and 90%) were studied. A relationship was established by a linear regression technique. It is valid in the 24°–34° C range, for air velocity =0.2 m·s⁻¹, clothing insulation =0.077° C·m²·w⁻¹ (0.5 clo), metabolic rate =64 w·m⁻² (1.1 met) and radiant temperature = air temperature. In these conditions =28.125+0.021P _w+0.210T _a (P _w: ambient water vapour pressure in mb). It shows a small humidity influence. The influences of sex, transition from one condition to the next, and air velocity were also studied. Measurements in Africa confirmed the small influence of humidity. Ethnic life-style differences indicated that a high precision in determination is difficult to achieve.     

Increased incidence of Lyme borreliosis in southern Sweden following mild winters and during warm, humid summers

The aim of the present study was to investigate the long-term incidence rate of Lyme borreliosis and, additionally, to determine whether a correlation exists between climatic factors and summer-season variations in the incidence of Lyme borreliosis. Climatic variability acts directly on tick population dynamics and indirectly on human exposure to Lyme borreliosis spirochetes. In this study, conducted in primary healthcare clinics in southeastern Sweden, electronic patient records from 1997–2003 were searched for those that fulfilled the criteria for erythema migrans. Using a multilevel Poisson regression model, the influence of various climatic factors on the summer-season variations in the incidence of erythema migrans were studied. The mean annual incidence rate was 464 cases of erythema migrans per 100,000 inhabitants. The incidence was significantly higher in women than in men, 505 and 423 cases per 100,000 inhabitants, respectively (p<0.001). The summer-season variations in the erythema migrans incidence rate correlated with the monthly mean summer temperatures (incidence rate ratio 1.12; p<0.001), the number of winter days with temperatures below 0°C (incidence rate ratio 0.97; p<0.001), the monthly mean summer precipitation (incidence rate ratio 0.92; p<0.05), and the number of summer days with relative humidity above 86% (incidence rate ratio 1.04; p<0.05). In conclusion, Lyme borreliosis is highly endemic in southeastern Sweden. The climate in this area, which is favourable not only for human tick exposure but also for the abundance of host-seeking ticks, influences the summer-season variations in the incidence of Lyme borreliosis.     

Oxygen uptake during running as related to body mass in circumpubertal boys: a longitudinal study

Summary To investigate the effect of endurance training on physiological characteristics during circumpubertal growth, eight young runners (mean starting age 12 years) were studied every 6 months for 8 years. Four other boys served as untrained controls. Oxygen uptake ( O ₂) and blood lactate concentrations were measured during submaximal and maximal treadmill running. The data were aligned with each individual's age of peak height velocity. The maximal oxygen uptake ( O _2max; ml · kg^–1 · min^–1) decreased with growth in the untrained group but remained almost constant in the training group. The oxygen cost of running at 15 km · h^–1 ( O ₂₁₅, ml · kg^–1 · min^–1) was persistently lower in the trained group but decreased similarly with age in both groups. The development of O _2max and O ₂₁₅ (1 · min^–1) was related to each individual's increase in body mass so that power functions were obtained. The mean body mass scaling factor was 0.78 (SEM 0.07) and 1.01 (SEM 0.04) for O _2max and 0.75 (SEM 0.09) and 0.75 (SEM 0.02) for O ₂₁₅ in the untrained and trained groups, respectively. Therefore, expressed as ml · kg^–0.75 · min^–1, O ₂₁₅ was unchanged in both groups and O _2max increased only in the trained group. The running velocity corresponding to 4 mmol · 1^–1 of blood lactate ( _la4) increased only in the trained group. Blood lactate concentration at exhaustion remained constant in both groups over the years studied. In conclusion, recent and the present findings would suggest that changes in the oxygen cost of running and O _2max (ml · kg^–1 · min^–1) during growth may mainly be due to an overestimation of the body mass dependency of O ₀₂ during running. The O ₂ determined during treadmill running may be better related to kg^0.75 than to kg¹.     

Stress-induced rise of body temperature in rats is the same in warm and cool environments

Several forms of psychological stress result in a rise in body temperature in rats. In this study, we report that rats housed at a low ambient temperature (11.1 degrees C) develop stress-induced rises in body temperature that do not differ from the responses seen when the animals are kept at a temperature within their thermoneutral zone (24.7 degrees C). These data support the hypothesis that stress-induced "hyperthermia" is a regulated rise in temperature (i.e., a rise in thermoregulatory "set-point," or fever), and is not simply the result of metabolic changes associated with the stress response itself.     

A modified,local sweat collector for warm and humid conditions

Summary The purpose of this study was to modify a previously described local sweat collector to facilitate the investigation of sweat rate and composition in a warm (30°C) and humid (relative humidity 80%) environment. The adherence of the collector to the skin was improved and a pouch was appended at the lower end of the collector. The limitations of the closed collector were examined by comparing the local sweat rate and the quantity of electrolyte excreted in sweat with those obtained using a second collector with a wide opening (to permit free evaporation) and by changes in the body mass. Eight subjects performed exercise on a cycle ergometer consisting of four equal periods of 15 min each, at 60% maximal oxygen consumption, with a rest of 5 min between each period. The sweat produced on a local skin area (85 cm², upper posterior thorax region) was collected at the end of each period, before measuring the body mass on a sensitive (±1 g) platform balance. The mean local sweat rate [2.61 (SEM 0.19) Mg-CM^–2. min^-1] was 2.4 times greater than the pro-rated whole body mass loss but the two were strongly correlated (r=0.82,P<0.01). Compared to the open collector, the greater quantity of electrolyte excreted into the closed collector would suggest that the conditions which prevailed in the closed collector, such as a higher local skin temperature, may have affected the function of the sweat gland. This method enabled the efficiency of local sweat evaporation to be assessed by measuring the difference between sweat volume collected in the open and in the closed collectors. Recovery of water volumes at rest indicated that no contamination and no apparent leakage occurred. This improved sweat collector is suitable for obtaining clean local sweat samples of up to 6 ml, and for measuring the sweat composition and also sweat rate during exercise in warm and humid conditions.     

Influence of training,sex, age and body mass on the energy cost of running

Summary To highlight the influences of age, sex, body mass (m _b) and running training on the energy cost of running (C _r) young basketball players [38 boys (BB) and 14 girls (BG), aged 14.2 (SD 0.3) and 12.2 (SD 1.9) years, respectively] were selected to be compared to middle-distance runners [27 men (MR) and 14 women (FR) aged 23.7 (SD 3.4) and 23.9 (SD 4.1) years, respectively]. TheC _r was measured during a maximal treadmill test. In each groupC _r and body mass (m _b) and body height were negatively and significantly correlated. A stepwise regression showed that among both the body dimensions measured,m _b was the most important factor in determining the variations ofC _r For the whole group (n=93) the correlation coefficient was 0.72 (P<0.0001). For a givenm _b, there was no significant difference between theC _r of BG, BB and MR: this result would support the hypothesis that the differences inC _r currently attributed to age, running training or sex differences are mainly related tom _b. On the other hand, for a givenm _b, FR showed a significantly lower Cr than the basketball players (P<0.01 for BG and BB) and than MR (P<0.05), thus suggesting that women decrease theirC _r as a response to running training more efficiently than do men.     

Acute and adaptive responses in humans to exercise in a warm, humid environment

 Acute and repeated exposure for 8–13 consecutive days to exercise in humid heat was studied. Twelve fit subjects exercised at 150 W [45% of maximum O₂ uptake (V.O₂,_max)] in ambient conditions of 35°C and 87% relative humidity which resulted in exhaustion after 45 min. Average core temperature reached 39.9 ± 0.1°C, mean skin temperature (T– _sk) was 37.9 ± 0.1°C and heart rate (HR) 152 ± 6 beats min^–1 at this stage. No effect of the increasing core temperature was seen on cardiac output and leg blood flow (LBF) during acute heat stress. LBF was 5.2 ± 0.3 l min^–1 at 10 min and 5.3 ± 0.4 l min^–1 at exhaustion (n = 6). After acclimation the subjects reached exhaustion after 52 min with a core temperature of 39.9 ± 0.1°C, T– _sk 37.7 ± 0.2°C, HR 146 ± 4 beats min^–1. Acclimation induced physiological adaptations, as shown by an increased resting plasma volume (3918 ± 168 to 4256 ± 270 ml), the lower exercise heart rate at exhaustion, a 26% increase in sweating rate, lower sweat sodium concentration and a 6% reduction in exercise V.O₂. Neither in acute exposure nor after acclimation did the rise of core temperature to near 40°C affect metabolism and substrate utilization. The physiological adaptations were similar to those induced by dry heat acclimation. However, in humid heat the effect of acclimation on performance was small due to physical limitations for evaporative heat loss. Received: 3 July 1996 / Received after revision: 26 September 1996 / Accepted: 7 January 1997     

Training errors in long distance running

Among the most common factors causing injury in long distance runners are training errors. Even the most elite runner may lose valuable workout time when training terrain, equipment, or the progression of workouts, is not appropriate. Through this article we hope to help athletic trainers identify specific errors in athletes' training and their related injuries. Additional suggestions are made that will aid the professional in workout modification in order to prevent injury or to expedite recovery.     

Prediction of mean skin temperature in warm environments

Summary The data collected by the authors in four experimental series have been analysed together with data from the literature, to study the relationship between mean skin temperature and climatic parameters, subject metabolic rate and clothing insulation. The subjects involved in the various studies were young male subjects, unacclimatized to heat. The range of conditions examined involved mean skin temperatures between 33‡ C and 38‡ C, air temperatures (T_a) between 23‡ C and 50‡ C, ambient water vapour pressures (P_a) between 1 and 4.8 kPa, air velocities (V_a) between 0.2 and 0.9 m · s⁻¹, metabolic rates (M) between 50 and 270 W · m⁻², and Clo values between 0.1 and 0.6. In 95% of the data, mean radiant temperature was within ±3‡ C of air temperature. Based on 190 data averaged over individual values, the following equation was derived by a multiple linear regression technique: ˉT_sk=30.0+0.138T_a+0.254P_a−0.57V_a+1.28 · 10⁻³ M−0.553 Clo. This equation was used to predict mean skin temperature from 629 individual data. The difference between observed and predicted values was within ±0.6‡ C in 70% of the cases and within ±1‡ C in 90% of the cases. It is concluded that the proposed formula may be used to predict mean skin temperature with satisfactory accuracy in nude to lightly clad subjects exposed to warm ambient conditions with no significant radiant heat load.