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Rain influences the physiological and metabolic responses to exercise in hot conditions a
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Ryo Ito , Naoyuki Yamashita , Eiko Suzuki & Takaaki Matsumoto a
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University Educational Center, Nihon Fukushi University, Mihama-cho, Japan
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Laboratory for Exercise Physiology and Biomechanics, Graduate School of Health and Sport Sciences, Chukyo University, Toyota, Japan Published online: 02 Jan 2015.
Click for updates To cite this article: Ryo Ito, Naoyuki Yamashita, Eiko Suzuki & Takaaki Matsumoto (2015): Rain influences the physiological and metabolic responses to exercise in hot conditions, Journal of Sports Sciences, DOI: 10.1080/02640414.2014.977938 To link to this article: http://dx.doi.org/10.1080/02640414.2014.977938
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Journal of Sports Sciences, 2014 http://dx.doi.org/10.1080/02640414.2014.977938
Rain influences the physiological and metabolic responses to exercise in hot conditions
RYO ITO1, NAOYUKI YAMASHITA2, EIKO SUZUKI2 & TAKAAKI MATSUMOTO2 1
University Educational Center, Nihon Fukushi University, Mihama-cho, Japan and 2Laboratory for Exercise Physiology and Biomechanics, Graduate School of Health and Sport Sciences, Chukyo University, Toyota, Japan
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(Accepted 10 October 2014)
Abstract Outdoor exercise often proceeds in rainy conditions. However, the cooling effects of rain on human physiological responses have not been systematically studied in hot conditions. The present study determined physiological and metabolic responses using a climatic chamber that can precisely simulate hot, rainy conditions. Eleven healthy men ran on a treadmill at an intensity of 70% VO2max for 30 min in the climatic chamber at an ambient temperature of 33°C in the presence (RAIN) or absence (CON) of 30 mm · h−1 of precipitation and a headwind equal to the running velocity of 3.15 ± 0.19 m · s−1. Oesophageal temperature, mean skin temperature, heart rate, rating of perceived exertion, blood parameters, volume of expired air and sweat loss were measured. Oesophageal and mean skin temperatures were significantly lower from 5 to 30 min, and heart rate was significantly lower from 20 to 30 min in RAIN than in CON (P < 0.05 for all). Plasma lactate and epinephrine concentrations (30 min) and sweat loss were significantly lower (P < 0.05) in RAIN compared with CON. Rain appears to influence physiological and metabolic responses to exercise in heat such that heat-induced strain might be reduced. Keywords: thermoregulation, hyperthermia, rain, heart rate, lactate
1. Introduction Outdoor exercise such as jogging, soccer, field hockey and rugby often proceeds in the rain. We previously showed that oesophageal temperature (Tes) decreases and oxygen consumption (VO2) and plasma lactate concentrations (La) increase while running in rain at 5°C (Ito, Nakano, Yamane, Amano, & Matsumoto, 2013). Several studies have addressed the effects of rain and cold on human physiological responses during exercise (Ito et al., 2013; Thompson & Hayward, 1996; Tikuisis, Ducharme, Moroz, & Jacobs, 1999; Weller, Millard, Stroud, Greenhaff, & Macdonald, 1997), but to the knowledge of the authors, rain also falls on hot days, and the effects of exercising under such conditions have not been examined. The influence of exposure to heat on human physiological responses during exercise is well-established (Gonzälez-Alonso et al., 1999; MacDougall, Reddan, Layton, & Dempset, 1974). Hyperthermia increases rates of muscle glycogenolysis (Febbraio, Snow, Stathis, Hargreaves, & Carey, 1994, 1996),
lactate production (Morris, Nevill, Boobis, Macdonald, & Williams, 2005; Schumacker, Rowland, Saltz, Nelson, & Wood, 1987), imposes cardiovascular strain (Gonzälez-Alonso et al., 1999) and induced central fatigue (Brück & Olschewski, 1987; Nybo & Nielsen, 2001). Some studies have examined thermoregulatory responses to wetting the skin during prolonged exercise, although the effects of rain were not evaluated. Gisolfi and Copping (1974) and Davies, Brotherhood, and Zeidifard (1976) showed that sponging water onto the skin does not affect rectal temperature in the heat. Sherman and Deutsch similarly reported that spraying the skin with water does not affect rectal temperature while running on a treadmill in a hot environment (Sherman & Deutsch, 1983). However, the body and clothing will become completely and continuously soaked in rain, and a generated headwind will increase the amount of heat lost from the body. Moreover, Sawaka, Cheuvront, and Kenefick (2012) recently reported that hot skin (>35°C) associated with high skin blood flow requirements can reduce aerobic exercise
Correspondence: Ryo Ito, University Educational Center, Nihon Fukushi University, Okuda, Mihama-cho, Chita-gun, Aichi-pref, Mihama-Cho 4703295, Japan. E-mail: [email protected] © 2014 Taylor & Francis
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performance. Thus, the cooling effect of rain at the skin surface reduces physiological heat strain in individuals who exercise in heat. The present study aimed to determine the impact of 30 min of steadystate running in rain and heat on thermal and physiological responses. We hypothesised that rain would lower core and skin temperature and suppress the physiological and metabolic responses during steady-state running under hot conditions. 2. Methods
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2.1. Ethical approval The Human Subjects Committee at Chukyo University Graduate School of Health and Sport Sciences approved the study, which conformed to the standards established in the 2008 revision of the Declaration of Helsinki. 2.2. Participants Eleven healthy men (age, 22.9 ± 1.5 years; height, 174.3 ± 6.9 cm; body mass, 67.4 ± 6.4 kg; VO2max, 55.5 ± 6.4 mL · kg−1 · min−1; means ± 1 s) who were non-smokers and not under medication participated in this study. All of the participants were active and recreationally exercised at least three times weekly. They were fully informed about the nature and purpose of the study before providing written informed consent to participate. 2.3. Simulated rain conditions Outdoor rainy conditions were simulated in terms of temperature, humidity, rain, and wind using a TBR-12A4PX climatic chamber (ESPEC, Osaka, Japan). The participants ran for 30 min and were continuously soaked with water (rain) delivered at 30 mm · h−1 from five nozzles placed on a wall 1.5 m in front and 2.2 m above a treadmill. A headwind equal to running speed was generated from a mesh panel located 1.5 m in front of the treadmill. The temperature of the “rain” emerging from the nozzle was ~3°C below the ambient temperature of 33°C. 2.4. Experimental protocol The participants initially performed an incremental running test to the point of volitional exhaustion on a BM-200 treadmill (S & ME, Tokyo, Japan) at 20°C to determine VO2max. VO2max was considered to have been achieved if at least two of the following criteria were met: respiratory exchange ratio (RER) ≥1.15; VO2 plateau during the last stage of exercise and maximal heart rate
(HR) within ±10 beats per min of age-predicted values. All participants underwent two trials in the TBR-12A4PX climatic chamber at intervals of 7– 10 days. All of them fasted for at least 12 h, abstained from alcohol or caffeine and from vigorous physical activities for 24 h before each trial and presented at the laboratory between 8 and 9 A.M. They were weighed in the nude and then dressed in identical 100% polyester half-sleeved running shirts and running shorts and rested while seated for about 30 min in a comfortable environment (26°C). During this period, an ST24S thermistor probe (Senser Technica, Seto, Japan) was inserted via the nasal passage to a distance equivalent to 25% of the participant’s height to measure Tes (Bruning, Dahmus, Kenney, & Alexander, 2013). ST-23S thermistors (Senser Technica, Seto, Japan) were also attached to the chest, upper arm, thigh and calf to measure skin temperature. Weighted mean skin temperature (Tsk) was calculated as described by Ramanathan (1964). A JS-N2325RSP indwelling catheter (JMS, Hiroshima, Japan) was inserted into a superficial forearm vein, and blood samples were withdrawn through three-way valves flushed with heparinised saline before entering the climatic chamber, after 10 min of rest in the chamber (0 min of exercise), and after 10, 20 and 30 min of exercise. The electrocardiography findings were monitored using a QI-236P radiotelemeter (Nihon Kohden, Tokyo, Japan), and HR was recorded every 30 s. Expired air was analysed every 30 s using an AE300S automatic gas analyser (Minato Medical Science, Osaka, Japan), and the rating of perceived exertion (RPE) was reported every 10 min using the Borg category scale (Borg, 1973). The participants dried themselves with a towel and were weighed in the nude after exercise. Sweat loss was calculated by subtracting body weight after exercise from that before. The participants entered the environmental chamber (ambient temperature, 33°C; relative humidity, 50%) and rested while seated for 10 min and not exposed to rain or wind. Thereafter, the participants started running on the treadmills for 30 min at 70% VO2max (velocity: 3.15 ± 0.19 m · sec−1) in the presence (RAIN) or absence (CON) of rain. The relative humidity was set 50% during rest and exercise in CON, but in RAIN, the relative humidity increased to 85 ± 5% during exercise. A headwind equal to running speed was provided during both trials. Expired air, HR, Tes and Tsk, were measured after 5 min of rest in the chamber to allow time to waterproof the measuring equipment. The experimental trials were administered in randomised order at least 7 days apart.
Effect of rain during exercise in heat
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2.5. Analytical techniques All blood samples (10 mL) were collected from a superficial forearm vein using a syringe. Portions (2 mL) were dispensed into chilled tubes containing EDTA-2Na and sodium fluoride and were separated by centrifugation (4°C, 3000 rpm, 15 min). Concentrations of La and plasma glucose (YSI 2300 STAT PLUS, Yellow Springs, OH, USA) were measured in the supernatant. Larger portions (4 mL) were also dispensed into tubes containing only EDTA-2Na and separated by centrifugation as described above. Plasma epinephrine and norepinephrine concentrations were measured by HPLC (FP-1520S, JASCO International Co., Hachioji, Japan). The remainder (4 mL) of the blood samples were dispensed into plain tubes and separated by centrifugation (at 20°C, 3000 rpm, 15 min), and then serum levels of free fatty acids (FFA) and triglycerides (TG) were determined using an automated analyser (7600-01, Hitachi, Tokyo, Japan). All parameters except La and plasmas glucose were measured by SRL (http://www.srl-group.co.jp/ english/index.html). 2.6. Statistical analysis All data were statistically analysed using SPSS software Version 10 (SPSS, Chicago, IL, USA) and checked for normality. Experimental data were compared using two-way (trial by time) repeated measures ANOVA. Where sphericity was significantly violated, F values were adjusted using Huynh-Felf corrections as appropriate. Significant main or interaction effects were assessed using Fisher’s least significant difference post hoc test. Differences in sweat volume between RAIN and CON were assessed using a paired t-test. Significance was taken at P < 0.05. The magnitude of the difference in significant effects was compared using Cohen’s d effect size (ES) and thresholds (0.5 (small), 0.5–0.79 (moderate), or ≥0.8 (large)) (Chohen, 1988). Mean values with s are presented in tabular and graphic formats. The primary outcome of interest in this experiment was Tes. Based on a post hoc power analysis (G*power 3.1.3 Macintosh Edition (Faul, Erdfelder, Lang, & Buchner, 2007)), the statistical power (1 - β) of the Tes variables in this test exceeded 0.80, and a sample size of eight for each condition was adequate.
Figure 1. Oesophageal temperature (°C) during rest and exercise for 30 min at 70% VO2max with (RAIN) and without (CON) 30 mm · h−1 of precipitation at 33°C. Values are presented as means ± s (n = 11). *Significantly different from control (P < 0.05).
Figure 2. Mean weighted skin temperature (°C) during rest and exercise for 30 min at 70% VO2max with (RAIN) and without (CON) 30 mm · h−1 of precipitation at 33°C. Values are presented as means ± s (n = 11). *Significantly different from control (P < 0.05).
reached a maximum of 37.7°C. The Tes was significantly lower in RAIN than in CON during the exercise (P < 0.05) (ES = 0.33–0.52) (Figure 1). Tsk in CON essentially remained at 34.5°C throughout the exercise. Tsk in RAIN decreased from 34.4°C to 30.4°C during the first 5 min and remained significantly lower than that in CON throughout the exercise (P < 0.05) (ES = 0.36– 0.83) (Figure 2). VO2 and minute ventilation (VE) did not significantly differ between RAIN and CON, whereas RER in RAIN and CON
3. Results Exercise resulted in a progressive increase in Tes from 36.5°C to 38.4°C in CON. The Tes in RAIN decreased from 36.5°C to 36.0°C during the first 5 min of exercise, started to increase thereafter and
Figure 3. Heart rates (bpm) during rest and exercise for 30 min at 70% VO2max with (RAIN) and without (CON) 30 mm · h−1 of precipitation at 33°C. Values are presented as means ± s (n = 11). *Significantly different from control (P < 0.05).
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Table I. Plasma lactate, glucose, epinephrine and norepinephrine, serum triglyceride and FFA concentrations at rest and during 30 min of exercise. Exercise duration (min) Condition Lactate (mmol · L−1) Glucose (mmol · L−1) Epinephrine (pmol · L−1) Norepinephrine (nmol · mL−1) TG (mmol · L−1)
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FFA (mmol · L−1)
RAIN CON RAIN CON RAIN CON RAIN CON RAIN CON RAIN CON
Rest 1.26 1.51 5.03 4.83 315.6 324.9 1.79 1.80 0.71 0.70 0.44 0.33
± ± ± ± ± ± ± ± ± ± ± ±
0.30 0.41 0.24 0.43 98.8 179.7 0.38 0.54 0.44 0.44 0.20 0.26
Rest (33°C) 1.11 1.20 5.07 5.15 486.7 469.6 1.67 1.88 0.71 0.69 0.39 0.36
± ± ± ± ± ± ± ± ± ± ± ±
0.19 0.32 0.22 0.61 155.6 172.5 0.27 0.92 0.50 0.45 0.22 0.22
10 4.05 4.48 4.95 5.16 841.4 825.0 7.76 7.24 0.74 0.75 0.42 0.39
± ± ± ± ± ± ± ± ± ± ± ±
20 1.40 1.68 0.28 1.06 487.0 231.5 2.83 1.74 0.51 0.46 0.27 0.24
4.57 5.49 5.70 5.17 1300.0 1582.3 13.22 13.46 0.73 0.76 0.49 0.42
± ± ± ± ± ± ± ± ± ± ± ±
30 1.94 2.91 0.75 1.11 1117.1 1117.1 6.80 5.09 0.50 0.47 0.34 0.23
4.95 6.96 6.85 6.99 991.0 1888.1 12.49 14.49 0.66 0.71 0.71 0.62
± ± ± ± ± ± ± ± ± ± ± ±
2.54* 3.29 2.11 1.90 609.3* 1675.1 5.58 4.35 0.50 0.44 0.26 0.37
Notes: RAIN, 30 mm · h−1 of precipitation; CON, control without precipitation. Values are presented as means ± s (n = 11). *Significantly different from CON (P < 0.05).
transiently increased by about 1.00 during the first 5 min of exercise, then decreased to about 0.15 until the exercise finished. Figure 3 shows that the HR was significantly lower in RAIN than in CON at 20, 25 and 30 min of exercise (178 ± 12 vs. 188 ± 10, 181 ± 10 vs. 190 ± 9 and 181 ± 10 vs. 194 ± 9 bpm, respectively; P < 0.05 for all, ES = 0.24–0.44), although the RPE did not significantly differ at 10, 20 and 30 min between RAIN and CON (12.0 ± 0.8 vs. 12.6 ± 1.7, 13.7 ± 1.1 vs. 14.1 ± 2.7, 14.7 ± 2.2 vs. 15.6 ± 3.2, respectively). The La values were significantly lower in RAIN than in CON at 30 min of exercise (4.95 ± 2.54 vs. 6.96 ± 3.29 mmol · L−1; P < 0.05, ES = 0.55; Table I). Plasma epinephrine concentrations were significantly lower in RAIN than in CON at 30 min (991.0 ± 609.3 vs. 1888.1 ± 1675.1 pmol · L−1; P < 0.05, ES = 0.61; Table I). Plasma levels of norepinephrine and glucose and FFA and TG did not significantly differ between trials (Table I). Sweat loss was significantly lower in RAIN than in CON (712 ± 183 vs. 1101 ± 203 g; P < 0.05, ES = 0.52). 4. Discussion To our knowledge, this study is the first to investigate the influence of rain and heat on physiological and metabolic responses while running in a climatic chamber that can precisely simulate hot rainy conditions. Exercise at 70% VO2max for 30 min at 33°C resulted in reduced Tes, HR, epinephrine, La and sweat loss levels in RAIN vs. CON. These results indicated that rain reduces thermal strain while running at 70% VO2max intensity under a hot ambient temperature. Therefore, rain provides advantages
during exercise in heat compared with cold (Ito et al., 2013; Weller et al., 1997). 4.1. Thermal responses Both Tes and Tsk were significantly lower in RAIN than in CON throughout the exercise. The Tes increased in both trials, but transiently decreased from 36.5°C to 36.0°C during the first 5 min in RAIN. A headwind equal to running speed would have increased rainwater evaporation from the body, enhancing evaporative heat loss and blunting the increase in Tes throughout exercise in RAIN. Velocity in running sports typically ranges from 2.5 to 5.5 m · s−1, and thus the thermal and cardiovascular responses from studies using wind velocity within this range are applicable to most sports (Mora-Rodriguez, Del Coso, Aquado-Jimenez, & Estevez, 2007). Adams, Mack, Langhans, and Nadel (1992) showed that increasing air velocity from 0 (still air) to 3.0 m · s−1 in a heated environment during submaximal cycle ergometer exercise attenuated rates of heat storage, primarily because the ability to evaporate water improved (still air vs. 3.0 m · s−1: 306 vs. 352 W · m−2). Pugh (1966) also showed that wind and rain increased the rate of heat loss through evaporation and reduce clothing insulation. Increasing the wet area in RAIN intensified water evaporation from the skin surface, which increased evaporative heat loss compared with CON. Relatively little heat was produced during the early period of exercise and heat loss exceeded heat production, causing Tes to decrease, whereas heat production offset heat loss, causing Tes to increase during the later period in RAIN. Additionally, the rate of the increase in Tes between CON and RAIN appeared similar after 5 min. This
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Effect of rain during exercise in heat indicates that rates of heat loss were similar between the trials, except during the first 5 min. Moreover, an initial fall in body temperature was generated in RAIN. Hong and Nadel (1979) described an initial fall in Tes during exercise in the cold and considered that a shift of cool blood from cutaneous blood vessels to the core causes a transient fall in Tes. We found here that Tsk was significantly lower in RAIN than in CON and decreased over a period of 5 min. Running thus shifted cooled blood from cutaneous vessels to the core, which caused a transient fall in Tes during the first 5 min of exercise. The increase in evaporative heat loss and the initial fall in body temperature might have been responsible for the decreased Tes in RAIN. These findings are similar to those of our previous study of simulated rainy conditions (Ito et al., 2013). Additionally, the present study found that significantly less sweat was lost in RAIN compared with CON. Soaking the skin with rain and headwinds appeared to increase water evaporation from the skin surface. However, increase in skin wettedness decreases sweat rates at rest (Nadel & Stolwijk, 1973), and water sponged onto skin reduces evaporative sweat loss during exercise (Davies et al., 1976). Enhanced water evaporation from the skin increased heat loss, which lowered Tsk and increased the temperature gradient from the skin to the air. A lower core temperature in RAIN resulted in less sweat loss in the present study. Sawaka et al. (2012) reported that hot skin is associated with high skin blood flow requirements, and hypohydration is associated with reduced cardiac filling, both of which act to reduce aerobic reserve. Therefore, a lower skin temperature or decreased sweat loss in rain might ameliorate heat-induced strain. Levels of epinephrine were significantly lower in RAIN than in CON at 30 min of exercise. Exercise alone can increase epinephrine secretion, but release of this hormone during exercise is also augmented by heat stress (Febbraio et al., 1994; Hargreaves, Angus, Howlett, Conus, & Febbraio, 1996). Judging from the lower Tes and Tsk, the cooling effect of rain blunted the increase in epinephrine secretion in RAIN. Levels of La were significantly lower in RAIN than in CON at 30 min of exercise. Many studies also have found higher La levels under heat stress (Dimri, Malhotra, Gupta, Kumar, & Arora, 1980; Macdougall et al., 1974). However, although La concentrations were higher in RAIN, RER did not significantly differ between trials, indicating no evidence of a difference in substrate oxidisation. The present results indicate that rain suppressed increases in core temperature in hot conditions, which might decrease thermal strain via hyperthermia.
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4.2. Cardiorespiratory responses Neither VO2 nor TE significantly differed between our participants who ran at 70% VO2max intensity in both RAIN and CON, indicating that the amounts of metabolic energy produced were similar under both conditions. The correlation between VO2 and HR during exercise is generally positive and linear (Morgan & Bennett, 1976). However, HR was significantly lower in RAIN than in CON, regardless of the similar VO2 values between trials. HRs invariably increase during exercise in a hot environment, and this is most likely derived from increased blood flow to the skin via vasodilation and the redistribution of central blood volume towards the periphery, resulting in a reduction in stroke volume (Rowell, Marx, Bruce, Conn, & Kusumi, 1966). HRs increase in an effort to maintain cardiac output (Nadel, Cafarelli, Roberts, & Wenger, 1979), which is called cardiovascular drift. On the other hand, some studies have found that the vasoconstriction of vessels beneath cooled areas might improve venous return and stroke volume and, consequently, decrease HR in the heat (Booth, Marino, & Ward, 1997; Mündel, Bunn, Hooper, & Jones, 2007). In the light of significantly lower Tsk in RAIN of about 1.7–3.4°C, less blood flowed to the skin in RAIN than in CON during exercise (Rowell, 1986). It was surmised that vasodilation at the area cooled by rain might be diminished, and as a result, venous return improved. Improved venous return and stroke volume suppressed cardiovascular drift and consequently decreased HR in RAIN. The RPE did not significantly differ between CON and RAIN. A critically high body temperature (Gonzälez-Alonso et al., 1999) or hot skin temperature (Sawaka et al., 2012) limits endurance performance. Core temperature increased in both CON and RAIN but did not reach a critically high level because of the relatively short exercise duration in the present study. However, given the lower Tes in RAIN and the similar between-condition rate of rise in Tes, it is possible that had the exercise protocol been extended, attainment of a critical core temperature and the development of fatigue would have been delayed during exercise under hot and rainy conditions.
4.3. Limitations The exercise duration was relatively short. The Tes similarly increased in both RAIN and CON, and core temperature did not become critically high. The impact of rain on physiological and metabolic responses might become more obvious during a longer duration of exercise.
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5. Conclusions and practical applications Rain suppressed increases in core temperature, HR, La and sweat loss in men who ran at 70% VO2max under simulated hot, rainy conditions in a climatic chamber. Rain in a hot environment thus facilitated body heat loss and suppressed heat-induced physiological heat strain. It is speculated that the practical implication of these findings for individuals exercising under similar conditions in the heat is that rain may increase the duration or intensity of exercise. However, further studies are required to gain a deeper understanding of the physiological and performance implications of exercise under such conditions, but which may influence training or event preparation.
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Gisolfi, C. V., & Copping, J. R. (1974). Thermal effects of prolonged treadmill exercise in the heat. Medicine and Science in Sports, 6, 108–113. Gonzälez-Alonso, J., Teller, C., Anderson, S. L., Jensen, F. B., Hyldig, T., & Nielsen, B. (1999). Influence of body temperature on the development of fatigue during prolonged exercise in the heat. Journal of Applied Physiology, 86, 1032–1039. Hargreaves, M., Angus, D., Howlett, K., Conus, N. M., & Febbraio, M. (1996). Effect of heat stress on glucose kinetics during exercise. Journal of Applied Physiology, 81, 1594–1597. Hong, S. I., & Nadel, E. R. (1979). Thermogenic control during exercise in a cold environment. Journal of Applied Physiology, 47, 1084–1089. Ito, R., Nakano, M., Yamane, M., Amano, M., & Matsumoto, T. (2013). Effects of rain on energy metabolism while running in a cold environment. International Journal of Sports Medicine, 34, 707–711. MacDougall, J. D., Reddan, W. G., Layton, C. R., & Dempset, J. A. (1974). Effects of metabolic hyperthermia on performance during heavy prolonged exercise. Journal of Applied Physiology, 36, 538–544. Mora-Rodriguez, R., Del Coso, J., Aquado-Jimenez, R., & Estevez, E. (2007). Separate and combined effects of airflow and rehydration during exercise in the heat. Medicine & Science in Sports & Exercise, 39, 1720–1726. Morgan, D. B., & Bennett, T. (1976). The relation between heart rate and oxygen consumption during exercise. The Journal of Sports Medicine and Physical Fitness, 16, 38–44. Morris, J. G., Nevill, M. E., Boobis, L. H., Macdonald, I. A., & Williams, C. (2005). Muscle metabolism, temperature, and function during prolonged, intermittent, high-intensity running in air temperatures of 33 degrees and 17 degrees C. International Journal of Sports Medicine, 26, 805–814. Mündel, T., Bunn, S. J., Hooper, P. L., & Jones, D. A. (2007). The effects of face cooling during hyperthermia exercise in man: Evidence for an integrated thermal, neuroendocrine and behavioural response. Experimental Physiology, 92, 187–195. Nadel, E. R., Cafarelli, E., Roberts, M. F., & Wenger, C. B. (1979). Circulatory regulation during exercise in different ambient temperatures. Journal of Applied Physiology, 46, 430–437. Nadel, E. R., & Stolwijk, J. A. (1973). Effect of skin wettedness on sweat gland response. Journal of Applied Physiology, 35, 689– 694. Nybo, L., & Nielsen, B. (2001). Hyperthermia and central fatigue during prolonged exercise in humans. Journal of Applied Physiology, 91, 1055–1060. Pugh, L. G. (1966). Accidental hypothermia in walkers, climbers, and campers: Report to the medical commission on accident prevention. BMJ, 1, 123–129. Ramanathan, N. L. (1964). A new weighting system for mean surface temperature of the human body. Journal of Applied Physiology, 19, 531–533. Rowell, L. B. (1986). Human circulation: Regulation during physical stress. New York, NY: Oxford University Press. Rowell, L. B., Marx, H. J., Bruce, R. A., Conn, R. D., & Kusumi, F. (1966). Reductions in cardiac output, central blood volume, and stroke volume with thermal stress in normal men during exercise. The Journal of Clinical Investigation, 45, 1801–1816. Sawaka, M. N., Cheuvront, S. N., & Kenefick, R. W. (2012). High skin temperature and hyphydration impair aerobic performance. Experimental Physiology, 97, 327–332. Schumacker, P. T., Rowland, J., Saltz, S., Nelson, D. P., & Wood, L. D. (1987). Effects of hyperthermia and hypothermia on oxygen extraction by tissues during hypovolemia. Journal of Applied Physiology, 63, 1246–1252. Sherman, B. A., & Deutsch, D. T. (1983). Effect of external cooling on selected physiological measures during prolonged
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running in the heat. Medicine & Science in Sports & Exercise, 15, 167–168. Thompson, R. L., & Hayward, J. S. (1996). Wet-cold exposure and hypothermia: Thermal and metabolic responses to prolonged exercise in rain. Journal of Applied Physiology, 81, 1128–1137. Tikuisis, P., Ducharme, M. B., Moroz, D., & Jacobs, I. (1999). Physiological responses of exercised-fatigued individuals
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exposed to wet-cold conditions. Journal Applied Physiology, 86, 1319–1328. Weller, A. S., Millard, C. E., Stroud, M. A., Greenhaff, P. L., & Macdonald, I. A. (1997). Physiological responses to cold stress during prolonged intermittent low- and high-intensity walking. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, 272, R2025–R2033.
Journal of Sports Sciences Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/rjsp20
Rain influences the physiological and metabolic responses to exercise in hot conditions a
b
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Ryo Ito , Naoyuki Yamashita , Eiko Suzuki & Takaaki Matsumoto a
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University Educational Center, Nihon Fukushi University, Mihama-cho, Japan
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Laboratory for Exercise Physiology and Biomechanics, Graduate School of Health and Sport Sciences, Chukyo University, Toyota, Japan Published online: 02 Jan 2015.
Click for updates To cite this article: Ryo Ito, Naoyuki Yamashita, Eiko Suzuki & Takaaki Matsumoto (2015): Rain influences the physiological and metabolic responses to exercise in hot conditions, Journal of Sports Sciences, DOI: 10.1080/02640414.2014.977938 To link to this article: http://dx.doi.org/10.1080/02640414.2014.977938
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Journal of Sports Sciences, 2014 http://dx.doi.org/10.1080/02640414.2014.977938
Rain influences the physiological and metabolic responses to exercise in hot conditions
RYO ITO1, NAOYUKI YAMASHITA2, EIKO SUZUKI2 & TAKAAKI MATSUMOTO2 1
University Educational Center, Nihon Fukushi University, Mihama-cho, Japan and 2Laboratory for Exercise Physiology and Biomechanics, Graduate School of Health and Sport Sciences, Chukyo University, Toyota, Japan
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(Accepted 10 October 2014)
Abstract Outdoor exercise often proceeds in rainy conditions. However, the cooling effects of rain on human physiological responses have not been systematically studied in hot conditions. The present study determined physiological and metabolic responses using a climatic chamber that can precisely simulate hot, rainy conditions. Eleven healthy men ran on a treadmill at an intensity of 70% VO2max for 30 min in the climatic chamber at an ambient temperature of 33°C in the presence (RAIN) or absence (CON) of 30 mm · h−1 of precipitation and a headwind equal to the running velocity of 3.15 ± 0.19 m · s−1. Oesophageal temperature, mean skin temperature, heart rate, rating of perceived exertion, blood parameters, volume of expired air and sweat loss were measured. Oesophageal and mean skin temperatures were significantly lower from 5 to 30 min, and heart rate was significantly lower from 20 to 30 min in RAIN than in CON (P < 0.05 for all). Plasma lactate and epinephrine concentrations (30 min) and sweat loss were significantly lower (P < 0.05) in RAIN compared with CON. Rain appears to influence physiological and metabolic responses to exercise in heat such that heat-induced strain might be reduced. Keywords: thermoregulation, hyperthermia, rain, heart rate, lactate
1. Introduction Outdoor exercise such as jogging, soccer, field hockey and rugby often proceeds in the rain. We previously showed that oesophageal temperature (Tes) decreases and oxygen consumption (VO2) and plasma lactate concentrations (La) increase while running in rain at 5°C (Ito, Nakano, Yamane, Amano, & Matsumoto, 2013). Several studies have addressed the effects of rain and cold on human physiological responses during exercise (Ito et al., 2013; Thompson & Hayward, 1996; Tikuisis, Ducharme, Moroz, & Jacobs, 1999; Weller, Millard, Stroud, Greenhaff, & Macdonald, 1997), but to the knowledge of the authors, rain also falls on hot days, and the effects of exercising under such conditions have not been examined. The influence of exposure to heat on human physiological responses during exercise is well-established (Gonzälez-Alonso et al., 1999; MacDougall, Reddan, Layton, & Dempset, 1974). Hyperthermia increases rates of muscle glycogenolysis (Febbraio, Snow, Stathis, Hargreaves, & Carey, 1994, 1996),
lactate production (Morris, Nevill, Boobis, Macdonald, & Williams, 2005; Schumacker, Rowland, Saltz, Nelson, & Wood, 1987), imposes cardiovascular strain (Gonzälez-Alonso et al., 1999) and induced central fatigue (Brück & Olschewski, 1987; Nybo & Nielsen, 2001). Some studies have examined thermoregulatory responses to wetting the skin during prolonged exercise, although the effects of rain were not evaluated. Gisolfi and Copping (1974) and Davies, Brotherhood, and Zeidifard (1976) showed that sponging water onto the skin does not affect rectal temperature in the heat. Sherman and Deutsch similarly reported that spraying the skin with water does not affect rectal temperature while running on a treadmill in a hot environment (Sherman & Deutsch, 1983). However, the body and clothing will become completely and continuously soaked in rain, and a generated headwind will increase the amount of heat lost from the body. Moreover, Sawaka, Cheuvront, and Kenefick (2012) recently reported that hot skin (>35°C) associated with high skin blood flow requirements can reduce aerobic exercise
Correspondence: Ryo Ito, University Educational Center, Nihon Fukushi University, Okuda, Mihama-cho, Chita-gun, Aichi-pref, Mihama-Cho 4703295, Japan. E-mail: [email protected] © 2014 Taylor & Francis
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performance. Thus, the cooling effect of rain at the skin surface reduces physiological heat strain in individuals who exercise in heat. The present study aimed to determine the impact of 30 min of steadystate running in rain and heat on thermal and physiological responses. We hypothesised that rain would lower core and skin temperature and suppress the physiological and metabolic responses during steady-state running under hot conditions. 2. Methods
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2.1. Ethical approval The Human Subjects Committee at Chukyo University Graduate School of Health and Sport Sciences approved the study, which conformed to the standards established in the 2008 revision of the Declaration of Helsinki. 2.2. Participants Eleven healthy men (age, 22.9 ± 1.5 years; height, 174.3 ± 6.9 cm; body mass, 67.4 ± 6.4 kg; VO2max, 55.5 ± 6.4 mL · kg−1 · min−1; means ± 1 s) who were non-smokers and not under medication participated in this study. All of the participants were active and recreationally exercised at least three times weekly. They were fully informed about the nature and purpose of the study before providing written informed consent to participate. 2.3. Simulated rain conditions Outdoor rainy conditions were simulated in terms of temperature, humidity, rain, and wind using a TBR-12A4PX climatic chamber (ESPEC, Osaka, Japan). The participants ran for 30 min and were continuously soaked with water (rain) delivered at 30 mm · h−1 from five nozzles placed on a wall 1.5 m in front and 2.2 m above a treadmill. A headwind equal to running speed was generated from a mesh panel located 1.5 m in front of the treadmill. The temperature of the “rain” emerging from the nozzle was ~3°C below the ambient temperature of 33°C. 2.4. Experimental protocol The participants initially performed an incremental running test to the point of volitional exhaustion on a BM-200 treadmill (S & ME, Tokyo, Japan) at 20°C to determine VO2max. VO2max was considered to have been achieved if at least two of the following criteria were met: respiratory exchange ratio (RER) ≥1.15; VO2 plateau during the last stage of exercise and maximal heart rate
(HR) within ±10 beats per min of age-predicted values. All participants underwent two trials in the TBR-12A4PX climatic chamber at intervals of 7– 10 days. All of them fasted for at least 12 h, abstained from alcohol or caffeine and from vigorous physical activities for 24 h before each trial and presented at the laboratory between 8 and 9 A.M. They were weighed in the nude and then dressed in identical 100% polyester half-sleeved running shirts and running shorts and rested while seated for about 30 min in a comfortable environment (26°C). During this period, an ST24S thermistor probe (Senser Technica, Seto, Japan) was inserted via the nasal passage to a distance equivalent to 25% of the participant’s height to measure Tes (Bruning, Dahmus, Kenney, & Alexander, 2013). ST-23S thermistors (Senser Technica, Seto, Japan) were also attached to the chest, upper arm, thigh and calf to measure skin temperature. Weighted mean skin temperature (Tsk) was calculated as described by Ramanathan (1964). A JS-N2325RSP indwelling catheter (JMS, Hiroshima, Japan) was inserted into a superficial forearm vein, and blood samples were withdrawn through three-way valves flushed with heparinised saline before entering the climatic chamber, after 10 min of rest in the chamber (0 min of exercise), and after 10, 20 and 30 min of exercise. The electrocardiography findings were monitored using a QI-236P radiotelemeter (Nihon Kohden, Tokyo, Japan), and HR was recorded every 30 s. Expired air was analysed every 30 s using an AE300S automatic gas analyser (Minato Medical Science, Osaka, Japan), and the rating of perceived exertion (RPE) was reported every 10 min using the Borg category scale (Borg, 1973). The participants dried themselves with a towel and were weighed in the nude after exercise. Sweat loss was calculated by subtracting body weight after exercise from that before. The participants entered the environmental chamber (ambient temperature, 33°C; relative humidity, 50%) and rested while seated for 10 min and not exposed to rain or wind. Thereafter, the participants started running on the treadmills for 30 min at 70% VO2max (velocity: 3.15 ± 0.19 m · sec−1) in the presence (RAIN) or absence (CON) of rain. The relative humidity was set 50% during rest and exercise in CON, but in RAIN, the relative humidity increased to 85 ± 5% during exercise. A headwind equal to running speed was provided during both trials. Expired air, HR, Tes and Tsk, were measured after 5 min of rest in the chamber to allow time to waterproof the measuring equipment. The experimental trials were administered in randomised order at least 7 days apart.
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2.5. Analytical techniques All blood samples (10 mL) were collected from a superficial forearm vein using a syringe. Portions (2 mL) were dispensed into chilled tubes containing EDTA-2Na and sodium fluoride and were separated by centrifugation (4°C, 3000 rpm, 15 min). Concentrations of La and plasma glucose (YSI 2300 STAT PLUS, Yellow Springs, OH, USA) were measured in the supernatant. Larger portions (4 mL) were also dispensed into tubes containing only EDTA-2Na and separated by centrifugation as described above. Plasma epinephrine and norepinephrine concentrations were measured by HPLC (FP-1520S, JASCO International Co., Hachioji, Japan). The remainder (4 mL) of the blood samples were dispensed into plain tubes and separated by centrifugation (at 20°C, 3000 rpm, 15 min), and then serum levels of free fatty acids (FFA) and triglycerides (TG) were determined using an automated analyser (7600-01, Hitachi, Tokyo, Japan). All parameters except La and plasmas glucose were measured by SRL (http://www.srl-group.co.jp/ english/index.html). 2.6. Statistical analysis All data were statistically analysed using SPSS software Version 10 (SPSS, Chicago, IL, USA) and checked for normality. Experimental data were compared using two-way (trial by time) repeated measures ANOVA. Where sphericity was significantly violated, F values were adjusted using Huynh-Felf corrections as appropriate. Significant main or interaction effects were assessed using Fisher’s least significant difference post hoc test. Differences in sweat volume between RAIN and CON were assessed using a paired t-test. Significance was taken at P < 0.05. The magnitude of the difference in significant effects was compared using Cohen’s d effect size (ES) and thresholds (0.5 (small), 0.5–0.79 (moderate), or ≥0.8 (large)) (Chohen, 1988). Mean values with s are presented in tabular and graphic formats. The primary outcome of interest in this experiment was Tes. Based on a post hoc power analysis (G*power 3.1.3 Macintosh Edition (Faul, Erdfelder, Lang, & Buchner, 2007)), the statistical power (1 - β) of the Tes variables in this test exceeded 0.80, and a sample size of eight for each condition was adequate.
Figure 1. Oesophageal temperature (°C) during rest and exercise for 30 min at 70% VO2max with (RAIN) and without (CON) 30 mm · h−1 of precipitation at 33°C. Values are presented as means ± s (n = 11). *Significantly different from control (P < 0.05).
Figure 2. Mean weighted skin temperature (°C) during rest and exercise for 30 min at 70% VO2max with (RAIN) and without (CON) 30 mm · h−1 of precipitation at 33°C. Values are presented as means ± s (n = 11). *Significantly different from control (P < 0.05).
reached a maximum of 37.7°C. The Tes was significantly lower in RAIN than in CON during the exercise (P < 0.05) (ES = 0.33–0.52) (Figure 1). Tsk in CON essentially remained at 34.5°C throughout the exercise. Tsk in RAIN decreased from 34.4°C to 30.4°C during the first 5 min and remained significantly lower than that in CON throughout the exercise (P < 0.05) (ES = 0.36– 0.83) (Figure 2). VO2 and minute ventilation (VE) did not significantly differ between RAIN and CON, whereas RER in RAIN and CON
3. Results Exercise resulted in a progressive increase in Tes from 36.5°C to 38.4°C in CON. The Tes in RAIN decreased from 36.5°C to 36.0°C during the first 5 min of exercise, started to increase thereafter and
Figure 3. Heart rates (bpm) during rest and exercise for 30 min at 70% VO2max with (RAIN) and without (CON) 30 mm · h−1 of precipitation at 33°C. Values are presented as means ± s (n = 11). *Significantly different from control (P < 0.05).
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Table I. Plasma lactate, glucose, epinephrine and norepinephrine, serum triglyceride and FFA concentrations at rest and during 30 min of exercise. Exercise duration (min) Condition Lactate (mmol · L−1) Glucose (mmol · L−1) Epinephrine (pmol · L−1) Norepinephrine (nmol · mL−1) TG (mmol · L−1)
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FFA (mmol · L−1)
RAIN CON RAIN CON RAIN CON RAIN CON RAIN CON RAIN CON
Rest 1.26 1.51 5.03 4.83 315.6 324.9 1.79 1.80 0.71 0.70 0.44 0.33
± ± ± ± ± ± ± ± ± ± ± ±
0.30 0.41 0.24 0.43 98.8 179.7 0.38 0.54 0.44 0.44 0.20 0.26
Rest (33°C) 1.11 1.20 5.07 5.15 486.7 469.6 1.67 1.88 0.71 0.69 0.39 0.36
± ± ± ± ± ± ± ± ± ± ± ±
0.19 0.32 0.22 0.61 155.6 172.5 0.27 0.92 0.50 0.45 0.22 0.22
10 4.05 4.48 4.95 5.16 841.4 825.0 7.76 7.24 0.74 0.75 0.42 0.39
± ± ± ± ± ± ± ± ± ± ± ±
20 1.40 1.68 0.28 1.06 487.0 231.5 2.83 1.74 0.51 0.46 0.27 0.24
4.57 5.49 5.70 5.17 1300.0 1582.3 13.22 13.46 0.73 0.76 0.49 0.42
± ± ± ± ± ± ± ± ± ± ± ±
30 1.94 2.91 0.75 1.11 1117.1 1117.1 6.80 5.09 0.50 0.47 0.34 0.23
4.95 6.96 6.85 6.99 991.0 1888.1 12.49 14.49 0.66 0.71 0.71 0.62
± ± ± ± ± ± ± ± ± ± ± ±
2.54* 3.29 2.11 1.90 609.3* 1675.1 5.58 4.35 0.50 0.44 0.26 0.37
Notes: RAIN, 30 mm · h−1 of precipitation; CON, control without precipitation. Values are presented as means ± s (n = 11). *Significantly different from CON (P < 0.05).
transiently increased by about 1.00 during the first 5 min of exercise, then decreased to about 0.15 until the exercise finished. Figure 3 shows that the HR was significantly lower in RAIN than in CON at 20, 25 and 30 min of exercise (178 ± 12 vs. 188 ± 10, 181 ± 10 vs. 190 ± 9 and 181 ± 10 vs. 194 ± 9 bpm, respectively; P < 0.05 for all, ES = 0.24–0.44), although the RPE did not significantly differ at 10, 20 and 30 min between RAIN and CON (12.0 ± 0.8 vs. 12.6 ± 1.7, 13.7 ± 1.1 vs. 14.1 ± 2.7, 14.7 ± 2.2 vs. 15.6 ± 3.2, respectively). The La values were significantly lower in RAIN than in CON at 30 min of exercise (4.95 ± 2.54 vs. 6.96 ± 3.29 mmol · L−1; P < 0.05, ES = 0.55; Table I). Plasma epinephrine concentrations were significantly lower in RAIN than in CON at 30 min (991.0 ± 609.3 vs. 1888.1 ± 1675.1 pmol · L−1; P < 0.05, ES = 0.61; Table I). Plasma levels of norepinephrine and glucose and FFA and TG did not significantly differ between trials (Table I). Sweat loss was significantly lower in RAIN than in CON (712 ± 183 vs. 1101 ± 203 g; P < 0.05, ES = 0.52). 4. Discussion To our knowledge, this study is the first to investigate the influence of rain and heat on physiological and metabolic responses while running in a climatic chamber that can precisely simulate hot rainy conditions. Exercise at 70% VO2max for 30 min at 33°C resulted in reduced Tes, HR, epinephrine, La and sweat loss levels in RAIN vs. CON. These results indicated that rain reduces thermal strain while running at 70% VO2max intensity under a hot ambient temperature. Therefore, rain provides advantages
during exercise in heat compared with cold (Ito et al., 2013; Weller et al., 1997). 4.1. Thermal responses Both Tes and Tsk were significantly lower in RAIN than in CON throughout the exercise. The Tes increased in both trials, but transiently decreased from 36.5°C to 36.0°C during the first 5 min in RAIN. A headwind equal to running speed would have increased rainwater evaporation from the body, enhancing evaporative heat loss and blunting the increase in Tes throughout exercise in RAIN. Velocity in running sports typically ranges from 2.5 to 5.5 m · s−1, and thus the thermal and cardiovascular responses from studies using wind velocity within this range are applicable to most sports (Mora-Rodriguez, Del Coso, Aquado-Jimenez, & Estevez, 2007). Adams, Mack, Langhans, and Nadel (1992) showed that increasing air velocity from 0 (still air) to 3.0 m · s−1 in a heated environment during submaximal cycle ergometer exercise attenuated rates of heat storage, primarily because the ability to evaporate water improved (still air vs. 3.0 m · s−1: 306 vs. 352 W · m−2). Pugh (1966) also showed that wind and rain increased the rate of heat loss through evaporation and reduce clothing insulation. Increasing the wet area in RAIN intensified water evaporation from the skin surface, which increased evaporative heat loss compared with CON. Relatively little heat was produced during the early period of exercise and heat loss exceeded heat production, causing Tes to decrease, whereas heat production offset heat loss, causing Tes to increase during the later period in RAIN. Additionally, the rate of the increase in Tes between CON and RAIN appeared similar after 5 min. This
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Effect of rain during exercise in heat indicates that rates of heat loss were similar between the trials, except during the first 5 min. Moreover, an initial fall in body temperature was generated in RAIN. Hong and Nadel (1979) described an initial fall in Tes during exercise in the cold and considered that a shift of cool blood from cutaneous blood vessels to the core causes a transient fall in Tes. We found here that Tsk was significantly lower in RAIN than in CON and decreased over a period of 5 min. Running thus shifted cooled blood from cutaneous vessels to the core, which caused a transient fall in Tes during the first 5 min of exercise. The increase in evaporative heat loss and the initial fall in body temperature might have been responsible for the decreased Tes in RAIN. These findings are similar to those of our previous study of simulated rainy conditions (Ito et al., 2013). Additionally, the present study found that significantly less sweat was lost in RAIN compared with CON. Soaking the skin with rain and headwinds appeared to increase water evaporation from the skin surface. However, increase in skin wettedness decreases sweat rates at rest (Nadel & Stolwijk, 1973), and water sponged onto skin reduces evaporative sweat loss during exercise (Davies et al., 1976). Enhanced water evaporation from the skin increased heat loss, which lowered Tsk and increased the temperature gradient from the skin to the air. A lower core temperature in RAIN resulted in less sweat loss in the present study. Sawaka et al. (2012) reported that hot skin is associated with high skin blood flow requirements, and hypohydration is associated with reduced cardiac filling, both of which act to reduce aerobic reserve. Therefore, a lower skin temperature or decreased sweat loss in rain might ameliorate heat-induced strain. Levels of epinephrine were significantly lower in RAIN than in CON at 30 min of exercise. Exercise alone can increase epinephrine secretion, but release of this hormone during exercise is also augmented by heat stress (Febbraio et al., 1994; Hargreaves, Angus, Howlett, Conus, & Febbraio, 1996). Judging from the lower Tes and Tsk, the cooling effect of rain blunted the increase in epinephrine secretion in RAIN. Levels of La were significantly lower in RAIN than in CON at 30 min of exercise. Many studies also have found higher La levels under heat stress (Dimri, Malhotra, Gupta, Kumar, & Arora, 1980; Macdougall et al., 1974). However, although La concentrations were higher in RAIN, RER did not significantly differ between trials, indicating no evidence of a difference in substrate oxidisation. The present results indicate that rain suppressed increases in core temperature in hot conditions, which might decrease thermal strain via hyperthermia.
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4.2. Cardiorespiratory responses Neither VO2 nor TE significantly differed between our participants who ran at 70% VO2max intensity in both RAIN and CON, indicating that the amounts of metabolic energy produced were similar under both conditions. The correlation between VO2 and HR during exercise is generally positive and linear (Morgan & Bennett, 1976). However, HR was significantly lower in RAIN than in CON, regardless of the similar VO2 values between trials. HRs invariably increase during exercise in a hot environment, and this is most likely derived from increased blood flow to the skin via vasodilation and the redistribution of central blood volume towards the periphery, resulting in a reduction in stroke volume (Rowell, Marx, Bruce, Conn, & Kusumi, 1966). HRs increase in an effort to maintain cardiac output (Nadel, Cafarelli, Roberts, & Wenger, 1979), which is called cardiovascular drift. On the other hand, some studies have found that the vasoconstriction of vessels beneath cooled areas might improve venous return and stroke volume and, consequently, decrease HR in the heat (Booth, Marino, & Ward, 1997; Mündel, Bunn, Hooper, & Jones, 2007). In the light of significantly lower Tsk in RAIN of about 1.7–3.4°C, less blood flowed to the skin in RAIN than in CON during exercise (Rowell, 1986). It was surmised that vasodilation at the area cooled by rain might be diminished, and as a result, venous return improved. Improved venous return and stroke volume suppressed cardiovascular drift and consequently decreased HR in RAIN. The RPE did not significantly differ between CON and RAIN. A critically high body temperature (Gonzälez-Alonso et al., 1999) or hot skin temperature (Sawaka et al., 2012) limits endurance performance. Core temperature increased in both CON and RAIN but did not reach a critically high level because of the relatively short exercise duration in the present study. However, given the lower Tes in RAIN and the similar between-condition rate of rise in Tes, it is possible that had the exercise protocol been extended, attainment of a critical core temperature and the development of fatigue would have been delayed during exercise under hot and rainy conditions.
4.3. Limitations The exercise duration was relatively short. The Tes similarly increased in both RAIN and CON, and core temperature did not become critically high. The impact of rain on physiological and metabolic responses might become more obvious during a longer duration of exercise.
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5. Conclusions and practical applications Rain suppressed increases in core temperature, HR, La and sweat loss in men who ran at 70% VO2max under simulated hot, rainy conditions in a climatic chamber. Rain in a hot environment thus facilitated body heat loss and suppressed heat-induced physiological heat strain. It is speculated that the practical implication of these findings for individuals exercising under similar conditions in the heat is that rain may increase the duration or intensity of exercise. However, further studies are required to gain a deeper understanding of the physiological and performance implications of exercise under such conditions, but which may influence training or event preparation.
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