Exercise in a hot environment provides a stress that leads to rapid physiological responses, as evidenced by a faster rise in heart rate (HR), higher core temperature (Tc), enhanced skin blood flow, and an increased release of neuroendocrine hormones (15). Fatigue in endurance-trained subjects during exercise in the heat has been related to a higher body temperature (10), generally believed to be the principle contributing factor that stimulates the secretion of cortisol (20). Hosick et al. (12) reported that during exercise in a hot environment (38° C), at 65% of peak oxygen uptake (VO2peak), cortisol was increased when Tc was elevated by approximately 1° C above resting Tc, with a gradual decrease within 24 hours after exercise in well-trained subjects. Prolonged elevation of cortisol after high-intensity exercise is believed to be detrimental to health. Previous studies reported that the cortisol released in response to increased Tc may influence inflammation and regeneration of tissue damaged after acute stress (12,19). Therefore, faster decreases in Tc and cortisol levels after strenuous exercise are believed to benefit recovery in athletes.
Cold-water immersion (CWI), ice massage, ice-vest, and ice or cold gel pack application have been examined as post-exercise cooling strategies to enhance recovery (25). Compared with other cooling methods, CWI is the most efficient recovery intervention after exercise in hot environments by lowering Tc, HR, and cortisol levels (11,12,17,25). However, the application of CWI requires ice, a large bathtub, and the treatment space that may not be widely available. Hence, an accessible alternative cooling recovery, which imparts similar physiological responses to the CWI method, may be needed.
Taking a shower is an activity that millions of people partake in as part of their routine after exercise. Eglin and Tipton (7) showed that initially undertaking repeated cold showering of the upper body (1–3 minutes at 10° C) may partly habituate subjects to subsequent head out CWI at the same water temperature. The authors suggested that the similar rates of decrease in skin temperatures between the cooling methods were associated with a reduced respiratory drive (breathing frequency) during the CWI. In addition, Juliff et al. (14) observed that in elite netball players, contrast showering (alternating 1 minute at 38° C and 1 minute at 15° C for 14 minutes) after a netball-specific circuit session decreased skin temperatures compared with a passive recovery condition. However, despite the contrast showers having a psychological impact (recovery perception scale), there was no effect on Tc or performance measures (repeated agility). Buijze et al. (2) reported that regular hot-to-cold showering (30, 60, 90 seconds at 10–12° C) for at least 30 days could reduce self-reported sickness absence in healthy adults. Furthermore, a recent study by Butts et al. (3) demonstrated that a 15-minute 20° C cold shower, applied to treat exertional hyperthermia, resulted in a greater rate of decrease in Tc and HR, and lower ratings of thermal sensation, compared with passive rest. Although past studies have determined the effects of cold showering on several physiological parameters, to our knowledge, the cortisol response after a cold shower intervention has not been examined. Therefore, the aim of this study was to investigate the magnitude of change in the salivary cortisol response between a cold shower intervention and passive rest, applied after exercise under hot conditions.
Experimental Approach to the Problem
To determine the recovery benefits of a cold-water shower (CWS) from high-intensity cycling in the heat, this study examined the effects of 2 different interventions; a 15-minute (15° C, CWS) or passive recovery in 25° C room (SIT25) on HR, core temperature (Tc), salivary cortisol, and thermal comfort sensation (TCS) after 45 minutes of cycling in a hot environment (35° C room temperature, 40–60% relative humidity) at 65% of peak oxygen uptake. This mode of exercise and relative intensity has previously been shown to significantly elevate Tc (>1° C) and cortisol levels above resting values under thermal stress (12). A similar cold shower water temperature and duration of exposure (15 minutes at 20° C) selected in our study has previously been shown to reduce exercise-induced hypothermia through greater rate of Tc reduction compared with passive rest (3). The experimental design is shown in Figure 1. The study used a randomized crossover design with the 2 recovery interventions 7 days apart.
Nine healthy male subjects aged between 20 and 25 years (mean ± SD; age: 20.9 ± 1.0 years; body mass: 70.3 ± 11.2 kg; height: 175.6 ± 8.4 cm; body mass index = 22.9 ± 1.8 kg·m−2, % body fat = 16.6 ± 6.1, VO2peak = 32.6 ± 4.1 ml·kg−1·min−1) participated in this study. Inclusion criteria included being habitually active and typically engaged in at least 30 minutes of moderate exercise per day for a minimum of 3 days per week in the previous 3 months before the study. After explanation of the testing procedures, and benefits and possible risks of the study, informed consent forms were signed. Subjects were instructed to refrain from additional exercise, alcohol and caffeine intake for at least 48 hours before the testing sessions. None of the participants were under pharmacological treatment during the study. The experimental procedure was approved by the Human Research Ethics Committee at Mahidol University (MU-IRB 2014/12.2001).
In the first visit, anthropometric data were collected; height (cm), body mass (kg), body mass index (kg·m−2), and % body fat. Peak oxygen uptake (Vo2peak) was then assessed using an incremental cycling test to establish the exercise intensity of 65% Vo2peak for the experimental trials. During the incremental cycling test on a Monark cycle ergometer (Ergomedic 828 E; Monark Exercise AB, Vansbro, Sweden), expired air was sampled with a mouthpiece attached to an online gas collection system (Moxus modular oxygen uptake system; AEI Technologies, Inc., Pittsburg, PA, USA). The incremental cycling protocol began at 0.5 kp at 60 rpm for 2 minutes; thereafter, the intensity increased by 0.5 kp every 2 minutes until volitional fatigue. VO2peak was recorded as the highest 30-second average.
For each experimental condition, participants arrived at the laboratory at 07:00 hours. Breakfast consisted of a tuna sandwich, 250 ml of water and 250 ml of orange juice, which was provided 90 minutes before the experiment. The testing session was divided into 3 main periods: (a) 45 minutes of cycling; (b) 15 minutes of a randomized intervention period (CWS or SIT25); and (c) 2-hour recovery period in a 25° C room. Before starting the experiment, urine specific gravity (USG) was measured using a handheld refractometer (model Master-SUR/NM; Atago Co Ltd., Tokyo, Japan) to ensure the subjects were not dehydrated (USG > 1.020). Participants were provided with an individual water bottle (500 ml; temperature range between 20 and 22° C) and were encouraged to drink water ad libitum throughout the experiment. Exercise was commenced after 30-minute rest in a controlled room (temperature = 25 ± 1.0° C; relative humidity (rh) range between 40 and 60%). The participants completed 45 minutes of cycling in a hot room (temperature = 35.7 ± 1.0° C; rh range between 40 and 60%) at a constant load of the individual's predetermined kilopond of 65% of Vo2peak. After 45 minutes of cycling, participants underwent either a 15-minute CWS or SIT25. In CWS, participants took a 15 ± 2.0° C water shower on 2 areas (back and chest), alternating between each area every 3 minutes for 15 minutes. The flow rate of the shower was 5 L·min−1. The angle of the shower stream was individually adjusted to allow the water from the base of the neck downward, ensuring the head was not showered. After 15 minutes, participants used bath towels to dry themselves before sitting in a room set at a temperature of 25° C for a period of 2 hours (recovery period). In SIT25, the participants sat passively in a controlled room (temperature = 25 ± 1.0° C; rh range between 40 and 60%) throughout the intervention and recovery periods (Figure 1).
Thermal comfort was measured using a 9-point thermal comfort scale ranging from very uncomfortable (−4) to very comfortable (4), with 0 representing the transition from discomfort to comfort (26).
Heart rate was measured every 15 minutes until the end of the experimental trial through telemetry from a chest strap and a wrist watch receiver (FS1; Polar Electro Oy, Kempele, Finland).
Salivary samples were collected for cortisol analysis immediately postintervention (Post-Int), and at 1 and 2 hours during the recovery period. Before the first saliva collection, contamination with food debris was avoided by rinsing the mouth with water. Salivary samples were collected using a cotton swab and saliva-collecting tube (Salivette; Sarstedt, Newton, NC, USA). After saliva collection, saliva-collecting tubes were centrifuged at 3,000 rpm for 15 minutes at 4° C and then stored at −80° C. Saliva samples were assayed for cortisol in duplicate using a highly sensitive enzyme immunoassay (Salimetrics, State College, PA, USA) (20). The test used 25 μL of saliva per determination, which has a lower limit of sensitivity of <0.19 nmol/L, and average intra-assay and inter-assay coefficients of variation of 4.13 and 8.89%, respectively.
Core temperature (Tc) was measured with an ingestible temperature sensor with a radio transmitter to a data logger (CoreTemp, HQ, Inc., Palmetto, FL, USA). Participants took the capsule 5 hours before exercising (17).
Statistical analysis was conducted using the Statistical Package for Social Sciences (SPSS v 17.0; SPSS Inc., Chicago, IL, USA). As the data were normally distributed, all variables were analyzed with repeated-measures analyses of variance (condition × time), where the between-group factor was recovery condition (CWS vs. SIT25) and the within-groups factor was time point for the exercise period (Pre-Exercise vs. Post-Exercise), the intervention period (Post-Exercise vs. Post-Int), and the recovery period (Post-Int, Post 30 min, Post 1 h, and Post 2 h). For cortisol, the 30-minute time point during the recovery period was not presented because we did not collect the samples at this time point. To account for differences in the recovery period values between conditions, HR, Tc, and cortisol values were expressed as percentage changes from the preceding recovery time point value using the formula: [(recovery time point − preceding recovery time point)/recovery time point] × 100. Significance was accepted at the p ≤ 0.05 level.
Pre-exercise to Post-exercise
Pre-exercise, there were no differences in HR, Tc, TCS, or cortisol levels between conditions (p > 0.05; Table 1). The cycling exercise increased HR and Tc from pre-exercise values (p = 0.001) in both conditions to a similar extent (p > 0.05; Table 1). Thermal comfort sensation rating was significantly lower (p < 0.001) at post-exercise (−1; just uncomfortable) compared with pre-exercise (+2; slightly comfortable) in both conditions (p < 0.001; Table 1). Cortisol levels did not significantly change from pre-exercise in either condition (p > 0.05; Table 1).
Post-exercise to Postintervention
Heart rate, Tc, and cortisol values were not different between CWS and SIT25 conditions immediately Post-Int (p > 0.05); however, CWS elicited a higher TCS rating (+4; very comfortable) compared with SIT25 (+1; just comfortable; p = 0.04; Table 1). Within-group comparisons found that HR was significantly decreased from post-exercise values immediately after both interventions, but no significant effect for time was observed in Tc or cortisol levels in either condition over the same period (p > 0.05; Table 1). Thermal comfort sensation rating was significantly rated higher at Post-Int compared with post-exercise in both conditions (p < 0.05; Table 1).
Postintervention to Post 2 h Recovery
There was no difference in HR between conditions at any time point during the recovery period (p > 0.05). Heart rate values were similar after the Post-Int period in both conditions (CWS, 90 ± 3 b·min−1 and SIT25, 84 ± 3 b·min−1) (p = 0.10). In the CWS condition, HR values were reduced over time, immediately decreasing after 30-minute Post-Int (73 ± 2 b·min−1, p < 0.0001) until the end of the 2-hour recovery period. Whereas, in the SIT25 condition, significant decreases in HR values were observed only after 1-hour Post-Int (73 ± 3 b·min−1, p = 0.001) until the end of the 2-hour recovery period (Table 1).
Intestinal temperature was observed to be similar between conditions throughout the recovery period (p > 0.05). At 30-minute Post-Int, Tc was significantly decreased in both conditions (CWS, 37.09 ± 0.17° C and SIT25, 37.34 ± 0.11° C, p > 0.05) and remained below Post-Int Tc values until the end of the 2-hour recovery period in SIT25 (37.15 ± 0.08° C, p < 0.001) and CWS (37.03 ± 0.04° C, p < 0.001), respectively (Table 1). Cooling rates were also calculated through the difference in Tc from Post-Int at 30-minute, 1-, and 2-hour recovery (Tc divided by the recovery time). However, no main effect for condition was observed (p > 0.41; data not shown).
There was no difference in salivary cortisol between conditions during the recovery period (p > 0.05). In the CWS condition, cortisol was observed to decrease at 2 hours after recovery from Post-Int (p = 0.04). However, there was no change in the SIT25 condition (p = 0.13) (Table 1).
There was no difference in TCS ratings (p > 0.05) between conditions at any time point during the recovery period. Thermal comfort sensation ratings were significantly improved at 1 h (+3; comfortable, p = 0.03) and 2 h (+3; comfortable, p = 0.03) (p = 0.02) compared with Post-Int (+1, just comfortable) in the SIT25 condition, but there was no change in the CWS condition (p > 0.99) (Table 1).
As shown in Figure 2A, %change in HR values for the CWS condition demonstrated a faster decrease after 30-minute Post-Int compared with those for the SIT25 condition (CWS, 18.34 ± 2.35% and SIT25, 7.03 ± 4.65%, p < 0.01). Whereas, there was no difference observed for %change in Tc or salivary cortisol at any time point between conditions (p > 0.05) (Figure 2B, C). Within-group comparisons found that % change in HR values were gradually increased after 30 minutes until the end of 2-hour recovery period in the CWS condition (p < 0.02), but there was no change at any time point in the SIT25 condition (p > 0.05) (Figure 2A). Percent change in Tc values were decreased at 30-minute Post-Int (p < 0.01) and then increased until 2 hours after recovery for both conditions (p < 0.05) (Figure 2B). For % change in salivary cortisol, there was a significant decrease after 1-hour Post-Int (p < 0.01). Then, the values were increased until the end of the 2-hour recovery period (p < 0.05) (Figure 2C).
This study investigated the effects of a CWS intervention on recovery after cycling in a hot (∼36° C) environment. The major findings were that taking a 15° C CWS for 15 minutes could promote a positive thermal comfort sensation immediately at the end of the intervention period and facilitate HR recovery within the first 30 minutes after showering. However, the CWS protocol in our study had no effect on Tc or salivary cortisol responses in healthy male subjects.
The TCS results demonstrated that CWS could induce a comfortable rating sensation compared with SIT25 at the end of the intervention period. Our findings may partly be explained by the participants' TWS rating improving through return of their body temperature toward resting values, upon removal from the cold exposure (9). Moreover, the rating of temperature change, as pleasant or unpleasant, is dependent on the preferable homeostatic state. For example, a cold stimulus applied to the skin when the Tc is elevated is likely to be identified as pleasant, whereas under hypothermic conditions, the same stimulus may be judged as unpleasant (4,18). Therefore, a reduced skin temperature during the CWS intervention after exercise in the heat may have resulted in a satisfied rating (3). Our findings are consistent with previous studies that observed changes in TCS after taking a cold shower, which suggested an associated psychological impact (14,22). It is possible that CWS stimulated the temperature-sensory receptors on the skin, sending afferent thermal input signals to specific brain areas, resulting in an improvement in the participants' thermal comfort sensation (8).
This study did not observe a difference in HR response between CWS and SIT25 during the preintervention to postintervention period (CWS = 64 b·min−1 and SIT25 = 72 b·min−1). This outcome contrasts to that of Butts et al. (3) whom reported lower HR values at 5 and 10 minutes during a 20.8° C CWS compared with passive rest in a heat chamber (CWS = 45 b·min−1 and control = 27 b·min−1). A possible explanation for the discrepancy in our findings may be that the passive control in Butts et al.'s (3) study was undertaken in a heat chamber (33.4 ± 2.1° C), which was at a temperature somewhat higher than the control used in our study (25 ± 1.0° C). This may likely explain the contrasting HR responses between our studies (25).
Our study showed that the recovery effects of CWS were present after the first 30 minutes during the recovery period, with CWS eliciting faster recovery of HR than SIT25. The present findings revealed that a CWS can lower HR by 18% (∼17 b·min−1) compared with 7% (∼7 b·min−1) (p < 0.013) for a SIT25 intervention. Our data compare favorably with those of Buchheit et al. (1) who observed that CWI (14° C for 5 minutes) can decrease HR by 15% (∼13 b·min−1), 7 minutes after removal from CWI compared with being seated in a heat chamber (33.4 ± 2.1° C). Therefore, our results seem to indicate that CWS has a recovery cooling effect by reducing cardiovascular strain after exercise in the heat similar to that observed with CWI (6,25). We would highlight that this effect was only observed at 30 minutes after the CWS intervention and not beyond this time point. It seems possible that these results are due to either one or both of the following reasons. First, this study demonstrated a comfortable sensation of CWS over SIT25 after 15 minutes of the intervention period. This finding provides evidence that the faster HR recovery may also be due to cold showering causing the reduction of HR by a larger parasympathetic activity to counteract exercise-induced sympathetic dominance (14,22). Second, it is well known that cold water temperatures can cause the vasoconstriction of cutaneous blood vessels to maintain core temperature, which in turn increases peripheral resistance and arterial blood pressure, consequently enhancing venous return and increasing venous pressure with a corresponding decrease in HR (13,25).
In this study, the CWS was shown to have no recovery effect on reducing Tc values after cycling in a hot environment. This finding supports previous research performed by Juliff et al. (14), which also observed no differences in Tc values between contrast showers and a passive recovery intervention. A previous study by Butts et al. (3) also reported no significant changes in rectal temperature (Tre) after cold showering compared with a passive recovery after a 15-minute intervention. However, the authors reported a significantly faster cooling rate over the first 5 minutes of showering. In our study, we did not record Tc values over 5-minute segments during the intervention period, but the cooling rate values at the end of the 15-minute intervention period were not different between conditions. Although we also calculated cooling rate throughout the recovery period, we did not observe a difference between the CWS and control conditions. Our findings contrast with Halson et al. (11), who reported that CWI (11.5 ± 0.3° C for 60 seconds × 3) can decrease Tc values faster than sitting in a 24.2° C room temperature at 20 minutes after intervention. In line with Halson et al.'s (11) findings, CWI (14.03 ± 0.28° C for 12 minutes) has been shown to be 38% more effective in Tc cooling after immersion than a passive recovery intervention (5). Therefore, our results indicate that the CWS did not have a recovery cooling effect on Tc values that is comparable with CWI (5,10,11). A possible explanation for this may be the different mechanisms of heat loss between showering and immersion. During CWS, heat loss occurs through running water over the skin and is dependent on the water temperature and water flow through a convection mechanism (15). Alternatively, during CWI, conduction is the primary mechanism for heat loss when the skin or body parts are immersed in cold water (16). Indeed, the conductive cooling effect can cause a substantial reduction in Tc both during and after cold exposure, whereas convective heat loss is relatively minimal in comparison (24). In addition, the body surface area (BSA) of subjects exposed to the cold stimulus during CWI is considerably larger than during CWS (approximately 90% of the BSA in CWI vs. 34% of the BSA in CWS) (7).
Cortisol, a stress hormone, is an indicator of the physical stress and inflammatory response after strenuous exercise (12,20). To the authors' knowledge, this is the first study that has reported on the effects of cold showers on the salivary cortisol response after cycling in a hot environment. This study did not identify any changes in measures of salivary cortisol between 15° C CWS and SIT25, indicating that CWS was not effective in reducing cortisol during recovery after exercising in the heat. A few studies have used CWI to investigate the changes in cortisol levels on recovery after body cooling (17,23). Minett et al. (17) showed that a 20-minute exposure to 10° C CWI decreased Tc values (1 h post CWI ∼1.7° C) and cortisol levels at 1 hour after exercise compared with sitting in a hot room (35° C). However, in contrast to these CWI findings, we observed no evidence that cortisol secretion was affected by CWS. The contradictory results may be due to the strong association between reductions in cortisol levels with reductions in Tc (12). Therefore, the lack of a cortisol response in our study is likely due to the lack of a Tc change (1 h post = ∼0.94° C) after CWS (21).
Our results indicate that a 15° C cold shower for a duration of 15 minutes is an adequate cooling intervention for promoting a positive thermal comfort sensation by facilitating HR recovery at 30 minutes after cold showering. However, cold showering was not effective in reducing Tc or salivary cortisol during post-exercise recovery as observed with CWI. Consequently, the application of cold showering may have limited benefits as a cooling treatment for patients suffering from heat stress and as a recovery intervention for elite athletes regaining sports performance from subsequent exercise bouts. Because cold showering is energizing, easy to use, hygienic, and has some perceptual beneficial effects, it could be recommended as a post-exercise practice to refresh the body and recover somewhat from the associated cardiac stress, in healthy individuals.
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