Ereq during the recovery sessions was not significantly different between groups (P = 0.457). EHL, THL, and DHL were similar between recovery periods and did not significantly differ between groups.
Change in Body Heat Content
The changes in body heat content during the exercise and recovery periods are presented in Figure 2. The amount of heat stored in the body increased with repeated exercises (P < 0.001), whereby a significantly lower amount of heat was stored during Ex2 compared with Ex1 in young (75.4 ± 2.4 [Ex2] vs 114.0 ± 4.9 kJ [Ex1], P = 0.001) and older females (92.3 ± 3.6 [Ex2] vs 135.9 ± 5.2 kJ [Ex1], P = 0.001). No differences in body heat content between Ex2, Ex3, and Ex4 were observed in either group. The change in body heat content was also significantly different between groups (P < 0.001). The older females stored significantly more heat than the younger females at the end of Ex1 (P = 0.004), Ex2 (P = 0.001), and Ex3 (P = 0.017), whereas no significant differences were measured at the end of Ex4 (P = 0.547).
The change in body heat content was significantly different during successive recovery periods (P = 0.001). The older females had a reduced rate of heat storage during R2 compared with R1 (−30.0 ± 7.1 [R2] vs −9.7 ± 5.8 kJ [R1], P = 0.001). A similar trend was observed in young females (−43.0 ± 4.1 [R2] vs −27.5 ± 5.7 kJ [R1], P = 0.068). No differences in the change in body heat content were measured between R2, R3, and R4. Moreover, the changes in body heat content during the recovery periods were not significantly different between age groups (P = 0.126).
Cumulative change in body heat content
The older females had a significantly greater cumulative change in body heat content compared with younger females (269.7 ± 20.1 vs 166.1 ± 20.3 kJ, P = 0.001) after the 2-h intermittent exercise bout.
Results for rectal (Trec) and telemetric pill temperatures (Tpill) during the exercise and rest periods are presented in Table 3. The main effect for the increase in core temperature with successive exercise periods was significant for both Trec (P < 0.001) and Tpill (P < 0.001). The rise in Trec was greater at the end of Ex2 compared with Ex1 in young females (P < 0.001). In older females, the rise in Trec was greater during Ex2 versus Ex1 (P < 0.001), Ex3 versus Ex2 (P < 0.001), and Ex4 versus Ex3 (P = 0.027). Although the rise in Tpill was not found to be significantly different between successive exercises in young females, older females had a greater rise in Tpill at the end of Ex2 versus Ex1 (P < 0.001), Ex3 versus Ex2 (P = 0.037), and Ex4 versus Ex3 (P = 0.049). Trec and Tpill did not significantly differ between young and older females (P = 0.773 and P = 0.814, respectively). The change in Trec from baseline was not significantly different between groups at end of each exercise bout (P = 0.680). Conversely, the change in Tpill from baseline was significantly greater in older females at end of Ex3 (0.58°C ± 0.04°C vs 0.38°C ± 0.06°C, P = 0.023) and Ex4 (0.65°C ± 0.05°C vs 0.43°C ± 0.08°C, P = 0.037), whereas the difference in Tpill at the end of Ex2 was just short of statistical significance (0.51°C ± 0.04°C vs 0.36°C ± 0.06°C, P = 0.056).
The main effect for the increase in core temperature during the rest periods was significant for both Trec and Tpill (both P < 0.001). The rise in Trec was greater at the end of R2 compared with R1 in young females (P = 0.044). In older females, the rise in Trec was greater during R2 versus R1 (P < 0.001), R3 versus R2 (P = 0.002), and R4 versus R3 (P = 0.022). Tpill was only found to be significantly greater at the end of R2 compared with R1 in older females (P < 0.001). The main effect for age group was not significant for either measures of core temperature during the recovery periods (TrecP = 0.426 and TpillP = 0.449). The change in Trec from baseline, however, was significantly greater in older females compared with young females at the end of R3 (0.56°C ± 0.07°C vs 0.36°C ± 0.06°C, P = 0.028) and R4 (0.63°C ± 0.08°C vs 0.40°C ± 0.06°C, P = 0.024). In addition, the change in Tpill from baseline was significantly more elevated in older versus young females at the end of R2 (0.41°C ± 0.04°C vs 0.18°C ± 0.05°C, P = 0.005), R3 (0.44°C ± 0.04°C vs 0.24°C ± 0.06°C, P = 0.018), and R4 (0.46°C ± 0.05°C vs 0.24°C ± 0.08°C, P = 0.014). The difference in Tpill between older and young females at the end of R1 was just short of significance (0.23°C ± 0.02°C vs 0.15°C ± 0.03°C, P = 0.064).
Mean Skin Temperature
Mean skin temperatures throughout the experimental protocol are presented in Table 3. Tsk significantly increased with successive exercise bouts (P = 0.023). However, Tsk was only found to be significantly different at the end of Ex2 compared with Ex1 in older females (P = 0.018). The main effect for age group was also significant (P = 0.019); however, this was likely due to the significantly greater mean skin temperature in younger females at baseline. When Tsk was expressed as a change from baseline (Fig. 3C), the main effect for group was no longer significant (P = 0.537).
Tsk did not significantly change during the recovery periods (P = 0.263), but there was a tendency for mean skin temperature to be different between groups (P = 0.054). When expressed as a change from baseline values, the main effect for age group was no longer significant P = 0.213).
Local Heat Loss Responses and HR
Exercise (local sweat rate)
The results for local sweat rate, SkBF, and HR are presented in Table 3. Three older females were excluded from the analysis of local sweat rate due to failure of the ventilated sweat capsule to remain attached to the participant during the experiment. The data presented are therefore from 11 young females and 10 older females. Local sweat rate measured on the upper back significantly increased with successive exercise periods (P < 0.001). Local sweat rate was greater at the end of Ex2 compared with Ex1 in young (P = 0.004) and older females (P = 0.015) as well as at the end of Ex3 versus Ex2 in older females only (P = 0.041). However, local sweat rate was not significantly different between age groups (P = 0.451). The main effect for age group remained nonsignificant when local sweat rate was expressed as a change from baseline (P = 0.166).
Local sweat rate significantly increased with successive recovery periods (P < 0.001). Local sweat rate was only found to be significantly greater at the end of R2 compared with R1 in older females (P = 0.038). The main effect for age group was not significant (P = 0.708), and this remained true when expressed as a change from baseline (P = 0.831).
SkBF did not significantly increase between exercise periods (P = 0.134) and was not significantly different between groups (P = 0.898). The main effect for age group remained nonsignificant when SkBF was expressed as a change from baseline (P = 0.442).
SkBF did not significantly increase with sequential recovery bouts (P = 0.732) and was not different between age groups (P = 0.777). The change in SkBF from baseline during the rest periods was also not different between young and older females (P = 0.245).
The HR responses of young and older females were analyzed as a percentage of maximal HR (%HR) to account for differences between age groups in maximal HR achieved during the maximal exercise test. However, both absolute HR values and %HR are presented in Table 3. HR significantly increased with repeated exercise periods (P < 0.001). HR response was found to be significantly greater at the end of Ex2 versus Ex1 (P = 0.043) and Ex3 versus Ex2 (P = 0.036) in older females. HR response, however, did not significantly differ between groups (P = 0.733).
HR did not significantly differ between recovery periods (P = 0.283) and was not significantly different between young and older females (P = 0.387).
The main findings of the present study confirm our hypothesis that older females have a reduced rate of whole-body heat loss during short duration intermittent exercise in the heat compared with younger females matched for fitness, body composition, and body surface area. At the end of the 2-h intermittent exercise, the older females had a greater cumulative change in body heat content, which was paralleled by a greater change in core temperature. This was a consequence of a marked reduction in their ability to dissipate heat only during exercise given that both the young and older females showed a similar attenuated rate of whole-body heat loss during the postexercise recovery periods. The observed lower whole-body heat dissipation during exercise was not paralleled by lower local heat loss (sweating and SkBF).
Our knowledge of how aging affects thermoregulatory responses is largely based on studies that compared young and older males (13,14,30,32,33,36,37). Recent evidence suggests that sex may be an important modulator of human temperature regulation during exercise in the heat (9,11,12). Recently, Gagnon and Kenny (12) showed that young females had a reduced capacity to dissipate heat, as evidenced by a marked attenuation in whole-body sudomotor activity, compared with males during exercise in the heat performed at a fixed rate of metabolic heat production exceeding 250 W·m−2. The lower sudomotor activity observed in females was subsequently ascribed to a peripheral modulation at the level of the sweat gland rather than centrally through potential differences in neural activity (8). The extent to which aging may alter this pattern of response is unclear. Some insight may be gleaned by an earlier study by Shoenfeld et al. (33). They reported lower mean sweat rates per body surface area between males and females ranging in age from 18 to 63 yr during a 10-min passive exposure to extreme heat (80°C–90°C). The males had a mean whole-body sweat rate of 287 ± 117 g·m−2, whereas in females, it was 212 ± 89 g·m−2. Although sex comparisons were not performed by age groups, the reported mean sweat rate in older females (167 ± 94 g·m−2) was lower compared with the rates recorded in older males (267 ± 98 g·m−2). Nonetheless, given the short and extreme nature of the passive heat stress, these findings should be interpreted with caution. Ultimately, it remains unclear how responses in older males and females would differ during a thermal challenge associated with exercise in the heat. However, when combined with the findings by Gagnon et al. (8,12), the evidence suggest that age-related differences in thermoregulatory function should be examined separately in males and females.
To date, very few published studies have examined age-related changes in the body’s ability to dissipate heat in females during exercise in the heat. One study by Drinkwater and Horvath (6) examined the effects of chronological age (12–68 yr) on heat tolerance during exercise in the heat by compiling data from four separate studies. On the basis of results from regression analyses, they reported that older females tended to have reduced heat tolerance. Given that no preplanned comparison was performed between a subgroup of young versus older adult females, it is not possible to determine the extent of the age-related impairments in thermoregulatory control, if any. To the best of our knowledge, the study by Anderson and Kenney (1) is the only study to compare thermal responses between young (25 ± 4 yr) and older females (56 ± 4 yr) while exercising at the same rate of heat production. The older females had a more pronounced rise in rectal temperature and greater heat storage than the younger females after 2 h of treadmill walking at 35%–40% V˙O2max in a hot dry environment (48°C, 10% RH). In parallel, they showed that older females had an attenuated sweat production as measured by lower local sweat rate at 30 min of exercise, which was maintained for the duration of the 2-h exercise protocol. Taken together, their observations demonstrated that older females had a reduced sudomotor capacity relative to their younger female counterparts, which was attributed mainly to a lower sweat output per gland. When the maximal evaporation possible within a given environment does not limit an individual’s ability to achieve heat balance, and therefore a stable core temperature, the level of sudomotor activity achieved during exercise is driven by the required evaporation for heat balance (4,17). Thus, given that the older females showed a marked reduction in local sweat rate for the duration of exercise, which was paralleled by a markedly greater rise in core temperature, indicates that the combined metabolic and environmental heat load exceeded the physiological capacity of the older females to dissipate heat. Given that the authors did not report data before what was recorded after the first 30 min of exercise (when the rate of whole-body heat loss is still increasing), it is unclear if these age-related differences in heat loss are only evidenced at and beyond 30 min of exercise when steady-state levels of whole-body sweat production are typically observed (27).
As seen in Figure 1, the rate of metabolic heat production increases immediately at the start of exercise and is not initially offset by an increase in the rate of whole-body heat loss, thus giving rise to a pronounced increase in body heat content during the early stages of exercise (38). Whole-body EHL increases in an exponential fashion during the early stages of exercise (i.e., non–steady-state period), with a time constant of approximately 12 min and reaching steady state after approximately 30 min of continuous exercise under compensable heat stress conditions (27). We show a marked attenuation in whole-body sudomotor activity during the non–steady-state period in the older females relative to their younger counterparts occurring as early as 10 min after the onset of exercise. This pattern of response was consistently observed in Ex1, Ex2, and Ex3 with rates of whole-body EHL reduced by 17%, 9%, and 9%, respectively, in older compared with younger females. This was paralleled by a greater change in body heat content (i.e., Ex1, 16%; Ex2, 18%; and Ex3, 11%). It is important to note that while there was no statistically significant difference between age groups measured in the final exercise bout, the difference in the rate of whole-body EHL was comparable between Ex3 and Ex4 for each group, albeit the separation between groups was marginally reduced (i.e., Ex3 vs Ex4; young, 273.8 ± 11.3 vs 272.4 ± 11.0 W; older, 248.9 ± 4.7 vs 254.2 ± 7.5 W). The greater heat storage measured in older females during the exercise periods resulted in a greater cumulative increase in body heat content during the 2-h intermittent exercise. In keeping with the progressively greater thermal drive, we observed a progressive albeit marginally higher rate of whole-body heat loss measured at the end of each successive 15-min exercise bout.
It is noteworthy that the amount of heat stored after the first exercise bout was reduced to a similar extent in the young (34%) and older (32%) females relative to the second exercise bout. This greater rate of increase in whole-body heat loss during successive exercise bouts is consistent with previous reports demonstrating that the thermal inertia (i.e., the time taken to balance the differential rates of heat production and heat loss) is reduced when core temperature is elevated and the body is already warm and much of the heat already stored during the first exercise/recovery cycle remains (10,23,24). As a result of the enhanced rate of local and whole-body heat loss, the net amount of heat gained during intermittent exercise is significantly reduced in the successive exercise bouts (23). We show that this response, known as the priming effect (10), occurs despite any apparent age-related impairment in thermoregulatory function. As in the case of exercise where the greater thermal drive with successive exercise enhanced the rate of heat loss, a greater decrease in body heat content would have been expected with each successive recovery period. We observed a marked attenuation of whole-body heat loss after the first exercise bout such that only 25% and 7% of the total heat gained during exercise was dissipated during the first 15-min recovery period in the young and older females, respectively. Noteworthy, a greater decrease in body heat content was measured during the second recovery period compared with the first recovery in the older females as shown in Figure 2. Nevertheless, the change in body heat content during the subsequent recovery bouts was not different despite our observation that the cumulative heat storage was greater at the end of each successive exercise bout (Fig. 2). In parallel, we measured a rapid decay in whole-body EHL at the cessation of each exercise bout of similar magnitude in both the young and older females. These observations add to the growing body of evidence that factors of nonthermal origin have an overriding influence on heat loss responses during the postexercise period irrespective of the changes in thermal status of the individual (23–27) and, as evidenced by our study findings, the level of age-related impairment in thermoregulatory function. For a more comprehensive review of the current state of knowledge pertaining to the effects of nonthermal influences on postexercise thermoregulation, the reader is referred to a recent review on this topic (26).
In addition to measuring whole-body heat loss responses and heat storage, we also measured local heat loss responses (i.e., local sweating and SkBF) as well as core and skin temperature responses between young and older females. Although reduced whole-body EHL in older females was measured by whole-body calorimetry, this was not paralleled by similar differences in local heat loss responses. In light of our observations of marked reductions in whole-body heat loss, these results may simply reflect regional differences in local sweat rate and SkBF, consistent with the results reported in previous studies (15,16). Segmental differences in vasomotor and sudomotor function have been well documented during rest and exercise (3,29). These differences are more pronounced with greater levels of hyperthermia (29) and can be exacerbated by the high degree of heterogeneity in local sweating and SkBF responses with aging (15,16) such that these measurements may not accurately reflect changes in whole-body heat loss capacity in older adults. On the other hand, the progressively greater change in body heat content with successive exercises eventually resulted in a significantly greater change in rectal and visceral pill temperature in older females. Differences in the increase in rectal temperature between age groups became apparent by the end of the third recovery period, whereas differences in visceral temperature were seen at the end of the second recovery period and remained significantly greater until the end of the experimental session. As depicted in Figure 3, for those subjects with both measures of body heat content and core temperature (young, n = 9; older, n = 12), the greater cumulative change in body heat content in older females was paralleled by a greater change in both rectal and visceral core temperature at the end of the 2-h experiment compared with young females. Specifically, the change in body heat content measured by calorimetry and the change in rectal temperature were 33% and 38% greater in older females compared with young females. Similarly, the change in both body heat content and visceral temperature were 30% and 48% greater, respectively, in older versus young females. Moreover, as a consequence of a greater increase in body heat content during exercise, both rectal and visceral temperature increased progressively from Ex1 to Ex2, Ex2 to Ex3, and Ex3 to Ex4 in older females. This was paralleled by a progressive elevation in rectal temperature from R1 to R2, R2 to R3, and R3 to R4. In contrast, in young females, rectal temperature was only significantly more elevated at the end of Ex2 compared with Ex1 and R2 compared with R1. No significant differences in core temperature were measured between Ex2, Ex3, and Ex4 as well as R2, R3, and R4. It is likely that with longer exercise bouts, greater series of exercise/recovery cycles, and/or greater heat load (metabolic and/or environment), the differences in core temperature between young and older females would have been even more pronounced.
The older females who volunteered for this study were probably more active and physically fit than the average aging female. As such, thermoregulatory capacity may be further reduced in older females with lower cardiorespiratory fitness or chronic medical conditions (28). We did not attempt in the present study to control for menopause or menstrual cycle. However, we looked at the cumulative change in body heat content between the eight older females that were postmenopausal and the five older females that were perimenopausal and found comparable results. In fact, the overall change in body heat content was 294 ± 26 kJ in perimenopausal women and 258 ± 18 kJ in postmenopausal women. This is in sharp contrast to younger females who had a change in body heat content of 166 ± 20 kJ. The effects of circulating estrogen and progesterone levels are reported to affect baseline core temperatures (2,35). There were no significant differences in baseline Trec (37.20°C ± 0.05°C [Y] and 37.13°C ± 0.12°C [O]) or Tpill (37.36°C ± 0.05°C [Y] and 37.23°C ± 0.07°C [O]) between our age groups. In addition, previous studies have shown that menstrual cycle does not play a major role in a women’s response to exercise in the heat (39,40). Menopause on the other hand is a natural occurrence with aging; thus, it seems irrelevant to control for this when studying the effects of aging on thermoregulatory responses in aging females. It remains evident that older females, postmenopausal or not, have a reduced capacity to dissipate heat relative to younger females. On a different note, it has been suggested that differences in hydration state could account for the differences in sweating capacity and, consequently, EHL in older versus young females (22). We encouraged our participants to hydrate before the experimental session, and no fluid replacement was provided during the session. Urine-specific gravity was not significantly different between groups at baseline (Y, 1.016 ± 0.002; O, 1.012 ± 0.002; P = 0.181) or at the end of the experiment (Y, 1.018 ± 0.002; O, 1.019 ± 0.002; P = 0.767). The percent change in plasma volume was not significantly different between groups (Y, −2.7 ± 0.7%; O, −3.6 ± 0.9%; P = 0.414). Therefore, the observed differences in EHL appear unrelated to differences in hydration state between groups.
Given the intermittent nature of many daily and work-related activities, it was important to determine whether aging females experienced greater thermal strain during short bouts of exercise. Anderson and Kenney (1) showed a marked difference in heat dissipation during prolonged and continuous exercise in a very hot ambient condition (48°C). Both the exercise and ambient conditions used in their study, however, are not representative of typical environmental conditions or activity patterns consistent with everyday life. On the other hand, the present study demonstrated that age-related differences in sudomotor activity were evident as early as 10 min after the start of exercise during moderate heat stress (i.e., ambient temperature of 35°C). As demonstrated in the present study, age-related impairments in heat dissipation during short exercise/rest cycles can result in a substantially greater increase in body heat content and consequently, a greater change in core temperature. This is exacerbated by the fact that older females appear to have a similar attenuation in local and whole-body heat loss shortly after the end of exercise. Taken together, our findings support the notion that older females may be more susceptible to heat-related injury not only during prolonged exercise in the heat as demonstrated in previous studies, but during short intermittent bouts of exercise as well.
In conclusion, whole-body EHL was reduced in older females during the first, second, and third exercise bouts compared with younger females, which led to greater changes in body heat content during exercise. Because a similar attenuation in local and whole-body heat loss during the postexercise periods was observed in both age groups, the greater cumulative change in body heat content measured in older females, which was paralleled by a greater change in body core temperature, was due to reduced sudomotor activity evident in the early stages of exercise. Consequently, older females maybe more susceptible to heat-related injury while performing work or exercise of short duration in the heat compared with younger females.
This project was funded by the Workplace Safety and Insurance Board of Ontario (grant no. 10001) and a Natural Sciences and Engineering Research Council Discovery Grant (RGPIN-298159-2009) (grants held by Dr. G. Kenny).
The authors are indebted to the study participants and to Mr. S. Nagaraja, Ms. N. Baala, Ms. J. Barrera, Mr. M. Poirier, and the entire Human and Environmental Physiology Research Unit team for their contribution to the success of this project.
The authors declare no conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:© 2013 American College of Sports Medicine
AGING; HEAT STRESS; THERMOREGULATION; SKIN BLOOD FLOW; SWEATING; CALORIMETRY