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rates between the two groups. Women had the greatest tolerance to the exercise and lowest rectal temperature prior to acclimation if they were in the pre-ovulatory phase, whereas post-ovulation, rectal temperature was similar to men while sweat rate and heart rate continued to be significantly lower. In women there was also a lag period before sweating began in the post-ovulatory phase, resulting in core temperatures rising above those seen in the pre-ovulatory phase. The differences seen in sweat rate pre-and post-ovulation in women, however, were not of the same magnitude as those seen when they were compared to men (Avellini et al., 1980).

Studies comparing men to women in hot environments have shown that women acclimated to the same work load as men demonstrate decreased sweat rates, but similar core (rectal) temperatures (Avellini et al., 1980; Wyndam, 1965). Other studies comparing men and women exercising in hot environments (Dill et al., 1977; Morimoto et al., 1967; Weinman et al., 1967) have consistently demonstrated less elevation of total body sweat rates (in milliliters per meter squared per hour) among women. Although heat and dehydration affect thermoregulatory responses such as sweating in men more severely than in women (Grucza et al., 1987), it is not gender but an individual's surface area, fitness or aerobic capacity, and acclimatization status that determine the relative heat strain in a given environment (Havenith and van Middendorp, 1990).

Overall, therefore, women do not seem to have less heat tolerance than men when they are exercising at equivalent intensities in relation to their aerobic capacities. Whereas women sweat less, they rely on circulatory cooling to a greater extent for heat dissipation. Therefore well-trained, heat acclimatized women show similar responses to hot-humid and hot-dry environments as do men.

Effect of Age

Apparent heat intolerance among the aged has been attributed to a reduction in sweating capacity and a decline in aerobic fitness (see Kenney and Gisolfi, 1986, for a review). It appears that the development of the decline in heat tolerance normally associated with men and women beginning around age 50 to 60 can be attributed to reduced cardiovascular fitness and a lack of prior heat exposure that would allow for heat acclimatization. One study (Robinson, et al., 1986) cited by Gisolfi (Chapter 5) demonstrated decreased sweating capacity in four men age 44 to 60 compared with measurements made 21 years earlier. However, this decline did not affect the ability of the older men to become acclimated to a hot-dry environment (as defined by a decreased body core temperature after 6 to 8 days) and to work at the same level and intensity as they had previously worked.

In contrast, a study done with five older men (aged 61 to 67), in which

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