Curr Rev Musculoskelet Med (2008) 1:131–136 related to the importance of the environment during sports competitions [22]. The risk of heat illness during high intensity or endurance activities is signiﬁcantly increased in hot and/or humid environments [5]. Acclimatization to hot and/or humid conditions is generally noted to take 10– 14 days [5]. However, beneﬁts have been noted in 5– 6 days with more time needed to acclimatize in humid environments [23, 24]. This adjustment to extreme tem- peratures enables the highly trained competitor to tolerate adverse conditions longer than someone who has not acclimatized. The deleterious effects of a hot environment were notable during the 2007 Chicago Marathon. Although most of the news coverage focused on the increased number of participants having heat-related illnesses, contributed to by a lack of acclimatization, even the elite runners were affected, ﬁnishing 4 min slower than in their competition just 1 year prior [25]. According to Ely et al. [26], the results of the Chicago Marathon should have been expec- ted, with their research revealing progressive slowing of marathon performance as temperature increases. This contrasts to data collected from the 1996 Atlanta Olympic Marathon in which the hot and humid temperatures were not found to signiﬁcantly affect ﬁnishing times [27]. The signiﬁcance of these conﬂicting results is likely secondary to acclimatization. Elite competitors still suffer from the same side effects as their non-elite counterparts. These effects, however, are attenuated due to their improved ﬁt- ness and their ability to ﬁnish faster, thereby reducing the overall environmental strain. Overall, the environment plays an important role during activities. Hot environments result in signiﬁcant heat stress, which is compounded by dehydration. The amount of time needed to reach exercise-related exhaustion is decreased with increasing temperature, humidity, and dehydration [5]. In contrast, the effects of dehydration on endurance exercise performance are diminished in cold compared to temperate environments [28]. The etiology of this is likely related to the increased temperature gradient associated with exercise in cool environments. As previously men- tioned, radiation, conduction, and convection are signiﬁcant thermoregulatory mechanisms at rest, while evaporation becomes a greater contributor with activity and increasing temperature [10]. However, this evaporative thermoregulatory mechanism is signiﬁcantly impaired when the relative environmental humidity is high as the partial pressure of water in the air is closer to or at its saturation point. Activities in cooler environments rely less on evaporative thermoregulation or sweating, thereby attenuating the effects of impaired performance associated with dehydration. These continued ﬁndings point to heat strain as a signiﬁcant contributing factor in exercise impairment. 133 Theories abound as to how the body acclimates to hot environments to attenuate the performance deﬁcits often seen during activity with adverse environmental pressures. Sweat increases in hot environments to help maintain physiologic normal core temperatures, but if the amount and concentrations do not change then the person will quickly become dehydrated and have electrolyte imbal- ances. In addition to the thermoregulatory effects, hydration status also affects hemodynamic stability. Dehydration can result in tachycardia through impaired plasma volume and decreased maximal cardiac output, which can lead to impaired work capacity as the exercising muscles require signiﬁcant blood ﬂow during activity [10, 29]. Increasing dehydration then results in decreased sweat rates subsequently leading to increased core temperatures. As dehydration continues the athlete’s body will eventually be unable to compensate for continued environmental stressors. Acclimatization allows for adjustments of how the body reacts to these stressors. One theory of acclima- tization to heat stress and sweating is that the body becomes more efﬁcient with its sweat production by having sweat glands excrete in a patchwork skin pattern to main- tain thermoregulation, while minimizing ﬂuid loss [30, 31]. Other stressors not related to heat strain are of importance as well. Although the effect of altitude on hydration and per- formance is complex, it is important to note the effects of acclimatization. Signiﬁcant numbers of endurance athletes train in this type of environment to improve their perfor- mance, both at sea level and in these environments themselves [31]. Altitude acclimatization has signiﬁcant physiologic effects on the human body. Living or training at elevated altitude creates a relative hypoxia, which results in the stimulation of erythropoietin. This leads to increas- ing red blood cells, and hemoglobin. Subsequently, the body will have increased oxygen carrying capacity [32]. However, altitude taken to the extreme as in space travel can potentially have negative performance effects. Lack of gravity has been shown to cause a relative diuresis with subsequent decreases in circulating plasma volumes [33]. The future expansion of space travel will result in more people being affected by these effects of microgravity. Electrolytes Acclimatization during training and competition affects more than how the body maintains hydration. The body will also conserve sodium by signiﬁcantly decreasing sweat sodium concentration [34]. Hyponatremia, deﬁned as serum sodium less than 135 mmol, is one of the most dangerous ailments of athletes because if the condition is not recognized or treated incorrectly, dire consequences