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physical performance, especially during exercise in high temperatures, with
high sweat losses and peripheral vasodilation (Cheuvront, Carter, Castellani
& Sawka, 2005).
Habitual physical activity appears to provide benefits to health, includ-

ing resistance to disease (Dishman et al., 2006); however, connections
between excessive training and increased disease susceptibility remain ten-
tative. Although there are anecdotally based claims that excessive training
(“overtraining”) may lead to an increased frequency of upper respiratory
infections, studies designed to test this hypothesis have failed to confi rm
the claim (Nieman, 2000). Rather, studies of prolonged continuous exertion
suggest that preparatory training, adequate energy intake, and rest help to

prevent any significant loss of function and protect against decrement in
performance (Frykman et al., 2003; Hoyt & Friedl, 2006). In the 3 week-
long Tour de France cycling race and in ambitious polar expeditions, peak
performances have been achieved over prolonged periods of high-intensity

exertion. Reports of significant physical breakdown are usually found related
to inadequate energy intake (e.g., Hoyt & Friedl, 2006; Stroud, Jackson &
Waterlow, 1996).

Even within well-reasoned models of the effects of prolonged exertion
and inadequate energy intake, dramatic variation between the individuals’

points to the importance of individual differences in physiological and psy-
chological resilience. For example, two soldiers with the lowest initial body
fat composition (~6% body fat) in a group of volunteers who were undergo-
ing the U.S. Army’s 8-week Army Ranger course provided extreme contrasts
in their responses to the stressors. As part of their training in this course, the

soldiers’ food intake is deliberately restricted. These two very lean soldiers

demonstrated dramatically different metabolic and behavioral responses.
One of the soldiers lost the largest amount of lean mass and 23% of his body
weight, while the other actually gained a small amount of lean mass (Friedl
et al., 1994). The adaptive responses and genetics behind such diff erences

were not readily explainable in this experiment but can include any of a full
range of factors, including behavioral responses that improve the economy of
motion and energy requirements, as well as the pattern of cytokine responses

affecting catabolism, inflammatory, and infectious challenges, all of which

were present in varying degrees in the men in this training model.

Thermal Strain

Physiological adaptation to cold is less dramatic than that observed in
response to heat. This may be due to human tropical evolutionary heritage

or the fact that behavioral responses (e.g., clothing) obviate the need for

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