Page 171 - Biobehavioral Resilence to Stress

148 Biobehavioral Resilience to Stress

High Work Intensity and Energy Balance

Optimal physical performance involves a unique interplay of physiology,
motivation, and environmental constraints. Factors that are important
to physical endurance performance include aerobic capacity (e.g., cardiac
output, hemoglobin P50, muscle mitochondrial density, vascularity), mus-
cle fiber type (e.g., proportion of Type 1 muscle fi bers), biomechanics, sub-

strate availability, and energy metabolism (e.g., fat metabolic effi ciency),
hydration and thermoregulation, training, and genetics. Nevertheless,
psychological motivation may be the most important variable to predict
success. First and foremost, motivation is essential for committed physi-
cal training, which is in turn essential to the making of an elite competi-
tor. Only through motivation and commitment is it possible to endure the
sort of long-term, intense training that is necessary to produce measurable
physiological advantage. For example, effective training leads to an increase

in type I muscle fibers. By such intense training, Tour de France cycling
champion, Lance Armstrong, was able to achieve an 18% improvement in
his steady-state power/kg body weight over a period of 7 years (Coyle, 2005;
Mena et al., 1991).
Human beings can function in a wide variety of environmental conditions
that vary by temperature, humidity, and altitude, but optimal performance
is possible only within a fairly narrow range of these environmental factors
without sustained training, exposure, and protective clothing and equip-
ment. Hot-dry (desert) and hot-wet (tropic) environments severely restrict

work output. Upon first arrival or experimental exposure to such environ-
ments, most individuals are severely limited in their ability to accomplish
a physical task without risking heat injury (heat exhaustion or heat stroke).
Pandolf and Young (1992) demonstrated that with first exposure to a desert

environment (120°F, 20% relative humidity), human subjects could not com-
plete a 100 min walk on a treadmill at a moderate pace (3.5 mph) without
experiencing an increase in core body temperature beyond safe limits. For
reasons discussed earlier in this chapter, altitude also dramatically aff ects
physical performance. Reduced atmospheric pressure affects the body’s abil-

ity to obtain oxygen from the atmosphere. Physical performance requires
an increasing percentage of maximum oxygen uptake as maximum oxygen
consumption decreases with increasing altitude (by some measurements as
much as 8% for every 1000 m of elevation; Fulco & Cymerman, 2002). Th is
severely limits physical performance ability. For example, Fulco, Rock, and

Cymerman (1998) reported that even after 10 days of exposure and training
at altitude, human performance in 30 min competitive events such as swim-
ming and running was 5–15% slower at 2000–4000 m, respectively, than at
sea level elevation. Hydration status is also a key determinant of sustained

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