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144 Biobehavioral Resilience to Stress

The Physiological Basis of Performance Limits and Injury

Hochachka, Gunga, and Kirsch (1998) have suggested that the human
ancestral phenotype arose under conditions that favored endurance per-
formance under increasingly cold, hypoxic, and arid conditions. In their
analysis, Hochachka et al. proposed that physiological responses, which
developed originally through oxygen sensing to mitigate hypobaric
hypoxia in Andean and Tibetan natives (Figure 7.1), are now common to
an up- regulated version of the human phenotype that enhances endurance
performance. For example, vascular endothelial growth factor (VEGF)
stimulates angiogenesis to better accommodate the need for oxygen trans-
port at altitude. Intramuscular VEGF response to the demands of physi-
cal training must also be important to the angiogenic responses that occur
in endurance-trained athletes. Ultimately, the key physiological predictor
of human performance is mitochondrial activity, which refl ects all other
critical functions (Hochachka et al., 1998; Hoyt & Friedl, 2006). In practi-
cal terms, sustained endurance activity in land-living mammals is limited
by mitochondrial/respiratory function to approximately five times resting

energy expenditure.*

High Altitude
As elevation increases, the partial pressure of oxygen decreases. In functional
terms, reduced oxygen pressure makes less oxygen available to the body. Th is
has profound consequences for the human body at altitudes of 8000 ft . and
above. Hypobaric hypoxia is detrimental to physical and cognitive functions.
However, hundreds of studies have demonstrated that the human body can
acclimatize quite well to hypobaric hypoxia. Initial ascent to high altitude

provokes a series of physiological changes that reflect an effort by the body to

maintain maximum oxygen saturation to bodily tissues. These changes are
†
observable as increases in heart rate, cardiac output, and ventilation rate.
Following acclimatization, heart rate and cardiac output return gradually
to their near baseline (sea level) values, and stroke volume decreases. Res-
piration rate remains elevated, however. Neuroendocrine and metabolic
processes support acclimatization to high altitude. During the first 4 days of

exposure to altitude, norepinephrine (NE) levels (found in urine) rise, and

* This is quite different than factors that determine performance and survival limits for
diving mammals (Hochachka et al., 1998).
† On initial ascent, an increase in hematocrit is also observed due to a loss of plasma
volume. This initial hematocrit increase is not related to an increase in the number of
red blood cells, which occurs more slowly over the course of several weeks or months in
response to erythropoeitin.

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