Exercise Physiology Laboratory

Internal energy liberated (dE) during breakdown of an organic molecule can either appear as heat (H) or be used to perform work (W). About 60% of the energy released appears immediately as heat, and the rest is used for work. The energy used for work must first be incorporated into molecules of ATP, the subsequent breakdown of which serves as the immediate energy source of the work. Heat released in its chemical reaction is used to maintain body temperature.

External work: muscle activity used to move external objects.
Internal work: all other form of work, including muscle activity not used to move objects. Internal work is also transformed to heat.
Total energy expenditure = internal heat production + external work + energy stored.

Metabolic rate (Total energy expenditure per unit time)
1kcal= amount of heat required to raise the temperature of 1 L of water by 1^oC. (energy expenditure while lying still awake =77kcal)

Most of the basal metabolic rate is expended by the heart, liver, kidneys and brain. Thyroid hormone and epinephrine have calorigenic effect (both increase oxygen consumption). Increased levels of epinephrine account for part of the greater heat production associated with emotional stress, although increased muscle tone also contributes.

The ingestion of food rapidly increases the metabolic rate by 10-20% for a few hours after eating: food-induced thermogenesis.
Altered skeletal-muscle activity can mostly increase metabolic rate. Strenuous exercise may increase energy expenditure more than 15-fold.
Energy stored = energy from food intake-(internal heat produced + external work).
Except in growing children, energy storage is mainly in the form of fat in adipose tissue. 1 g of fat contains 9 kcal, 1 g of carbohydrates or protein contains 4 kcal.

Body temperature

The rise in body temperature during exercise is due to retention of some of the internal heat generated by exercising muscles. Heat production rises immediately during the initial stage of exercise and exceeds heat loss. This rise in core temperature triggers reflexes, via central thermoreceptors, for increased heat loss. With increased skin blood flow and sweating, the discrepancy between heat production and heat loss start to diminish, but core temperature continues to rise until heat loss and heat production are equal. At this point, core temperature stabilizes at the elevated value despite continued exercise.

Immune system/Host resistance

Modest exercise and physical conditioning have net beneficial effects on the immune system and the host resistance.