Humans and animals expand energy at rest and in activity. About a quarter of the chemical energy used in muscle contractions goes into performing work against external forces. The rest is converted into heat. The primary method of assessing energy expenditure during an activity is through the evaluation of exchange of oxygen and carbon dioxide. Glucose and fat metabolism depend on the availability of oxygen. The amount of oxygen and carbon dioxide exchanged in the lungs normally should equal to that used and released by the body tissues in converting food energy into heat and mechanical work. The carbon and oxygen contents of carbohydrates, fats, and proteins differ dramatically, and therefore the amount of oxygen used during metabolism depends on the type of food fuel being oxidized.

The rate at which body uses energy is called the metabolic rate. At rest, the body usually burns a mixture of carbohydrate and fat. The average resting energy expenditure for a 70-kg man is about 2,000 kcal/day. This value reflects the minimum amount of energy required to carry out the body's essential physiological functions. The basal metabolic rate is directly related to the fat-free mass of the body because preserving fat requires almost no energy expenditure. The other factors that affect the basal metabolic rate are surface area of the body (the larger the surface area, the higher the rate of heat loss across the skin), age (metabolic rate decreases with age), body temperature, stress, and various hormones.

The body's ability to gauge muscle needs for oxygen during exercise is not perfect. At the beginning of exercise, the oxygen transport system (respiration and circulation) does not immediately supply the needed quantity of oxygen to the active muscles. The oxygen consumption requires several minutes to reach the required steady-state level while the body's oxygen requirements increase markedly the moment exercise begins. As a result, after the completion of the exercise, even though muscles are no longer actively working, oxygen demand does not immediately decrease.

This consumption exceeding what is usually required when at rest is referred to as the oxygen debt.

The amount of energy expended for different activities varies with the intensity and the type of the exercise. Some activities such as bowling or archery require only slightly more energy than when at rest. At the other extreme, sprinting requires so much energy expenditure that it can be maintained for only a few seconds. The energy expenditure per minute during high-speed running and crawl swimming is probably the highest among athletic activities, followed in order by handball, basketball, weight lifting, cycling, and so on. The metabolic cost of running increases with increasing speed of running.

The oxygen consumed during an athletic activity increases in proportion to the effort. For example, the oxygen uptake per minute is proportional to the speed of running. Eventually, as the speed of running further increases, the body reaches a limit for oxygen consumption. Even though the work intensity continues to increase, the oxygen consumption peaks and remains constant or drops. The peak value of oxygen consumption is called the maximum oxygen uptake. This parameter is regarded as a measure of cardiorespiratory endurance and aerobic fitness.

Part of the expenditure of energy during an athletic activity beyond that of the resting level results from the additional demands imposed on the heart and the rest of the circulatory system. The remaining part correlates with the state of activation of the skeletal muscles of the body, and may be intrinsically related to the work done by the muscles against the environment. In the following, we present an introduction to the study of energy transfer during athletic activity.

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