It is becoming increasingly clear that a person’s health and well-being are improved by physical activity as well as by a nutritious diet. Both physical activity and diet stimulate processes that, over time, alter the morphological composition and biochemical function of the body. Physical activity and diet are interrelated in that optimal adaptation to the stress of exercise-training usually requires a diet that is not lacking in various nutrients. Physical activity should therefore be viewed as providing stimuli that stress various systems of the body to differing degrees depending upon the type and intensity of exercise. Furthermore, the progressive adaptations to regular physical activity are very specific in response to the stress encountered during physical activity.
Stress of Physical Activity
The stress of physical activity is typically categorized in two ways: (1) by the metabolic aerobic stress it places on the cardiovascular system when exercise is performed for several minutes or longs (Table 1) and (2) by the percentage of the individual’s one-repetition maximum (1RM) for physical activity involving lifting weights for short periods of time (Table 2). Weightlifting is largely power by anaerobic metabolism.
|TABLE 1. Classification of Intensity of Aerobic Exercise1|
|Relative Intensity||Absolute Intensity in METSa|
|Classification||% Max Heart Rate||%Max VO2 or |
% Heart Rate Reserve
|Low||30-49||25-39||10-11||3.0- 4.7||2.0- 3.1|
|Moderate||50-69||40-59||12-13||4.8- 7.1||3.2- 4.7|
|High||70-89||60-85||14-15||7.20- 10.1||4.8- 6.7|
|Very High||> or = 90||> or = 85||> or = 16||> or = 10.2||> or = 6.8|
aAbsolute intensity, measured in metabolic equivalent units (METS), is an appropriate mean value for men.
Mean values for women are approxiamately 1-2 METS lower than those for men.
bBorg Rating of Perceived Exertion-original 7-20 scale.
cMaximum values are mean values during maximal exsercises for healthy adults.
Examples of the Stress of Various Types of Physical Activity and the Specific Adaptations
Low-intensity aerobic exercise such as walking (e.g., 25-40 percent VO2max) increases metabolism several fold above basal levels, with relatively large increases in free fatty acid mobilization and oxidation. 3. The humoral responses are small but physiologically significant, and carbohydrate metabolism is stimulated slightly. Although the cardiovascular stress is mild, it may be sufficient to stimulate acute and chronic adaptations.
Aerobic exercise performed at moderate to high intensity (50-90 percent VO2max) for 30-60 minutes stimulates a large increase in carbohydrate metabolism and oxidation with a continued stimulation of fat metabolism.
Moderate to heavy weightlifting stimulates neuromuscular recruitment to very high levels. A sufficient number of repetitions produces brief alterations in metabolic homeostasis within muscle and a postcontraction hyperemia. Chronic activity promotes muscle fiber hypertrophy, possibly from the stretch-overload activation of a promoter of protein synthesis (-actin),6 which requires certain humoral agents.7
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