BASIC EXERCISE PHYSIOLOGY
The primary physiologic event in exercise is contraction of skeletal muscle, or so-called
voluntary muscle. Muscles perform movements through a series of either concentric shortening or eccentric lengthening. Concentric
contractions draw the ends of the muscles closer together, shortening the fibers, while eccentric muscle contractions move the ends of the active muscles farther apart, elongating the fibers. Concentric contractions lift an object; eccentric contractions lower the object. Eccentric muscle contractions use more energy compared to concentric exercise. When a muscle contracts, it compresses the blood vessels in it, but between contractions, blood flow in exercising muscle is increased as much as thirtyfold. Extra oxygen must be carried to active cells and carbon dioxide away from them at high rates, resulting in increased circulatory and respiratory rates.
The energy for this activity comes from the two systems for different types of activity. The anaerobic system, which does not require oxygen, provides energy for short-term activity of moderate intensity. This system uses glycogen (glucose) from food as the fuel source. It is the major source of energy for the first minute and a half of exercise. Beyond the second minute of exercise, the oxygen-using aerobic system, for endurance activities, predominates. The aerobic system requires glycogen, fats, and proteins for fuel sources. Exercise that depends on stamina and endurance uses the aerobic system and has been found to contribute more to cardiovascular health. Most daily activities are aerobic since they require little power and occur over prolonged periods, but heavy labor is usually both aerobic and anaerobic.
The initial energy for muscle action comes from the manufacture of adenosine triphosphate (ATP). When energy is released by the breakdown of ATP, the muscle cells contract. This system provides energy for short, quick bursts of activity and is the major source of power for the first 20 to 30 seconds of intense exercise. After this the anaerobic and aerobic systems take over.
As the heart rate rises, cardiac output increases, and there is an increase in the systemic arterial blood pressure. Exercise causes an increased blood flow, which will increase oxygen delivery to muscles for work. If an area of the body is not used during the exercise program, peripheral resistance equalizes the blood pressure in parts not being used.
During exercise the body consumes oxygen in two different ways, one for the total body and the other for the myocardium, or heart muscles. Total body or ventilatory oxygen consumption (VO6) is the amount of oxygen consumed by the body as it performs work. Maximal VO6 is figured by multiplying maximal cardiac output and the maximal difference between the oxygen in the arteries and veins. Cardiac output is equal to the product of the stroke volume of the heart and the heart rate; this is used to determine exercise tolerance and to measure cardiac performance.
Contracting muscle cells may increase total heat production 10 to 20 times the normal
temperature and thus place severe demands on the mechanisms that regulate body temperature. As exercise continues over a period of time, the body temperature rises, causing the temperature-sensitive cells in the brain to be activated, which inhibits the nervous input of sympathetic outflow to the skin vessels. This stimulates the nerve fibers to the sweat glands, enlarging them so perspiration occurs, lowering the body temperature.