Exercise
Exercise is often recommended as a means of improving motor skills, fitness, muscle and bone strength, and joint function. Exercise has several effects upon muscles, connective tissue, bone, and the nerves that stimulate the muscles. One such effect is muscle hypertrophy, an increase in size. This is used in bodybuilding.
Various exercises require a predominance of certain muscle fiber utilization over another. Aerobic exercise involves long, low levels of exertion in which the muscles are used at well below their maximal contraction strength for long periods of time (the most classic example being the marathon). Aerobic events, which rely primarily on the aerobic (with oxygen) system, use a higher percentage of Type I (or slow-twitch) muscle fibers, consume a mixture of fat, protein and carbohydrates for energy, consume large amounts of oxygen and produce little lactic acid. Anaerobic exercise involves short bursts of higher intensity contractions at a much greater percentage of their maximum contraction strength. Examples of anaerobic exercise include sprinting and weight lifting. The anaerobic energy delivery system uses predominantly Type II or fast-twitch muscle fibers, relies mainly on ATP or glucose for fuel, consumes relatively little oxygen, protein and fat, produces large amounts of lactic acid and can not be sustained for as long a period as aerobic exercise. Many exercises are partially aerobic and partially anaerobic; for example, soccer involves a combination of both.
The presence of lactic acid has an inhibitory effect on ATP generation within the muscle; though not producing fatigue, it can inhibit or even stop performance if the intracellular concentration becomes too high. However, long-term training causes neovascularization within the muscle, increasing the ability to move waste products out of the muscles and maintain contraction. Once moved out of muscles with high concentrations within the sarcomere, lactic acid can be used by other muscles or body tissues as a source of energy, or transported to the liver where it is converted back to pyruvate. In addition to increasing the level of lactic acid, strenuous exercise causes the loss of potassium ions in muscle and causing an increase in potassium ion concentrations close to the muscle fibres, in the interstitium. Acidification by lactic acid may allow recovery of force so that acidosis may protect against fatigue rather than being a cause of fatigue.
Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising and subsides generally within two to three days later. Once thought to be caused by lactic acid buildup, a more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.
Famous quotes containing the word exercise:
“To read well, that is, to read true books in a true spirit, is a noble exercise, and one that will task the reader more than any exercise which the customs of the day esteem. It requires a training such as the athletes underwent, the steady intention almost of the whole life to this object.”
—Henry David Thoreau (1817–1862)
“I would not that death should take me asleep. I would not have him meerly seise me, and onely declare me to be dead, but win me, and overcome me. When I must shipwrack, I would do it in a sea, where mine impotencie might have some excuse; not in a sullen weedy lake, where I could not have so much as exercise for my swimming.”
—John Donne (c. 1572–1631)
“I will have no Parsons around me but such as drink deep, ride to Hounds and caress the Wives and Daughters of their Parishioners. A Virtuous Parson does nothing to test or exercise the Faith of his Flock.”
—Aldous Huxley (1894–1963)