Muscle activity

Exercise is muscular activity. When the word is used, there is almost always the additional implication of the activity being extended over time, but for how long is up to the user. More commonly explicit are the adjectives of intensity (mild, moderate, strenuous/high) and body region (leg, upper body/arm). An important distinction, from the point of view of physiological response, is between exercise predominantly involving movement (dynamic exercise) and that in which the muscles brace against each other or an unmoving outside load (static exercise). Static exercise is also known as isometric because the muscles stay at (approximately) constant length.

All exercise, then, starts with the activation of voluntary muscle. Whether there is significant movement depends on whether the force the muscle is producing exceeds, matches, or falls short of the load against which it is acting. The first situation produces dynamic exercise of the form we usually think of; technically, the muscles, successfully shortening, are said to be contracting concentrically. However, the last situation is dynamic too; here the muscles, extending under the greater external force, are active eccentrically (often pronounced ee-centrically). Only in the middle case, where muscle force equals that against which it is acting, will the exercise be static. Finally, it must be made clear that the muscles need not be working flat out in any of these situations. That will depend on their degree of activation by the nervous system; full activation is uncommon in daily life.

The chemical demands of the muscles underlie most of the other phenomena of exercise. In particular, ample supplies of oxygenated blood must be supplied to every active muscle. Both the heart and the circulation, and the respiratory system, respond accordingly. Scientific understanding of these responses, however, depends on our ability to measure both muscular performance and the metabolic energy input upon which it is based.

Measuring muscular performance and metabolic input

It is a fairly simple matter to measure isometric force production. All that is required is a spring balance or, better, an electronic strain gauge, against which the body-part of interest exerts force through a virtually inextensible wire or rigid lever system. Grip strength, bite force, elbow flexion, or knee extension are easily measured by dynamometers (force measurers) of this broad type.

In dynamic exercise, measuring force as such is not often sufficient for the physiologist, though transducers placed in bicycle cranks, or in force plates let into a rigid laboratory floor, are examples of instruments which can provide this information. The overall demand of dynamic exercise is, however, most completely indicated by the power output achieved by the body, for power embodies both the force and the rate of movement. Power output is assessed by ergometers (work measurers), and can be most readily measured for rhythmic movements against external load, such as in cycling or rowing.

The input of energy from metabolism can be estimated with reasonable precision when the exercise lasts long enough at a steady rate for breathing to come into balance with the muscles' demands (aerobic exercise). Then the effort may be considered to be entirely founded upon the burning of fuel molecules in oxygen. As all the body's fuels (carbohydrate, fat, and normally used to a much lesser extent protein) release rather similar amounts of energy when reacted with the same volume of oxygen, measurements of the volume of oxygen consumed per minute (Vo2) are the basis of the energy- input calculations. Such measurements are made by collecting the air breathed out by an exercising subject, assaying the percentage of oxygen left in that air, and subtracting that from the percentage of oxygen which would have been in the same volume of air when it was breathed in. The result gives the aerobic power of the subject performing that exercise. The maximum aerobic power a subject can achieve (Vo2max) is a fundamental indicator of exercise potential.

Changes in heart and circulation

Considering the heart first, its rate of beating rises appreciably even as we stand up and walk gently through the house. In the highest intensity exercise, the pulse rises to its maximum. This varies with the age of the individual, but negligibly with gender and, more surprisingly, only a little with fitness. The thumb rule is that maximum heart rate (HR) (in beats per minute) = 220 - (age in years). People who are trained to sustain high intensity dynamic exercise for periods of many minutes at a time (aerobic athletes) actually have maximum HRs 10-15 beats per minute lower than would be calculated by that formula. This seeming paradox makes more sense when it is considered that the amount of blood pumped by their hearts in every beat (their stroke volume, SV) is greater in any given state of rest or exercise than that of an untrained person; thus the aerobic athlete's resting pulse will be slower than the average person's by at least as much as the shortfall at maximum HR, and so allows a greater percentage increase from rest to maximum exercise.

Read the original post:
exercise: Definition, Synonyms from Answers.com