exercise

exercise

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 (V̇o₂) 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 (V̇o₂max) 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.

During the responses to increasing exercise intensity there is some increase of SV as well as of HR in everybody, so that in an untrained but healthy young adult, of 70 kg body weight (the standard textbook figure), pulse might rise about threefold, from say 70 beats per minute at rest to 200, SV by about 1.7 times, and thus total cardiac output (CO) from 5 to 25 litres/min. Equivalent figures for the internationally elite aerobic athlete might be from 45 to 185 beats per minute (HR) and 5 to 40 litres/min (CO), implying a near doubling of the already large SV. Notice, however, that the resting CO is the same for both, as the metabolic demands of sitting still are much the same for everybody of a given weight.

Nevertheless, even the élite athlete's eight-fold increase in CO is far from sufficient by itself to explain the total blood flow through each of the muscles that is working flat out. Modern indications are that muscle blood flow can increase by the order of 100-fold from the resting level. Great increases of flow through the active muscles are achieved by dilatation of blood vessels running through them, assisted to some extent by constriction of the vessels supplying organs, such as the gut and kidneys, which do jobs that can take second place during the exercise. (How vessels constrict and dilate is discussed under ‘Blood vessels’.) Finally, the active muscles' metabolism is enabled to increase by yet one more factor — enhanced extraction of oxygen and nutrients from each ml of blood flowing through them. In the case of oxygen, this increase is typically about three-fold.

The limit to maximum power output

Pursuing our figures, if muscle blood flow rose 100-fold and oxygen extraction/ml of blood rose threefold, 300 times as much oxygen would have to be extracted from the air each minute for all muscles in the body to be maximally active at once. Actually, this cannot happen: it has been calculated that the heart can only supply 30–40% of the total musculature, fully active, simultaneously. This puts a significant limitation on running and cycling, and an even more substantial one on activities demanding direct propulsive power from all four limbs — such as cross-country skiing and swimming. Tellingly we find that, if any one of the measures of whole-body effort (such as maximum CO, maximum power output, or maximum oxygen consumption — V̇o₂max) is considered, its values over all these exercises are within about 10% of each other — strongly indicating that the chief limitation on them all is a central function upon which each depends. One expression of this central limitation is the ceiling, just noted, on cardiac output.

Changes in breathing

The limit shows itself in respiratory function, too. However, it is not in the obvious feature, ventilation (the volume of air breathed in and out each minute); this increases several times more than CO — namely 15–35-fold, according to aerobic fitness. (Typical patterns of the increase of ventilation during the first few minutes of both moderate and strenuous exercise are described under breathing during exercise.) That the maximum ventilatory rate is more than sufficient to meet requirements is indicated by the fact that oxygen extraction from each litre of air goes slightly down, not up, at high exercise intensities. At such intensities the time available for oxygen to diffuse from the air in the lungs into the blood as it races past, begins to become a limiting factor. In normally healthy people near sea level the limitation is barely, if at all, detectable; but in top athletes racing at sea level the arterial blood, fresh from the lungs, falls clearly short of full saturation with oxygen — comparable to its condition in a resting person at the altitude of an Alpine ski resort.

Anaerobic exercise

A distinction which has been avoided until this point must now be confronted. The discussion has focused on exercise continued long enough (say 4 min or more) that it must be performed in balance with oxygen uptake. Any track race longer than 1500 metres is of this kind once the athlete's body has adjusted fully to the pace. Briefer activities (like a 400 metre race) can be more intense, but only on the basis of the extra power, greater than the aerobic maximum, being supplied via anaerobic metabolic pathways. Such very intensive, short-term exercise is termed ‘anaerobic’; but note that, while aerobic exercise, when we have settled into it, is totally aerobic, even the briefest high-intensity exercise is never wholly anaerobic.

Upper body exercise

Before leaving dynamic exercise, we should note that exercise using only the arms provides less power at a given HR than exercise predominantly using the legs. Among the reasons for this is that external (and therefore measurable) work done by the arms usually requires the trunk to be braced by muscular effort which needs energy but does not move the load.

Static versus dynamic

Bracing actions of the trunk muscles are in fact examples of static exercise. Other instances are the guardsman's posture at attention, the weight-lifter's few seconds of triumph with the bar above his head, and the dinghy crew's efforts to hold the body horizontal over the water, balancing the boat. In all these situations HR is raised (in the latter two instances, very considerably), yet compared with dynamic exercise giving the same HR — especially leg exercise — two things are markedly different:(i) oxygen consumption is much lower;(ii) blood pressure is higher, especially during diastole.

The first point is explicable chiefly by the fact that isometrically contracting muscles require substantially less oxygen than the same muscles cyclically shortening and lengthening. The second arises because, in dynamic exercise, blood flows through the active muscles during the periods of relaxation which alternate with their contractions; during the contraction phases it is impeded. There being no relaxation periods during a static exercise, blood pressure must be raised if any flow at all is to be forced through the tensed muscles. This rise is brought about by reflex mechanisms originating in the muscles themselves.

Hormonal adjustments

In addition to the cardiovascular and respiratory adjustments which the body makes in the face of exercise, substantial hormonal adjustments also occur. Adrenaline flow is elevated, especially in anticipation of vigorous exercise; and as exercise proceeds, cortisol and (particularly in really protracted efforts, such as marathon races) growth hormone concentrations are both substantially raised, and may not return to basal levels for some hours afterwards. All these promote mobilization of both carbohydrate and lipid fuels, and growth hormone also promotes tissue adaptation and repair when the activity is over. Insulin flow, however, is reduced during exercise. This at first seems a paradox, for the function of insulin is to promote glucose entry into tissues such as muscle, and exercising muscle surely needs its glucose? It is now clear that increased availability of glucose transporter molecules in the membranes of exercising muscle fibres enables them to take in glucose with less insulin than usual. Suitably controlled exercise therefore has special benefits for diabetics.

Fuel sources

In short bursts of intensive exercise, carbohydrates are the main fuels used. At lower intensities, fats contribute more and, as endurance efforts proceed, they become the major energy source. Four-fifths of carbohydrate storage is as ‘glycogen’ (animal starch) within the muscle fibres themselves. The rest is as glycogen in the liver, from which it can be released as glucose (blood sugar) when circulating levels fall. However, the brain, which uses no other fuel, makes priority demands, so blood-borne glucose does not contribute a major fraction of the energy used by the muscles in a long event unless its concentration is kept topped up by glucose drinks or carbohydrate food.

Fat is stored both within some muscle fibres and in fat cells. The balance, however, is the converse of that for carbohydrate: most activities seem to draw more upon the fat cells than the intramuscular stores.

Health benefits

Clearly, all exercise constitutes a degree of training for the muscles which it uses. All exercise also enhances cardiovascular and respiratory health to some extent, though aerobic exercise benefits these systems most. The hormonal and metabolic consequences of any but the most severe exercise are almost always advantageous too. Of these benefits, the cardiovascular ones are normally emphasized. Sustained aerobic exercise trains the heart, lowers blood pressure, tends to reduce body fat, and promotes a switch from ‘bad’ to ‘good’ lipids — from low to high density serum lipoprotein — thereby reducing the risk of atheromatous plaques.

How much exercise is necessary, and of what form, has naturally been much researched. Recent work indicates that the most marked gains, relative to a sedentary lifestyle, are achieved by a mere 30 min of moderate exercise (such as brisk walking), on each of 3 days a week. The more exercise is taken, within a normal lifestyle, the greater the health benefit; yet a law of diminishing returns applies.

As to the form of exercise, it is clearly undesirable for an unfit person to leap straight into short-term, high-intensity activity. Worse still, isometric exercise will always, in the short term, raise the blood pressure. So exercise for health, in those who have been sedentary, should be dynamic and essentially aerobic. Such exercise will not build up much muscle. Effort against high resistance, in the weights room or equivalent, is the way to achieve that; but such ‘resistance exercises’ are best not embarked upon by people who have not already achieved a fairly good aerobic fitness base.

Exercise in different cultures

Finally, it may be salutary to recall how rare, and for the most part recent, in human societies is the disposition to take exercise when it could have been avoided. Exercise has been toil, for the great majority of mankind, at least until an industrial revolution was well advanced in the society concerned. Wealth and status thus meant indolence and often corpulence, whether in medieval Europe or over a similar period in China. Yet in such civilizations as that of Sparta and Rome, and in sectors of Japanese society over many centuries, exercise was cultivated in the expectation of war. Perhaps it is ancient Athens that, in its attitudes to exercise as in so many other ways, most closely anticipated our own outlook: exercise for sport, for health, and to maintain/improve the body image were all recognized by the contemporaries of Plato, as they are once more by us. It is to be hoped, however, that our physiological understanding is at least a little better.

Neil Spurway

Bibliography

Bursztyn, P. (1990). Physiology for sportspeople: a serious user's guide to the body. Manchester University Press.
Noakes, T. (1991). Lore of running. Human Kinetics, Champaign, Illinois.
Wilmore, J. H. and and Costill, D. L. (2000). Physiology of sport and exercise. 2nd ed. Human Kinetics, Champaign, Illinois.

See also breathing during exercise; fatigue; fitness; sport.