CHAPTER I.

LOCAL FATIGUE.

Experimental Fatigue-Absolute and Relative Fatigue-Fatigue in Ordinary Conditions of Work; it is always Relative-Examples of Relative and of Absolute Fatigue-Causes of Sensation of Fatigue-Causes of Muscular PowerlessnessInfluence of Waste-Products of Combustion; Transmission of Fatigue to Muscles which have not been Working-Use of Fatigue-Part played by the Brain in Fatigue. Unconscious Movements cause less Fatigue than Voluntary Movements; Practical consequences.

I.

IF we isolate one of the muscles of a living animal and pass an electric current through it, we see that it contracts as long as the passage of the current lasts. But if the experiment is prolonged, the muscle after a time contracts more feebly; a little later there comes a stage when the muscle does not contract at all: it is fatigued.

Fatigue is at first only relative, and the muscle will contract afresh if stimulated by a current stronger than the first. But there comes a time when fatigue is absolute, that is to say the muscle has lost the property of contracting under the influence of the most powerful electric stimuli.

A human muscle never reaches, in consequence of work, the condition of absolute fatigue, of complete powerlessness, which we observe in animals under experiment. What prevents this is the painful sensation experienced by the man before the time when the muscle becomes absolutely incapable of action. Under the influence of the suffering which the contraction causes, the work is

stopped and the muscle rests. Here is the capital difference between true absolute fatigue, such as can be produced by experiment on animals, and the fatigue observed clinically in a man at work.

That which dominates in the fatigue of a man performing any exercise is the subjective element, the painful sensation which prevents him from continuing his work until the muscle becomes absolutely exhausted. We can represent the effort made by a powerful man to carry his exercise to the last possible limits, as a combat between the will which commands and the sensibility which rebels.

The most energetic will is unable to use up the contractile power of a muscle as completely as do mechanical or physical agents. When a fatigued man ceases the effort which he has long been making, we say that his muscles are exhausted: this is not yet so. The proof of this is as follows:

We know that one of the most tiring attitudes to assume is that which consists in holding the arm horizontally outstretched. The deltoid muscle in this case does most of the work. There are few men vigorous enough to be able to hold out an arm in this manner for more than five or six minutes. At the end of this time the deltoid can act no longer and the arm drops. But the muscle is not exhausted: its fibres still possess a great contractile force, and this is proved by the fact that certain agents, such as electricity, can bring into play this motor force over which the will has no longer any action. If, in a man who has been holding his arm outstretched, we wait till the sensation of fatigue becomes intolerable, and if, at the moment when the man declares he has used up all his power, and is about to let his arm fall, we apply a strong electric stimulus to the deltoid muscle, the fatigue seems to vanish, and the arm remains outstretched; the muscle had not yet lost its contractile power.

What is the cause of this local fatigue? A double answer is needed to this question; we must say why the work renders muscular contraction painful in a tired.

limb, and why also the muscle which has been long at work finally loses for a time the power of contraction.

Often repeated muscular contraction becomes painful mechanically, owing to the repeated shocks and disturbances occasioned in the muscle itself and in the neighbouring tissues. Every mechanical action submitting the muscular masses of the body to compressions, movements and shocks similar to those produced by work can bring about, just like work, a sensation of fatigue. What we call "massage is a series of manipulations to which muscles are submitted. After the action exercised on the muscles by the hand of the masseur, sensations of local fatigue are experienced, similar to those produced by muscular work. We must then conclude that the sensation of pain in a region which has worked, is due to the same cause as that in a region which has undergone massage, that is to a mechanical action.

And this action is easily explained. The muscle is traversed by a number of sensory nerve-filaments. These little twigs are rubbed and twisted by the movement of the muscular fibres which swell and harden during the energetic contraction of work. The muscular fibres are themselves pulled about, and the tendons, the aponeuroses of insertion, and the synovial membranes undergo repeated friction. There results, then, from very violent muscular work a genuine traumatism in the whole region which is the seat of the work, and the consequences of this traumatism may be the same as those due to external causes, as for instance, contusions. Frequently, as we shall see in speaking of the accidents of work, ruptures, inflammations, and even abscesses may result from excessive work.

But apart from these causes of discomfort, the muscle at work undergoes others less known and more interesting. In the muscular fibre, modifications of nutrition take place, due to the combustions accompanying contraction. Every muscle in contracting becomes heated, and this increase of temperature is due to the chemical combinations of which we have spoken in the

chapter on Combustions. The chemical actions which we call combustions, profoundly alter the structure of the tissues at the expense of which they occur, and from this change result new products which stay in the muscle for a certain time.

Now these products exercise on muscle a peculiar action which paralyses it and renders its contraction impossible. If we submit the muscles of a frog to the action of a powerful electric stimulus, and prolong this action until fatigue is complete, that is, till the limbs of the animal remain motionless under the most powerful stimulation, we shall have in the fatigued muscles the elements necessary for a most curious experiment. Their substance rubbed in a mortar, and made into a fine soup, contains a principle capable of producing in healthy muscle at rest the fatigue which had exhausted the first muscles. If we inject into a second frog this extract of fatigued muscles, we bring about in this animal all the phenomena of fatigue, and its limbs will fail to respond to electric stimuli.

Thus there are developed in the muscles, in the very act of work, certain products of dissimilation which have the power of doing away with the contractile force of the muscle-fibres. If these products are not formed in excessive quantity, they are rapidly carried off by the blood-current, and, if not renewed, the nutritive disturbances produced by work are promptly repaired. If, however, the work is continued too long, these products accumulate in the muscle in excessive quantity. They can then, for a time, abolish its contractility, and may further produce serious general consequences, of which we shall speak in the chapter on Overwork.

We must then conclude that the pain felt in a muscle which has been contracting for a long time, results from a number of small lesions, stretchings and rubbings, of the sensitive parts of the region which has been working, and that the absolute powerlessness which we notice is due to a nutritive disturbance, to the formation within the muscular tissue of products of dissimilation, the