'Thus we have reached the other fundamental factor in fatigue -- the consumption of the energy-yielding substance itself.Not only does tissue manufacture poison for itself in the very act of living, casting off chemical wastes into the circling bloodstream; not only are these wastes poured into the blood faster with increased exertion, clogging the muscle more and more with its own noxious products; but, finally, there is a depletion of the very material from which energy is obtained.The catabolic process is in excess of the anabolic.In exhaustion, the organism is forced literally to "use itself up."'5§3.So much for the physiological meaning of muscular fatigue.
Closely associated with muscular fatigue is nervous fatigue.For every voluntary muscular action receives its stimulus from a nervous centre.
Though the nature of this nervous energy, accumulated in the central nervous system and distributed in stimuli, is not well understood, its economy is gravely disturbed by conduct involving heavy muscular fatigue, as well as by work of a mental kind involving heavy drains on its resources.Aprocess of building up, storage, and dissipation of nerve tissue and energy-yielding material, corresponding to that which we have traced for muscle tissue, must be accepted as taking place.Fatigue of the nervous system will thus be attended by a similar accumulation of poisonous waste products, and an excessive consumption of substances needed for the maintenance of nervous activity.
Though physiologists are not agreed as to how and when fatigue acts on the nervous cells, there is no question of the reality and of the importance of this injury of excessive work to 'the administrative instrument of the individual' which 'directs' controls and harmonises the work of the parts of the organic machine and gives unity to the whole.'
Still confining our attention to purely physical conditions, we learn that work done in a state of muscular fatigue involves an increase of nervous effort.
'Mosso showed that a much stronger electric stimulus is required to make a wearied muscle contract than one which is rested.He devised an apparatus, the ponometer, which records the curve of nervous effort required to accomplish muscular action as fatigue increases.He showed that the nerve centres are compelled to supply an ever stronger stimulus to fatigued muscles.'6Professor Treves at Turin throws further light upon the relations between the muscular and the nervous economy.It is well known that in muscular activity there is an opening period during which efficiency, or practical response to nervous stimulus, increases.Before fatigue begins to set in, the muscle appears to gain strength, its working power being actually augmented.
This period of maximum efficiency continues for an appreciable time, then fatigue advances more and more until muscular contraction refuses any longer to respond to even a heightened nervous stimulus.This, of course, is also an epitome of the course of organic life itself, its rise towards maturity, its level of maximum power and its decline.
Now training or practice can notoriously affect this natural economy.
The muscular system, or some part of it, can by practice accommodate itself to increasing quantities of fatigue-poisons, and can draw from the general organic fund a larger quantity of material for repair of local muscular tissue and energy.But it has long been recognised that some real dangers attach to this excessive specialisation of muscular activities.The pathological nature of over-training in athletics has its plain counterpart in industry.
This, according to Professor Treves, lies in the failure of the supply of nervous energy to rise in proportion to the requirements for this higher pressure upon the muscular tissues.
'According to my experience, it has not been found that training has as favourable an effect upon [nervous] energy as upon muscular strength....
This fact explains why muscular training cannot go beyond certain limits and why athletes are often broken down by the consequences of over-exertion.
And this fact teaches also the practical necessity of preventing women, children, and even adult men from becoming subjected to labour, which, indeed, a gradual muscular training may make possible, but at the price of an excessive loss of nervous energy which is not betrayed by any obvious or immediate symptoms, either objective or subjective.'7A series of experiments has been directed to the more detailed study of the relations between activity and repose.Their general result is to prove that muscular work, done after fatigue has set in, not only costs more nervous effort but accomplishes less work.The ergograph, an instrument for measuring work, yields ample testimony to the recuperative effect of rest taken before exhaustion is reached, on the one hand, and the rapid rate of decline in achievement when activity is continued after the fatigue point has been reached.
§4.To this account of the physical costs of excessive work in muscular and nervous waste must be added the greater liability to accidents and the greater susceptibility to industrial and non-industrial diseases which fatigue entails.
The statistics of industry in various countries prove that fatigue is a very important factor in industrial accidents.Though fatigue is not always proportionate to duration of work, the number of hours worked without intermission is usually a valid index of fatigue.After a long stunt of work the attention of the worker and his muscular control are both weakened.
