In the preceding chapter we have seen that the power of a muscle for work, other things being equal, is in direct proportion to its thickness, but also that other things never are equal, and that in all power of doing work, especially for a long period, there are many other factors than the muscles. The conditions, especially of the circulatory and respiratory apparatus and of the nervous system, play important parts. Incidental conditions have also a great effect. Purely intellectual or psychic strain, depression, sleeplessness, fasting, losses of blood or anaemia from other causes lower the power of work. Work performed by separate muscle groups has to some extent a fatiguing, i.e., a force-diminishing, effect on the other muscle groups (Mosso, Maggiora, Adduco, Treves, Horwath, Brandis, and others). Lastly, practice has a very great influence, although it is possible and perhaps probable that the difference between equally thick practised and unpractised muscles is not in their force, but merely in the external expression of their work (see below). Comparative test of strength must therefore be minute, must be repeated several times, and must be concerned with both strength for the moment and power of endurance; of which more below.

The absolute strength of an individual is represented by E. F. Weber as the lowest weight that the individual quite fails to lift.

There are different instruments and methods for other tests of strength, most of which are concerned with the upper extremities, and are performed with apparatus constructed after the principle of Mosso's ergograph, designed for the fingers. The test is performed in such a way that the person tested lifts certain weights in a certain time, while a graphic representation is obtained in the usual way showing the height, time, and number of the liftings. Generally a higher work value is obtained in a certain time by lifting smaller weights quickly than by lifting larger weights more slowly (Stupin); on the other hand, to obtain the maximum work value one must make use of a certain weight value. By diminishing the weight lifted at a certain time-rate till the individual at that rate can go on lifting the weight for an indefinite time without diminishing the height of the contraction, one obtains, for that individual, the "final maximum weight" (Treves) with which he can work, as we all work with the heart and respiratory muscles.

If the weight or the rate of lifting is increased, by degrees fatigue (see pp. 44, 85, 86, 93, 114, 119) arises and shows itself by diminished functional power ( = weaker contractions) in the muscles, and by the feeling of fatigue, probably produced through their sensory nerves.

I will remind the reader that extreme over-strain can entirely destroy for the moment the power of the muscle to contract under the influence of the will, and that fatigue lasts much longer if it is driven to extreme (Maggiora); that this is probably accounted for by the collection of "fatigue products," and by a lack of supply of those substances which are consumed during muscle contraction. A dissected muscle which has been fatigued by electrical stimulation will recover to a certain extent after a short rest, and the recovery is less complete the less oxygen there is in the atmosphere surrounding it.

Further, we know definitely that the ganglion cells are the first parts of the neurons to tire, that after them probably the muscle end-plates are overcome by fatigue, but that the nerve fibres may receive millions of stimuli without any functional disturbances arising in them (Mosso, Bernstein, Wedenskij, Langendorf, Bow-ditch).

By practice one can increase one's strength and the external effect of work, as is easily understood. By exercise the muscles undergo a work-hypertrophy, and a thick muscle, other things being equal, is stronger than a thin. For this reason a practised worker is specially competent in his special work, since the muscles which are exercised in this work are those which hypertrophy. But this does not explain all the facts in regard to the effect of practice. It takes a long time to develop a work-hypertrophy, while by exercise one can increase one's power of work in a comparatively short time; an unpractised individual in a few weeks can by practice double his daily work. For this reason the difference in power of work between the unpractised and the practised worker is very much greater if one compares moderate work done for a long time rather than maximum or great efforts in a short time. While an unpractised but stronger person can produce a greater work-effect in a quarter of an hour than a practised but weaker person, the latter has a better prospect of excelling in a longer period of work, therefore especially in the question of a day's work. The difference is due to some extent to the power of the practised worker to perform with his arms and implements the movements most effective for his special work; this difference affects the simplest work. But other factors are also concerned, and there is a little series of phenomena which are worth considering, and which perhaps are all explained in the same way. Leo Zuntz states that metabolism during work (cycling) is greater during fatigue, other things being equal. Johansson and Koraen also observed that the production of carbonic acid is greater towards the end than in the beginning of long-continued muscular contractions. Schnyder found that a sick person or convalescent consumes more than a healthy person for the same work. Possibly the condition observed by Zuntz and Schumberg that work is more expensively performed (i.e., with greater metabolism) in excessive heat than at a cooler temperature has the same explanation.* It may be supposed that all these facts depend upon the varying inhibition of innervation of the antagonists of the working muscles discussed by Kohnstamm, and that one who is tired, ill, or working in great heat loses something of his power to completely inhibit the innervation of the antagonists.