The effect of a rise of temperature is to quicken the twitch and the rising phase of a tetanus. One effect of this quicker response is to make the maintenance of a contraction more expensive.
Much use has been made of the fact that muscles removed from the body may, under proper conditions, contract practically normally for many hours. The thinner the excised muscle the more satisfactory is its survival, since the usual provision of oxygen via the circulation must be replaced by diffusion from outside, which is a slow process over greater distances. It is com mon to immerse such muscles in "Ringer's fluid," a watery solu tion of salts in approximately the same concentrations as the blood.
The intrinsic strength of muscles is considerable. A frog's sar torius may exert a force of 2.5kilog. per sq.cm. of section, but this is small compared with the force of human muscles. In the human arm, for example, at a mechanical disadvantage of about 6 : i , a force of 4o kilograms may be exerted at the hand by the biceps and brachialis muscles, with a cross-section of about 25sq.cms., which is about iokilog. per square centimeter or , approximately i o atmospheres.
It was shown by Keith Lucas that the twitch of a muscle fibre is independent of the strength of the impulse which evokes it, though dependent upon the condi tion of the fibre in its external and internal relations (temperature, length, preceding twitch, condition of fatigue, etc.). Strictly de fined, this principle is universal and has the consequence that a muscular effort can be graduated in intensity only by adjusting the number of fibres employed in it, and not by adjusting the response of the individual fibres.
a muscle fibre is excited a wave of negative potential, a few hundredths of a volt in magni tude and a few cms. in length, starts off in both directions from the stimulated point. The wave can be recorded, either in an ex cised muscle, or during voluntary movements in the body. This "electro-myogram" has been used very largely in investigations of the heart ("electrocardiogram") and considerably in studies of natural muscular movement. By means of it all voluntary and most reflex contractions have been shown to be of a discontinuous tetanic nature, due to a rapid succession of impulses reaching the muscle from the nerve; while a few reflex contractions, chiefly "tendon-jerks," have been found to be single twitches.
In the maintenance of body posture, and in preserving a readiness for instant action, a certain amount of muscle "tone" is necessary. When a muscle contracts, the tone of its antagonist is inhibited. It has been supposed that tone repre sents a different type of contraction, not of a tetanic nature, and it is reported that muscle fibres are doubly innervated, by voluntary and by sympathetic nerves; section of the sympathetic has been stated to cause a loss of tone. The evidence is not yet con clusive and there seems to be at present insufficient reason to regard tonic contraction as different, in principle, from ordinary tetanus ; it is merely weaker, only a small fraction of the fibres being active at any given moment. Perhaps the fibres which are normally concerned with maintenance of tone are naturally slower than their fellows, and so more economical in maintaining a ten sion. Differences in speed in the different fibres of an individual muscle group are known to occur.
When a muscle contracts it liber ates heat. This can be measured directly by placing an isolated muscle on a thermopile connected to a sensitive galvanometer, when a rise of about 0.003° C. is found in a single twitch : or it can be studied by placing a complete animal in a calorimeter and causing it to work. "Myothermic" experiments have thrown much light on the internal mechanism of the muscle.
When a muscle deprived of oxygen is given a suc cession of shocks it gradually weakens and finally ceases to respond. When a muscle with its circulation intact, or adequately supplied with oxygen, is similarly treated, its response diminishes at first, but finally reaches a "steady state," in which recovery balances breakdown. The first case corresponds to very violent exercise in man, where energy requirement is far in excess of possible oxy gen supply; the second corresponds to steady work of moderate intensity where oxygen supply rapidly rises to meet energy re quirement.