Muscle and Muscular Exercise

MUSCLE AND MUSCULAR EXERCISE. Muscle makes up a considerable, often a major fraction of the body, and many of the other functions are devoted to its service. It con sists essentially of fibres, complete living units which are fre quently of considerable length and about 0.05mm. in diameter.

These fibres contain numerous minute fibrils embedded in the semi-liquid "sarcoplasm" inside the "sarcolernma." They are bound together by a connective tissue framework, and in the case of most voluntary muscles form anatomical units which are con nected by tendons to the bony levers. Involuntary muscle, serv ing the "domestic" arrangements of the body (digestion, excretion, circulation, etc.), is usually in a sheet-like form, the fibres being mosaiced or cemented together to cover the organ operated. Car diac muscle, intermediate between the other two types, consists of shorter fibres, apparently in direct physiological connection with one another, thus ensuring a co-ordinated response.

Animal movement is due to the shortening and thickening of these fibres, not to changes in their volume. This shortening is initiated, in the case of voluntary muscle, by an impulse from a nerve, but may be produced artificially by various means, chief of which is the electric shock. In cardiac muscle there is an inherent tendency to beat, and any piece of a heart, removed from con tact with the rest, will proceed to shorten in its own intrinsic rhythm. In the normally functioning heart the speed is set by the quickest portion, viz., that near the entrance of the great veins. whence the beat is conducted rapidly from fibre to fibre so that the whole contracts approximately together. In involuntary muscle the fibres may show a rhythm of their own ; more usually, like those of voluntary muscle, they require an impulse from a nerve; sometimes, as in the intestine, they possess also a primitive local nervous system co-ordinating their activity.

Voluntary muscle is richly supplied with blood vessels, bringing it fuel and oxygen. Even at rest the muscles require materials for combustion; they are the chief source of animal heat. If the rest ing heat production be insufficient, the animal either takes volun tary exercise or shivers, so increasing the output of heat from its muscles. At rest the majority of the blood capillaries are closed ;

during activity these open up and allow more blood to pass; this is supplied by the greater activity of the heart, and provided with oxygen by the enhanced efforts of the respiratory system.

The Mechanical Response.

The fundamental unit of mus cular response is the twitch, contraction followed by relaxation. It is possible (though not certain) that in some muscles, e.g., those holding closed the shells of bivalves, relaxation may not necessarily ensue; a semi-permanent change in length may occur on stimulation, the change requiring an expenditure of energy, but the maintenance of the new length requiring none. In the vast majority of muscles, however, the unit of response involves relaxa tion as well as contraction, and a prolonged contraction can be maintained only by the fusion of a number of twitches in succes sion, so requiring a prolonged expenditure of energy. There are vast differences in the speed of the single twitch, which may occupy a few thousandths of a second in insects, and minutes, or even hours in some smooth muscles. A single twitch of given strength in a given length of muscle is associated always with a fairly con stant liberation of total energy. If T dynes be the tension devel oped in the "isometric" twitch of a muscle fibre /cm. long, and if H ergs be the total energy set free (initial plus recovery) we have approximately, for all muscles, fi = . This relation is in dependent of the speed of response, so that in setting up a ten sion all muscles work similarly efficiently. In maintaining a tension for a given time, however, there are vast differences between muscles of different speed : those giving the quickest twitch re quire the highest frequency of stimulation to produce a properly fused summation, and expend correspondingly more energy. A muscle capable of giving a twitch in 0.003 second will liberate 20,000 times as much energy per second in maintaining a contrac tion as one whose twitch lasts for one minute. When, therefore, a force has to be maintained over long intervals a slow-moving muscle is essential. Ordinary voluntary movements are due to a fusion of twitches evoked by a rapid succession of impulses along the nerves.