Mechanical Changes Shape and volume When a muscle contracts it becomes shorter and thicker, but its volume remains unchanged. This statement holds true, however, only in an excised muscle or in the intact animal when only the muscle substance proper is under consideration. It is definitely that during muscular activity a greater volume of blood flows through the blood vessels of the contracting muscles. Because of this, a muscle as a whole will increase in size. That the muscle substance itself does not change in volume when it contracts may be demonstrated in the following way. A muscle or an entire limb (frog) to which is attached a pair of stimulating electrodes is placed in a wide-mouth bottle which has been filled with physiological saline (0.6 per cent sodium chloride solution). A tight fitting rubber stopper, through which has been inserted a capillary glass tube, is now adjusted so that the fluid stands at some level in the narrow tube. When the muscle is stimulated by means of an electric shock, it responds by contracting, but the level of the fluid in the capillary tube remains unchanged. The capillary tube gives to the apparatus a very high sensitivity so that even if small changes of volume occur in contracting muscles, they should alter the level of this fluid. Muscle contraction and its myogram The mechanical changes which take place in contracting muscle occur so rapidly that it is quite impossible to follow the various phases of the response with the naked eye. In order to analyze the time, course, and extent of the response, some form of mechanical device (myograph) must be employed. If the gastrocnemius (calf) muscle of a frog, together with its motor-nerve supply (sciatic nerve), is dissected away carefully and applied to the myograph where it can be stimulated artificially, a record of the contraction can be obtained on the smoked surface of a rapidly revolving drum (kymograph). The muscle must be so attached that its free or movable end is connected, by means of a thread, to the writing lever or stylus, the latter being applied to the surface of the smoked drum. When such a muscle is excited by the application of a single induction shock (electrical) to its motor nerve, it responds with a single twitch. The record formed on the rapidly moving smoked surface is known as a myogram. It will be observed that the mechanical response of the muscle does not begin immediately upon the application of the stimulus (point of stimulation SP), but only after a short interval--the latent period. The contraction curve proper will show an ascending limb representing the period of contraction and a descending limb representing the period of muscular relaxation. The shape of the curve will depend on the speed of revolution of the drum. If the latter is revolving rapidly, the curve will be elongated and drawn out so that the grades of ascent and descent will be slow and gradual while the maximal vertical height will be unaffected. Under these conditions, all phases of the mechanical response are magnified and readily analyzed. An ordinary tuning fork of one hundred vibrations per second will serve as a suitable timing device. By attaching a writing point to one arm of such a tuning fork, a satisfactory time tracing can be obtained by simply setting the fork into vibration and bringing the writing point into contact with the revolving smoked surface just below the abscissa of the myogram. In this manner it can be shown that in the simple twitch of a frog's muscle, the latent period has a duration of approximately 1/100 second, the contraction phase one of approximately 4/100 second, while the relaxation phase requires about 5/100 second. Although the response is somewhat different in man, the relative length of the various phases is probably the same. It can be seen that the contraction and relaxation phases are not evenly balanced in time. This may help to explain why sprinters "pull" muscles. The muscles of the body are arranged in pairs. When one muscle or group of muscles contracts, the antagonists must relax and vice versa. The contracting muscle requires 4/100 second for its reaction while the relaxing muscle requires 5/100 second for its coordinating response. Because of this discrepancy in time, the contracting muscle may get the "jump" on its antagonist and may "pull," strain, or even rupture it. Proper "warming up" of the athlete tends to diminish this discrepancy between the time of the contraction and relaxation phases and thus aids in preventing injury to the muscles by "pulling." When a muscle responds by contraction and shortening it is termed an isotonic twitch; on the other hand, when it responds by developing tension without shortening, as against a load or resistance which it cannot lift or move, it is known as an isometric twitch.		Home Classes of levers Varieties of muscle Smooth muscle The blood supply to muscles The neurone Cardiac muscle On the nomenclature of skeletal muscles The nerve supply to muscles Structure of a skeletal muscle Central nervous system The reflex arc and reflex action Physiologic properties of muscle Mechanical Changes The effects of temperature on muscular contraction Muscle glycogen, its source and fate The reaction of muscles Oxygen consumption of muscle Determination of the oxygen consumption of muscle The viscosity factor in muscle and muscle efficiency Significance of the recovery heat Production of Electricity