Our motor apparatus is constructed, owing to the short leverage of the muscles (especially in the extremities, but also on the trunk), more for quick than for strong movements. If, as an example, we still consider the elbow joint, it is obvious that biceps brachii and brachialis anticus, on account of their very short levers, need only contract very little in order to bring the arm from full extension to, e.g., flexion to a right angle, and that their insertions need only move a very little way in comparison with the distance which the hand has to move at the same time. The weight lever in the hand changes considerably like the physiological lever of the muscles on the bones, but in certain positions it is comparatively very great. When with the forearm horizontal I hold a weight in the hand with the elbow flexed to a right angle, and the products of the weight and length of the lever on either side are equal and therefore in complete equilibrium, it is obvious that in order to maintain the position I must exert considerable muscular force in comparison with the weight.

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The study of the effects of muscle action, especially in active movements, upon the muscles themselves, the bones, joints, and nerves connected with the motor apparatus (both motor and sensory), upon the circulation, respiration, metabolism, and other functions, disposes entirely of the not uncommon idea among people ignorant of physiology that the muscles are of little other use than as organs of movement.

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Julius Wolff has pointed out the importance of active movements for the development and architecture of the whole skeleton, and has demonstrated that the shape of the bones, which aims at fulfilling the mechanical requirements with the smallest possible bulk, partly depends upon the tension of the muscles on them.

Further details concerning the effect of muscle movements on the nutrient arteries of bone have not, to my knowledge, been investigated. But all points to the fact that muscle work, by the increase in blood pressure and rate of blood flow thereby produced in the muscles, does not diminish the flow in the nutrient arteries of the bone, but, on the contrary, produces an increased flow of blood through them, whereby the nutritive condition of the bones is improved. The fact that paralyses are followed by atrophic changes also in the bones supports this theory.

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Joints are exercised in their functions and maintained in good nutrition by both active and passive movements. Hygienically, active movements are of more importance in this respect, and keep the joints in better nutrition than other movements. But in the gymnastic treatment of diseases of joints passive movements are of most value. In diseases of joints which ought to be treated by medical gymnastics the object is usually to stretch contracted parts. For this purpose passive are more useful than active movements, since the latter are usually performed too feebly, partly on account of muscle atrophy; partly on account of changes in the joints, especially in the soft parts; partly also owing to the pain associated with the movements.

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Muscle action strongly affects the circulation in various ways.

The first phenomenon which arises is the dilatation of the small vessels of the working muscles, the arterioles and capillaries. Acceleration of the heart beat and rise of blood pressure also occur. These phenomena produce a blood stream three to five * times as great during moderate work, seven to ten times as great during hard work as in a muscle at rest.

With a stronger blood stream there follows always a stronger lymph stream.

An immediate result of the greater supply of nourishment in the stronger blood stream to the working muscle is the improved nutrition of the muscle. By repeated, systematic work, continued for a sufficiently long time, the muscle thus undergoes a "progressive change" in that its fibres increase in thickness. Thus by means of gymnastics an atrophied muscle may regain its normal thickness; a normal muscle may become thicker and reach that nutritive condition called "work-hypertrophy." Obviously all this can only be due to the increased blood stream, brought to the working muscle by the dilatation of its own blood vessels, not only bringing a sufficient quantity of fuel and oxygen for the increased combustion, but also leaving a surplus amount of nutritive material, especially of nitrogenous substances, which is stored up in the form of muscle protoplasm.

* E.g., the blood in the levator labii of a horse represented 17.5 per rent. during rest, 85 per cent. during work, of the whole weight of the muscle.

However, in extreme, continually repeated over-strain it may happen that the dilated vessels are not able to bring to the muscle sufficient nutrition to cover the expenditure, and it thus happens in rare cases, that long, repeated, excessive, muscle work produces atrophy in the over-strained muscles (see below).

But with continual and superfluous rest, i.e., insufficient work, the nutritive condition of the muscle always sinks, and we all recognise as the opposite of work-hypertrophy the atrophy of disuse found in paralysis, joint diseases, and other conditions where there is insufficient muscular activity.

The repleting and depleting effects of gymnastics are based upon the increased supply of blood to the working muscles. Both these effects are obviously important. When in a closed vascular system containing a certain quantity of blood (about 4 litres in an adult) a certain part receives a greater supply than before, another part or parts must receive a less supply than before; in the latter area either the path of the stream must become narrower, or the stream itself slower, or both these changes take place. We all know what effect the mesenteric vessels, with their powerful, continually changing stream, exercise upon the distribution of blood, as upon blood pressure. We also know that there is no better cure for hyperaemia of the head than to take a brisk walk, which by the enormous dilatation of the stream in the lower extremities depletes the upper parts of the body.