(a) The blood supply and nutrition of the working muscles are increased because the nutrient arteries as well as the vessels within the muscles dilate. At the same time metabolism and production of heat are increased. By continued exercise the muscles are developed and become stronger. If the actual length of a certain muscle is to be diminished, it is made to work in as strong shortening as possible, i.e., in the inner part of its range of movement.

(b) Because of the increased supply of blood to the working muscles the amount of blood in the other parts of the body is diminished. In this way we can by certain movements influence the distribution of blood in the body, and so, according to need, increase or diminish the quantity of blood in the different parts. By strong leg movements we can thus deplete the head.

(c) The mobility of the joints is increased, especially if the movements are taken to the extreme limit.

(d) The heart's contraction becomes stronger, partly by reflex action and partly through the influence of the products of metabolism.

(e) Circulation in the veins is assisted in various ways : -

(1) Because the contracting muscles, becoming thicker, press upon the veins in their vicinity and press their contents onward towards the heart (valves hinder the return towards the capillaries).

(2) Because the movements cause an alternate lengthening and shortening of the veins passing over the joints moved, a pumping of the blood takes place towards the heart, since the capacity of a blood vessel is alternately increased and diminished with moderate lengthening and shortening.

(3) By the presence of so-called "venous pumps," especially at the larger joints. These are due to the fact that the walls of the veins are partially adherent to the fascia or tendon sheaths, which, partly on account of the actual movement in the joint and partly through the contraction of their attached muscles, are alternately stretched and lifted from the tissues beneath and pressed down upon them. This, of course, causes an alternate increase and diminution of the capacity of the veins and consequent pumping. The best example of a venous pump is to be found at the knee-joint, where soleus arises from a tendinous band passing between the tibia and fibula, to which the posterior wall of the posterior tibial vein is adherent.

(f) The respiratory organs are also affected by active movements, the increased need of oxygen forcing them to work more strongly, so that the mobility of the ribs is increased and the respiratory muscles are developed and strengthened. By means of improved respiration both the pulmonary and systemic circulations are benefited.

In the systemic circulation the flow of blood to the large veins in the thorax is increased because negative pressure is increased by deeper inspirations. The pulmonary circulation is helped by the alternately increased and diminished expansion of the lungs and the consequent lengthening and shortening of the blood vessels, causing pumping of the blood.

(g) Because every active movement requires for its performance complicated exercise of the brain and nervous system, these organs, and especially those parts which are brought into action, receive increased blood supply and nutrition, and are also exercised and developed.

(h) Increased metabolism causes eventually increased need of nourishment, and, in connection with this, increased work for the digestive organs, and increased work for, and greater activity of, the organs of excretion. All this occurs partly by reflex action, partly by direct stimulation of the various organs by the resulting products of combustion.