Probably the first question asked by those starting on a systematic course of exercise for developing and strengthening the entire muscular organism is, “why do we develop?” A writer on the physiology of bodily exercise says, Dumbbell exercises, in spite of the great quantity of work they need, have little influence on the brain. They affect the functions of nutrition much more than those of innervation. The energetic and sustained muscular contractions which they render necessary draw blood to the muscles in great quantity and keep it there a long time.
The muscular fibers benefit from this and increase in size. On the other hand, the blood is enriched with a great quantity of oxygen, for increased respiratory need is the first effect of great expenditure of muscular force. This need finds free and easy satisfaction in the period of repose which inevitably follows each exercise. Finally the intensity of the combustions due to a great quantity of work, promotes the using up and prompt disappearance of the reserve materials, and the need of quick repair: whence increased appetite.
On the other hand, the repeated contractions of the abdominal muscles in frequently recurring efforts, performs a sort of massage on the intestines, which favors the onward movement of the feces and makes the bowels regular. Such exercises are then favorable to all nutritive functions. They increase energetically, and even violently the working of all the organs of the body, while leaving in relative repose the nerve-centers and physical faculties.
They tend to increase the weight of the subject. Observations of facts show that these exercises, when they are not beyond the strength of the subject, place him in the most favorable conditions of nutrition.
Exercise of this kind deserves the preference from the hygienic point of view, and it is in fact in the professions in which work is taken in large doses that we find the most vigorous persons.
Finally, it is necessary to avoid overwork, that the work should be gradually increased, and not done in the largest quantities till after complete training.
source: The Science and Art of Physical Development by W. R. Pope, 1902
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