of a series of partial efforts which are evolved at once in the arm and shoulder, the vertebral column and the thighs; and the stroke of the racket in itself does not represent the whole expenditure of force occasioned by the game. To it should be joined the motions preceding it and preparatory to it, or the player's changes of attitude. All those who have held the racket know how rapidly these motions have to be made. In less than a tenth of a second one must look ahead and up to catch the ball on the fly, or stoop to take it on the bound, or bend to one side to hit it a back stroke. In these rapid changes of attitude the center of gravity of the body is abruptly displaced, and equilibrium can not be preserved without bringing a large number of muscles into energetic play. The muscles of the thorax, the loins, and the pelvis contract and bring the bony parts forming the framework of the body into close action upon one another. The lower limbs, without leaving the ground, also furnish a considerable interior labor, the purpose of which is to assure the player a solid footing, a stability essential to the force of the racket-stroke; and even the feet seem to fasten themselves to the ground, with the assistance of the toes.
Thus, in the game of tennis, the exercise is distributed among a large number of muscles, and this fact enables us to explain how the effects of work may be very much accentuated without our being conscious of having made great efforts. In giving racket-strokes we make infinitely less efforts than in raising heavy chest-weights, yet we do not perform less work in a game of tennis than in a practice of gymnastic athletics.
In all natural movements we use a large number of muscles at once, and we sometimes bring into action those which are very remote from the point where the work appears to be localized. Active games constantly tend to the division of the work among a large number of muscles. This is the consequence of their very character of natural exercises. Being copied from instinctive acts of which they are simply the methodical regulation, they all present the same character of causing the human machine to execute much work without demanding much effort from it. The operation of the motions adopted by gymnastics proper is different. That does not tend, in general, to seek out the associations of muscles, called in physiology synergies, but rather to avoid them, with the view of augmenting the effort of the muscles that are brought into play by suppressing the co-operation of the other muscles.
The property of games is, then, to cause the production in the human body of much work without great effort. Now, the hygienic quality of exercise is not effort, but rather work. The more work we do, the more we stimulate the great vital functions,