122 THE HUMAN BODY 



raised the conditions become more and more favorable to the 

 muscle. Those who have practiced the gymnastic feat of raising 

 one's self by bending the elbows when hanging by the hands 

 from a horizontal bar know practically that if the elbow-joints 

 are quite straight it is very hard to start; and that, on the 

 other hand, if they are kept a little flexed at the beginning 

 the effort needed is much less; the reason being of course the 

 more advantageous direction of traction by the biceps in the 

 latter case. 



Experiment proves that the power with which a muscle can 

 contract is greatest at the commencement of its shortening, the 

 very time at which, we have just seen, it works at most mechanical 

 disadvantage; in proportion as its force becomes less the conditions 

 become more favorable to it. There is, however, it is clear, nearly 

 always a considerable loss of power in the working of the skeletal 

 muscles, strength being sacrificed for variety, ease, rapidity, ex- 

 tent, and elegance of movement. 



The Equilibrium of Opposing Muscles. The muscles are highly 

 elastic bodies, and on account of their elasticity exert, even while 

 not actively contracting, a definite elastic tension upon their 

 points of insertion. We have, therefore, at every joint, constant 

 opposing pulls of the resting muscles. The efficient manner in 

 which the elastic tensions* of opposing muscles balance each other 

 is often very striking, particularly when one considers the marked 

 differences in size and in mechanical advantage apt to exist in 

 opposing muscles. Take, for example, the muscles which move 

 the fore arm. The flexor muscle (biceps) is a larger and stronger 

 muscle than the extensor (triceps). It has, moreover, a better 

 leverage. Yet so far as elastic tension is concerned the two 

 muscles balance perfectly. One appreciates best the significance 

 of this equilibrium when a case is seen in which it has been lost. 

 The disease known as infantile paralysis injures or destroys the 

 nervous connections of the skeletal muscles, and often affects one 

 group while leaving the opposing group unimpaired. The normal 

 tension is lost in the affected muscles but persists in the unaffected 

 ones. Unless special precautions are taken to prevent the occur- 

 rence, the muscles which still exert elastic tension will gradually 

 shorten, pulling the joint into the position into which it is drawn 

 by the normal contractions of these muscles, and holding it there 



