100 THE HUMAN BODY 



gether. It must be borne in mind, too, that many apparently 

 long muscles are really short stout ones those namely in which 

 a tendon runs down the side or middle of the muscle, and has 

 the fibers inserted obliquely into it. The muscle (gastrocnemius) 

 in the calf of the leg, for instance (Fig. 40, B), is really a short stout 

 muscle, for its working length depends on the length of its fasciculi 

 and these are short and oblique, while its true cross-section is that 

 at right angles to the fasciculi and is considerable. The force 

 with which a muscle can shorten is very great. A frog's muscle 

 of 1 square centimeter (0.39 inch) in section can just lift 2,800 

 grams (98.5 ounces), and a human muscle of the same area more 

 than twice as much. 



The Beneficial Effect of Exercise. An interesting fact about 

 skeletal muscle, that is in the experience of every athlete, and can 

 also be demonstrated upon an isolated muscle, is that the response 

 to stimulation after a period of inaction is less vigorous than the 

 response to precisely the same amount of stimulation after the 

 muscle has been exercised for a while. This fact explains the ne- 

 cessity under which base ball pitchers and other athletes labor of 

 " warming up" before they can use their muscles effectively. In 

 the case of an isolated muscle stimulated at regular intervals the 

 effect is seen in a well-marked increase in the height of contraction 

 during the first dozen or so of the series. This increase from con- 

 traction to contraction is often very regular, suggesting a flight of 

 stairs. For this reason it is often spoken of as the "staircase phe- 

 nomenon." 



Fatigue of Muscle Contracture. If an isolated muscle is sub- 

 jected to a series of stimulations at fairly frequent intervals one 

 a second or oftener the period of the " staircase" described above, 

 is usually followed by a period in which the relaxation of the muscle 

 is definitely slowed. So far as can be seen the muscle contracts 

 as rapidly and forcibly as ever, but the relaxations are drawn out. 

 The effect of this in a regular series of stimulations is that the 

 muscle fails to relax completely from one stimulation by the time 

 the next one is sent in, so that it continues in a state of partial 

 contraction during the intervals between stimulations. This 

 condition is called contradure. It consists essentially of a slowing 

 down of the relaxation rate, and is the first indication of the con- 

 cjit on of impairment that we call fatigue. Aside from its signifi- 



