THE MUSCULAR SYSTEM. 301 



a number of interesting details as to the properties of contracting 

 muscles. 



The muscle does not begin to contract at the exact moment that 

 the stimulus is applied. A very short latent period (.01 sec.) 

 elapses between the stimulus and the beginning of the contrac- 

 tion. During this time the muscle is undergoing some internal 

 change which must precede the contraction. The period of active 

 contraction is relatively short (.04 sec.) and the period of relax- 

 ation somewhat longer (.05 sec.). The ordinary movements of 

 the body cannot obviously be of the nature of a single muscular 

 contraction, for they much exceed one-tenth of a second in dura- 

 tion. They are in fact produced by a prolonged contraction of 

 muscles caused by the fusions of several single contractions. This 

 is known as tetanic contraction, and it can easily be produced 

 in the muscle preparation described above by giving it a series 

 of electrical stimuli from an induction coil. If the stimuli 

 be properly timed, a contraction curve somewhat higher and 

 showing no relaxation phase will be produced. When the ex- 

 citation is discontinued, the muscle returns to its normal length. 



The amount of load which the muscle lifts has a peculiar effect. 

 Up to a certain point an increase in the load increases the effi- 

 ciency of the muscle and the muscle will actually perform more 

 work with a moderate load than with no load at all. After a 

 certain load is reached, the efficiency of the muscle begins to 

 diminish and further increase of the load decreases the work 

 accomplished by the muscle. The principle involved here is made 

 use of by fork and shovel manufacturers, who are careful to 

 make their implements carry the load best suited to develop the 

 maximal efficiency of the muscles of a normal average man. Al- 

 lowing the laborer to choose his own shovel is not always the 

 best for the laborer or for his employer. 



Another interesting fact is that a contracted muscle is more 

 clastic than a relaxed muscle. Equal weights attached to a con- 

 tracted and to a relaxed muscle will produce a greater elonga- 

 tion in the contracted than in the relaxed muscle. It is this prop- 

 erty which protects the muscle from sudden rupture when at- 

 tempts are made to lift loads that are too heavy. 



