THE FRAMEWORK OF THE BODY 



45 



by accident or by disease, as in poliomyelitis, the muscle shrinks and may 

 ultimately disappear. Exercise brings increased muscular efficiency not 

 merely by strengthening the muscles; the result is due in even greater 

 degree to improvements in circulatory and respiratory adjustments and 

 to better nervous coordination. 



tendons of 

 origin 



Fig. 3.10. Antagonistic muscles of the upper arm. 



It is important to note that muscles in action can only pull and never 

 push. The biceps can perform but one useful action — that of pulling its 

 origin and insertion closer together. It cannot push them apart again. 

 Except in rare instances, therefore, skeletal muscles are always arranged 

 in antagonistic pairs or sets. The action of each muscle is opposed by the 

 action of one or more other muscles. After one muscle has contracted 



