BODY LEVERAGE 55 



Examples of levers of the first class. Examples of 

 the first form of lever are found in the nodding of the 

 head, in the extension of the elbow by the triceps muscle 

 on the back of the arm, and in the raising of the body 

 upon the toes by means of the calf muscles. In the lat- 

 ter case, the pressure upon the floor by the weight of 

 the body is equivalent to the support, or pressure against 

 the foot, given by the floor. Since the floor support 

 is the external force counterbalanced by the pull of the 

 muscles, it is the weight. 1 



Examples of the third class. Of the third class of 

 levers, we find examples in the action of the biceps in 

 raising weights by the flexion of the elbow ; in the action 

 af the pectorals in drawing the arm inward ; and in the 

 action of the muscles which draw the arm downward, 

 as in hauling or climbing a rope. 



Body as a weight. A study of the problems in body 

 leverage shows that the force exerted by the muscles 

 is surprisingly large. It is further greatly increased 

 if the body gets heavier without a corresponding in- 

 crease in muscle power. For example, if a man weigh- 

 ing two hundred pounds climbs a stair, the quadriceps 

 muscle must exert, when his leg is bent, a pull amount- 

 ing to about a ton. A man weighing but one hundred 

 and fifty pounds would require of his muscle five hun- 

 dred pounds less force. 2 



1 Many text-books of physiology analyze this so that the floor 

 becomes the fulcrum and the body the weight applied at the 

 ankle. The error of this is seen when further analysis discloses 

 the fact that, were this true, two-thirds of the weight of the body 

 would be supported by the muscle and but one-third upon the 

 floor. In other words a man weighing one hundred and fifty 

 pounds, were he to rise on his toes on weighing scales, would press 

 down upon them with a force of but fifty pounds! 



2 In this study of leverage, we have considered the mechanical 

 relations only in those cases in which the forces act at right angles 

 to the length of the lever, in order to make the treatment as simple 



