206 HUMAN PHYSIOLOGY 



these individual active components we find the single result- 

 ant according to the law of the parallelogram of forces. 



Muscles acting in the same direction are said to be syner- 

 getic, while antagonistic muscles are those which act upon a 

 joint in opposite directions. 



We may analyze the movements of the bones according 

 to the laws of the lever, for all movable bones may be con- 

 sidered as one- or two-armed levers. The length of the 

 lever-arms from force to fulcrum and that from fulcrum to 

 weight is the distance from the axis of the joint to the point 

 at which the force and the weight work. 



In the body, the lever-arm of the force is generally smaller 

 than that of the weight. This is not unfavorable, however, 

 for by it we gain velocity, even though it be at the expense 

 of force. 



During the motion of the supposed movable bone, the 

 amount of the effective force frequently changes because of 

 changes in the lever-arm either of the force or of the weight. 



Of interest is the decrease of the weight-arm during the move- 

 ment, for thereby the muscle works more advantageously (see page 

 193). An example of a movement of the body during the unload- 

 ing [Entlastung] of a muscle is the elevating of the body by the 

 knee-joint. In the position with flexed knee, the lever-arm of the 

 weight is the horizontal distance of the axis of the knee-joint from 

 the perpendicular line through the centre of gravity of the body, 

 which lies at the .promontorium. This distance becomes smaller 

 as the body is elevated and, in the upright position, is zero. The 

 direction of pull of the quadriceps, which straightens the knee, 

 retains, during the movement, approximately the same distance 

 from the axis of the knee-joint. 



2. STANDING, WALKING, RUNNING 



The general erect position (standing) and the general 

 modes of locomotion (walking, running) have a typical form 

 in all people, for they are based upon a common principle, 

 that of the least muscular exertion. We are accustomed so 

 to stand and move that the muscles are as little exerted as 

 possible. 



