426 MOVEMENTS OF THE LIMBS 



arm may come to any position and remaiii there without the 

 expenditure of any extra energy (not taking into account gravity). 



Synergists. Movement does not usually take place merely by 

 the contraction of a muscle and the relaxation of its opponent. 

 There are numerous other muscles brought into play, synergists — 

 whose action, though secondary, helps the primary movements, 

 generally by altering the pose of the body as a whole, but some- 

 times by immobilising the bone to which the muscle is fixed. As 

 an example of the former, may be cited the action of the trunk 

 muscles holding the body erect while a weight is being held above 

 the head. The latter synergetical complex may be illustrated 

 by the various nuiscles brought into action in opening a table 

 drawer. " One hooks his fingers into the handle of the drawer 

 and if it opens easily enough, the contraction of the flexors of the 

 fingers is sufficient. If it works a little harder the flexors at 

 the elbow contract to hold the bones of the forearm up so that the 

 flexors of the fingers may have a firm origin. If still more force 

 be needed the latissimus and teres major spring into action to 

 support the humerus and rhomboids to hold the scapula. To 

 make a strong pull one pushes against the table with the other 

 arm and brings the extensors of the trunk into action, and finally 

 if this does not suffice, the legs are braced and the whole body is 

 converted by muscular action into a single solid piece in order 

 that the flexors of the fingers may exert all their power to open 

 the drawer." This description by Bowen shows clearly the 

 complexity of an apparently simple action. The student will 

 note too that as more muscles are called upon, the lever is 

 lengthened and the position of the fulcrum altered. 



Centre of Gravity. One of the first problems to be tackled is 

 the maintenance of the body in a vertical position in a state of 

 stable equilibrium. To produce this state of affairs the body must 

 be balanced over the two feet so that the centre of gravity lies 

 vertically over the area between the feet. In this position the 

 centre of gravity of the whole body lies just above and half-way 

 between the anterior sujyerior iliac spines. As the body is bent 

 forwards, backwards, or sideways the centre of gravity moves 

 accordingly, and may lie outside of the body. As long as the 

 vertical dropped from the centre of gravity lies within the area 

 between the feet (underpropping area) the body will be in equili- 

 brium. The maintenance of this balance is a function of the 

 skeletal muscles. Whenever the centre of gravity tends to move 

 beyond the underpropping area muscular contraction pulls it back. 



Standing. In order to maintain a vertical position the tibia has 

 to be balanced on the ankle-joint, the femur on the knee-joint, and 



