Antagonistic muscles
pull a body part in the same direction
always have the same strength
pull a body part in opposite directions
don't like each other

If it's a smaller child or infant, place one on the middle of the chest and on the child's back. (if it is a bigger child with enough room for two, place one on the upper right/your left of the chest, and one on the lower left/your right) the pads should never touch each other. 

To this question it will be choice one

Answer psychoneuroimmunology;

Sorry man that a really hard one you got there