Globally, the key micronutrient deficiencies include all of the following nutrients except
a. vitamin a deficiency
b. iodine deficiency
c. iron deficiency
d. sodium deficiency

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. 

Hyper extension is the extension of a limb/body part beyond its limits