Brace your BACK and it will brace you BACK
'Spine or Back' in general is used to show something as a base of something.
Example :
He is backbone of our company.
My father is back of our family. (and so on, such examples are commonly used in our daily conversation)
Backbone is synonym for a support system. Now just imagine if our support system stops functioning adequately what will happen to the whole system. Same way if our spine is or back is not in good health it reflects in our overall health. It starts compromising the overall health of the individual as the first thing which starts happening is it hinders the proper movement and functionality of our body.