the meaning of straighten one's shoulders

[optimistic pessimist]

Dear all,

"...Then the guard straightend his shoulders, took a deep breath, and blushed..."

In the sentence above, what does "straighten one's shoulders"?

Is this different from "to straighten one's spine" or "to straighten onself"?

Thank you!

OP

 

[Tdol]

I think it means he pushed his shoulders back, adopting a more formal position from a more relaxed one.

PS straightened