Usage patterns
背筋がまっすぐだone's back/spine is straight
Her back is perfectly straight, and she has a beautiful standing posture.
背筋にファスナーがあるto have a zipper along the back seam (of a garment)
This dress has a zipper along the back seam.
背筋に力を入れるto tense or engage the muscles along the back
I sat down in the chair with my back muscles engaged.
Expressions
背筋を伸ばすstraighten one's back; sit/stand up straight