expression, godan verb (-su)
straighten one's back; sit/stand up straight
Literally 'stretch the back muscles'. Used for physically correcting posture, and figuratively for adopting a serious, dignified attitude.
背筋を伸ばして座ると、腰が楽になる。
Sitting with your back straight eases your lower back.
面接の前は、背筋を伸
Before the interview, walk tall with your back straight.