This is the 9634th most frequent Japanese word.
背筋
"背筋" (せすじ) refers to the muscles along the spine or the posture of one's back.
In this sentence, 背筋 indicates the muscles along the spine, and the instruction emphasizes posture.
背筋を伸ばして座ってください。
Please sit up straight, keeping your back straight.
Here, 背筋 is used metaphorically to refer to the feeling along one's spine caused by cold or emotion.
寒い風が吹くたびに背筋がぞくぞくする。
Whenever the cold wind blows, I get chills along my spine.
In this context, 背筋 directly refers to the muscles in the back and spine, emphasizing physical exercise.