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muscles of the back; back muscles
「背筋」 has another reading with different meanings.
spine; line of the backbone
背筋せすじを鍛えようきたえよう。
Let's train our back muscles.
彼かれの背筋せすじは強いつよい。
His back muscles are strong.
背筋せすじが凍るこおるような話はなしだ。
It's a chilling story that makes your back muscles freeze.