She was a slender, small-breasted girl, with an erect carriage which she accentuated by throwing her body backward at the shoulders like a young cadet.

请问老师throwing her body backward at the shoulders在这里作何解?是不是相当于 throwing her shoulder backward。这里的at the should和 I caught him by the arm.中的by the arm是否是一样的用法，这两个介词短语是作地点状语么？