expression, godan verb (-su)
straighten up; sit upright
Refers to raising one's upper body from a bent or lying position into an upright posture.
彼はソファに寝転んでいたが、急に上体を起こした。
He was lying on the sofa, but suddenly sat upright.
腹筋
When raising your upper body during sit-ups, be careful not to strain your lower back.