He does 30 sit-ups every day to strengthen his abdominal muscles.

sit-ups 是复合名词，由动词 sit（坐）和副词 up（向上）组成，中间用连字符连接。字面意思是"坐起来"，指"仰卧起坐"这种锻炼动作，通过躺下再坐起的动作来锻炼腹部肌肉。