She tried to sit up straight and quickly realised all her muscles were stiff from non activity.

sit 是动词，意思是"坐"。 up 在这里是副词，表示"向上"的方向。 straight 是形容词，意思是"直的"，这里形容坐姿的状态。 sit up straight 是一个短语，意思是"坐直"，指保持背部挺直的坐姿。