The exercises are designed to strengthen your stomach muscles.

　　這些活動目的在于增強(qiáng)你的腹部肌肉。

　　Muscles contract and relax during physical activity.

　　身體活動時(shí)肌肉收縮放松。

　　He didn't move a muscle.

　　他一動不動地站著。

　　The heart muscles contract to expel the blood.

　　心臟肌肉收縮以擠壓出血液。

　　The exercise consists of stretching and contracting the leg muscles.

　　此項(xiàng)訓(xùn)練包括伸展和收縮腿部肌肉。