人在屈肘时肱二头肌舒张、肱三头肌收缩．______．
骨骼肌包括中间较粗的肌腹（红色）和两端较细的肌腱（乳白色），同一块骨骼肌的两端跨过关节分别固定在两块不同的骨上．骨骼肌有受刺激收缩的特性，骨骼肌只能收缩牵拉骨而不能将骨推开，因此每一个动作的完成总是由两组肌肉相互配合活动，共同完成的．例如屈肘动作和伸肘动作的产生．屈肘时肱二头肌收缩，肱三头肌舒张，伸肘时肱三头肌收缩，肱二头肌舒张．
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| I stood there listening to my father shout at the top of his voice. The look on his face said that one of us had done something wrong. "Which one of you did this?" We all looked down at the step. There was child's 1 in chalk there. I was very 2 . I hoped no one could see it. Would he 3 it was me? I was so scared. When he asked me, I lied, "Not me, Dad." The others denied (否认) it as well. Of course, we all knew that one of us 4 have done it. But as the youngest and smallest of the three, I just couldn't find the courage (勇气) to tell the 5 . To 6 out who had written on the step, Father gave us each a piece of paper and a pencil. "I want each of you to write exactly 7 you see on the step." I tried my best to write the words 8 . I didn't want dad to be able to tell it was me. He gathered up our pieces of paper and looked at them, "Since 9 of you will admit (承认) to have done it, then I will punish you all." I stood there and said nothing. The last thing I 10 was for Dad to punish me! "I did it." My sister stepped forward and said she had done something she hadn' t. She was grounded (受 处罚) for a month. We didn't talk about that day for many years. Not until we were all older and I knew it was 11 to finally tell my dad it was really me. I always felt 12 because of it. That was the last time I let anyone take the blame for me. | ||||
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