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Muscles work together to move your body. Put your arm straight down at your side. Now bend at your elbow and bring your hand up to your shoulder. Put your hand back down at your side. Hold your hand out in front of you now with the palm of your hand up and don't move it. When you moved your arm so your hand was touching your elbow and down again, the two muscles in your arm worked together to do that whole process. When you moved your hand up, the top muscle, from where you are looking at it now, or your upper arm contracted, and the bottom muscle on your upper arm stretched out. And when you put your arm back down again, the bottom muscle contracted, and the top one stretched. You can put your arm down now.
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