This is a great hands-on activity about paired muscles. In this activity, students will build a model using materials you have in your class. The one exception is that you probably don't have long "balloon animal" balloons or a balloon pump in your room. Be sure to track these down in advance, as you will probably not find what you need, or enough of what you need, in the local super-market or convenience store.
I suggest making one of your own first to get the hang of it. I have included a photo of a finished model.
It helps to have covered the bones of the arm and the bicep and tricep, so that students can label these. I also like to have discussed paired muscles first, but you can also do this as part of introducing this lesson.
I have done this activity with 3rd graders. It works better with 4th and above, but the younger students just need a little more help with some of the fabrication. 5th and 6th should have no problems, especially working with a partner.