This is actually a very complicated project, but don't despair! You might be able to have enough time to do some type of experiment (I'm not sure from your statement if your son has completed an experiment and you are just having trouble finding research, or if nothing has been completed yet). Everyone's muscles have developed differently, so for each person this would be different. It also depends upon which muscle you study. The heart, for example, beats until we die, but other muscles tire more quickly. I'm not sure how much your son is going to be able to finish in one night. You could try a quick experiment with the members of your family, doing different types of weight training (if you don't have weights you could use a heavy object, such as a book). Pushups, crunches, etc. Counting the number of movements each person can do until they become tired might work, although each person will also have a different tolerance for pain.
I tried to search for a few kid-friendly websites that had a good description of how muscles worked, and here they are. You might want to read them with him:http://health.howstuffworks.com/muscle.htm http://www.kidshealth.org/kid/body/muscles_noSW.html http://www.ebsinstitute.com/Baseball/EBS.crb3df.html
(this talks a bit about why muscles become tired)
There are also a few quick projects you might be able to do in time, if you don't want to do this one anymore. You would probably have to cut out some of the tests or modify them in order to finish in time.http://www.sciencebuddies.org/science-f ... ?from=Homehttp://www.sciencebuddies.org/science-f ... ?from=Homehttp://www.sciencebuddies.org/science-f ... ?from=Home
I hope that helps, and good luck! The most important part is that your son learns something from this, so even if the project is rushed you might want to discuss muscles with him later, too. (For future reference, Science Buddies doesn't always have people to answer questions 24/7, so it normally takes a few hours or more to get a response, especially if the person replying to Ask an Expert is not very familiar with the subject area. We rely on volunteers that donate any free time they may have. Although it sounds like your son's project will be rushed, I hope you will continue to discuss science with him. It's fun to do some quick projects every once in a while (especially if you don't have to write up everything in a professional manner, like for school))