According to this:

http://www.nlm.nih.gov/medlineplus/d...s/a682159.html

Ibuprofen works by stopping the body's production of a substance that causes pain, fever, and inflammation. Nothing about being a muscle relaxant.

I was in college when Ibuprofen became available without a prescription, and the professor I worked for had me do research on pain killers at the time (I don't remember why, since he was a marketing professor, but anyway.) The articles I read were about the dose-response relationship for Ibuprofen and Acetaminophen (Tylenol). For both drugs, the research found that the dose-response curve is not linear, and that the response levels off as the dosage increases. Which means if you take 500 mg, you get less than twice as much of the effect you get at 250 mg. That might explain why some people don't get much relief from acute pain with ibuprofen if they're already taking it regularly for something else.