Taking daily recommended dosages of ibuprofen and acetaminophen caused a substantially greater increase over placebo in the amount of quadriceps muscle mass and muscle strength gained during three months of regular weight lifting, in a study by physiologists at the Human Performance Laboratory, Ball State University.
Over three months, says Dr. Trappe, the chronic consumption of ibuprofen or acetaminophen during resistance training appears to have induced intramuscular changes that enhance the metabolic response to resistance exercise, allowing the body to add substantially more new protein to muscle.
Interesting read.
Although, they don't give near enough information in the write up, I would like to see the actual study. I'm just not in the mood to go looking for it.
Now, Here are two studies I do have book marked on the subject.
These results suggest that ibuprofen and acetaminophen have a comparable effect on suppressing the normal increase in PGF2{alpha} in human skeletal muscle after eccentric resistance exercise, which may profoundly influence the anabolic response of muscle to this form of exercise.
These results suggest that over-the-counter doses of both ibuprofen and acetaminophen suppress the protein synthesis response in skeletal muscle after eccentric resistance exercise. Thus these two analgesics may work through a common mechanism to influence protein metabolism in skeletal muscle.