That's what researchers at the University of Georgia have found in a recent study exploring why muscles hurt during exercise. The research group previously learned that aspirin, though commonly used to treat muscle pain, did not reduce muscle pain produced by vigorous exercise.
"Muscle contractions produce a host of biochemicals that can stimulate pain. Aspirin blocks only one of those chemicals," said Patrick O'Connor, professor of exercise science in UGA's College of Education. "Apparently the biochemical blocked by aspirin has little role in exercise-induced muscle pain."
The researchers' latest study, published in the August issue of the Journal of Pain, found that caffeine reduced thigh muscle pain during cycling exercise. Participants in the study, 16 nonsmoking young adult men, cycled for 30 minutes on two separate days. The exercise intensity was the same on both days and purposefully set to make the riders' thigh muscles hurt. Participants in the study took either a caffeine pill or a placebo pill one hour before the exercise. The riders reported feeling substantially less pain in their thigh muscles after taking caffeine compared to after taking the placebo.
This observation suggests that prior reports showing that caffeine improves endurance exercise performance might be explained partially by caffeine's hypoalgesic properties, according to O'Connor.
"Not all analgesics or combinations [acetaminophine and caffeine] are effective for every type of pain or every individual," he said. "Much of this is due to biological variation among people in receptors for the drugs as well as variation in pain receptors in different body tissues. For instance, brain tissue has no pain receptors so surgery can be done on the brain without anesthesia. Of course it will hurt getting through the skin and cranium."
Caffeine also seems to work less well for heavy caffeine users who habituate because of a change in receptors with caffeine use, O'Connor said.
Prior research has focused on other types of pain, such as headaches, joint or skin pain, toothaches or pain in damaged muscles at rest, maybe a few days after being injured during exercise. The UGA researchers' work focuses on pain that occurs naturally with muscles contracting during exercise.
"The next step is to learn how caffeine helps people feel less muscle pain during exercise" said Robert Motl, lead author of the study and an assistant professor of kinesiology at the University of Illinois. "Evidence suggests that caffeine works by blocking the actions of adenosine, however, we don't know yet whether the caffeine is acting on muscles or the brain."
Motl, who received his doctorate from UGA in 2002, also worked with Rod Dishman, professor of exercise science, on the research project.