Caffeine makes exercise less painful, according to a new US study. And it seems to be more effective in women than in men.

The researchers looked at the effects of two different doses of caffeine, compared to a placebo, on leg muscle pain in 11 college-aged women. The women completed 30 minutes of cycling on ergometers at approximately 60% of VO2max on three occasions, one to two weeks apart, under each of the following conditions:

• One hour after ingesting a moderate dose of caffeine (5mg per kg of body weight);
• One hour after ingesting a high dose of caffeine (10mg per kg of body weight);
• One hour after ingesting a placebo.

The women’s perceptions of leg muscle pain were recorded every five minutes during exercise, along with power output, heart rate and blood pressure. Leg muscle pain was rated on a scale from 0-10, with 0 being no pain at all and 10 being extreme, intense pain.
The mean pain intensity scores for the three exercise conditions were:

• 1.3 for 5mg of caffeine;
• 1.6 for 10mg of caffeine;
• 2.4 for placebo.

The difference between the effects of caffeine and placebo on pain perception was significant and large, according to the researchers. Despite the different pain scores, there was no statistical difference between the effects of the two doses of caffeine – ie higher doses didn’t offer greater benefits in terms of pain relief.

Previous research has shown that caffeine has similar effects on men. But the effects on women were more than twice as large as those observed for men. The reasons for this large sex difference are unclear – as, indeed, are the mechanisms by which caffeine exerts it pain-suppressing effects.

The researchers have called for more research on competitive male and female athletes to try to tease out some of the differences and also look at whether there is a link between perceptions of muscle pain and performance.

Med Sci Sports Exerc, vol 38, no 3, pp598-604