In a study on the effect of caffeine on muscle metabolism, nine male volunteers from a random group of men underwent arm exercise tests on two separate occasions. On one occasion the volunteer took a placebo capsule an hour before the test; on the other occasion he received a capsule containing caffeine. The order of these treatments was randomised for each participant. During each exercise test, the ratio of carbon dioxide produced to oxygen consumed (respiratory exchange ratio or "%RER") was measured.

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