Caffeine ingestion improves endurance time trial performance. However, the ergogenic mechanism of action remains unresolved. One potential explanation for caffeine`s performance-enhancing effect is an improvement in work for a given amount of muscle pain. Purpose: To test this hypothesis, participants performed two studies in which they regulated exercise intensity based on feelings of muscle pain. Methods: Thirteen young men were asked to regulate exercise intensity based on feelings of "moderate" muscle pain (a "3" on a 0-10 pain scale). After three familiarization trials, either caffeine (~5 mg/kg body weight) or placebo were administered before a moderate pain trial. Nine caffeine "responders" were retested and ask to regulate their exercise intensity at a "strong" pain level (a "5" on a 0-10 pain scale). A caffeine (~5 mg/kg body weight) or placebo was again ingested before exercise. Results: Participants performed more work (P = 0.008) and covered more distance (P = 0.008) at a higher average power output (P = 0.009) and V.O2 (P = 0.019), for an identical amount of "moderate" muscle pain in the caffeine condition. When exercising at a rating of a "5," caffeine did not increase total work, distance covered, or V.O2 for an identical amount of "strong" pain in the nine caffeine "responders." Conclusions: Our findings indicate caffeine increases work performed during exercise, eliciting a moderate amount of a pain. However, a threshold level of muscle pain may exist above which antagonism of adenosine receptors alone does not induce a hypoalgesic effect.
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