Muscle Pain as a Regulator of Cycling Intensity: Effect of Caffeine Ingestion

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 (similar to 5 mg.kg(-1) 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 (similar to 5 mg.kg(-1) 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) over dotO(2) (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) over dotO(2) 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.