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−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 (∼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˙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.