Caffeine ingestion can delay fatigue during exercise, but the mechanisms remain elusive. This study was designed to test the hypothesis that blockade of central nervous system (CNS) adenosine receptors may explain the beneficial effect of caffeine on fatigue. Initial experiments were done to confirm an effect of CNS caffeine and/or the adenosine A(1)/A(2) receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) on spontaneous locomotor activity. Thirty minutes before measurement of spontaneous activity or treadmill running, male rats received caffeine, NECA, caffeine plus NECA, or vehicle during four sessions separated by approximately 1 wk. CNS caffeine and NECA (intracerebroventricular) were associated with increased and decreased spontaneous activity, respectively, but caffeine plus NECA did not block the reduction induced by NECA. CNS caffeine also increased run time to fatigue by 60% and NECA reduced it by 68% vs. vehicle. However, unlike the effects on spontaneous activity, pretreatment with caffeine was effective in blocking the decrease in run time by NECA. No differences were found after peripheral (intraperitoneal) drug administration. Results suggest that caffeine can delay fatigue through CNS mechanisms, at least in part by blocking adenosine receptors.

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Background: The purpose of this study was to evaluate the effects of caffeine (CA) on multiple resistance training bouts on repetitions to failure. Methods: Resistance-trained men (n = 10) completed a double-blind, placebo (PL) controlled, randomized trial. A 10-repetition maximum was established to individualize workload. Participants completed two trials (4 sets to failure, 6 exercises [24 sets total]) including bench press, lat pulldown, shoulder press, bicep curl, tricep pushdown, and leg press following CA (6 mg$kg À1 ) or PL ingestion 1 hour prior to the trial (counterbalanced). Exercises were performed to failure (2-minute recovery between sets/exercises). Ratings of perceived exertion (RPE) (0-10 omnibus and is nit expanded on (OMNI) scale) was recorded after every set and session-RPE (S-RPE) was recorded 20 minutes after each trial. Results: For total repetitions per exercise, a 2-way (PL vs. CA) repeated measures analysis of variance revealed a significant main effect of CA on performance ( p £ 0.01). Post hoc analyses showed a significant difference for total repetitions with bench press ( p = 0.03) but no significant difference for lat pulldown, shoulder press, bicep curl, tricep pushdown, or leg press (CA vs. PL). No significant effect was observed ( p = 0.24) for total repetitions for all exercises and sets combined compared with PL. Post hoc analyses revealed significantly greater repetitions ( p = 0.003) for bench press (set 4) with CA. However, no significant difference was found for any other set ( p > 0.05). RPE was significantly higher for tricep pushdown ( p = 0.002) for CA versus PL. No significant difference was shown for RPE for all other exercises ( p > 0.05) or S-RPE ( p = 0.44) with CA (8.1 -0.8) versus PL (7.9 -0.9). Conclusions: Results indicate that CA did not improve overall resistance training performance.