Caffeine increases endurance capacity by sparing muscle glycogen (stored carbohydrate). It also increases high intensity, short-term performance by increasing adrenaline levels and activating the central nervous system.
Some scientists think that caffeine improves performance by reducing pain and speculate that aspirin might be an effective supplement for athletes. A Baylor University study, led by Geoffrey Hudson, showed that caffeine was more effective than aspirin in promoting muscle endurance during a high rep bench press test. Aspirin had no effect on weightlifting performance or the perception of effort or pain during exercise. Caffeine is an excellent pre-training supplement for bodybuilders. It helps provide motivation to train and provides a small boost in performance.