One of the most obvious differences between anytime energy and pre-workout formulas is caffeine dosage. Anytime options generally hold per serving levels to 80 or 100 mg; about what you'd get in a strong cup of coffee. This level allows multiple servings throughout the day and makes a much better fit for caffeine-sensitive individuals. Pre-workout formulas pour on a higher dose of caffeine, typically between 250 and 400 mg, with the goal of boosting short-term energy and focus for intense effort.
A study published in the journal Physiology & Behavior demonstrates how low doses of caffeine don't add much to athletic performance, at least in terms of accuracy. Tennis players working off 5 hours of sleep took 80 over-the-net serves into an area 43 x 71 inches. Some got 80 mg of caffeine half an hour beforehand while others got a placebo. Accuracy was impaired for both groups compared to fully-rested performance, and the effect was more pronounced in female subjects.
Whether a higher dose of caffeine would have improved the accuracy of serves in this sleep deprived study is unknown, but pre-workout levels of caffeine can help you push more reps to failure. Check out recent research on this topic by reading the Breaking News blog at optimumnutrition.com