Caffeine is one of the most popular sports supplements, and for good reason. Caffeine has been shown to improve muscle fatigueability, endurance performance, and may even increase pain tolerance. Try 3-6mg/kg bodyweight within an hour before your workout. Make sure to maintain hydration and electrolyte levels if supplementing with high amounts of caffeine.
Reference: Graham, T. E. (2001). Caffeine and exercise. Sports Medicine, 31(11), 785-807.
