One stimulant whose effects have been thoroughly studied is caffeine. A general consensus is that 3mg of caffeine per kg of bodyweight will improve aerobic performance. The effect of caffeine on power, sprint and strength events is still unclear.  Although, during a longer strength / power session the reduced sense of perceived exertion could be beneficial. As mentioned previously if it is dosed properly caffeine will improve aerobic performance. The negative effects of caffeine are that it impairs fine motor control, can lead to more impulsive decisions and can dehydrate the user as caffeine is a diuretic.