Amphetamines really don’t decrease fatigue on endurance tasks— they just mask it. This goes for most stimulants, although there may be an increase in fat mobilization for energy. Through stimulating the mind and euphoric centers of the brain, the exertion that you perceive is less intense and you feel less of the discomfort related to the fatigue.1 With that effect, you can go longer on endurance tasks. Further, the effect on the mind is one of increased focus, concentration and reaction times. Anaerobic activities like strength training are also enhanced by amphetamine use. This may be due to an increase in sensitivity of the muscle contractile tissues to calcium release, but the exact mechanism of increased strength is probably multifactorial. - See more at:
http://www.advancedmolecularlabs.com/the-real-dope-on-amphetamine-like-supplements-for-fat-loss-and-performance.