Some of the data are conflicting, which is in part due to how the experimental studies were designed and what methods were used. Caffeine can enhance performance in endurance sports. Glycogen is the principal fuel for muscles and exhaustion occurs when it is depleted. A secondary fuel, which is much more abundant, is fat. As long as there is still glycogen available, working muscles can utilize fat.
Caffeine mobilizes fat stores and encourages working muscles to use fat as a fuel.
This delays the depletion of muscle glycogen and allows for a prolongation of exercise. Glycogen saved at the beginning is thus available during the later stages of exercise. Although the exact method by which caffeine does this is still unclear, caffeine caused sparing in all of the human studies where muscle glycogen levels were measured. The effect on performance, which was observed in most experimental studies, was that subjects were able to exercise longer until exhaustion occurred. In addition to the beneficial effects on muscle, caffeine may alter the perception of how hard you are working.