Caffeine not only gives a person energy to exercise in the first place, but it can lessen muscle pain during a workout.
Caffeine affects the brain and spinal cord, which process pain in the body. It blocks adenosine, which helps with energy transfer and other biochemical processes. Caffeine has a greater effect than taking an aspirin before exercising.
The interesting thing is that it has the same level of pain reduction no matter how much caffeine a person consumes on a regular basis, (including non-caffeine consumers.) Therefore, caffeine tolerance has no correlation with this particular process.
Another result is increased endurance because there must be more strain on the muscles to produce the same amount of pain.