sprinting is an anaerobic excersice, this is an excercise that uses glucose in the body as oppose to oxygen, sprinting will, in no way, help you to punch faster
you need to train the fast twitch muscles in your arms if you want to punch faster
to do this, even though you will end up looking gay, go to a swimming pool and try to punch under water as fast as you can for 30 second bursts
aim for speed, not power
its very very physically demanding if you push your self hard
you want to be stood chin deep in the water so your feet are firmly planted!
the water actually slows your punches down and makes your fast twitch muscles work harder than they usually would
training fast twitch muscle fibers to fire faster is probably the hardest thing to do in sport
if you do it 3 times a week (maximum 5) you'll see a difference in 6 months
fighters do sprints for its cardio advantages, not to make them punch faster!!!