What is phosphagen system for exercise?

It is the quickest method your body can use to produce ATP (the energy molecule used by all our cells). The phosphagen system only lasts 10-15 seconds before it is depleted however so aerobic and anaerobic metabolism must take over. An all out spring for a few yards relies heavily on the phosphagen system. A 400 meter sprint on the other hand uses mainly aerobic metabolism after the first few seconds. A 1600 meter run in turn relies heavily on aerobic metabolism (us of oxygen).

I have a B.S. in Exercise Science from Ball State University so its a good answer ;)