this indeed is a very good topic, fast bowling is about bio mechanics, strength, power desire, technique, the list is endless, at the age of 15 my son was a incredibly fast bowler (80 m.p.h), then he went to durham county cricket club, who under the tuition of geoff cook and martin speight ripped his action from his bones limb from limb, at the time i was incredibly frustrated, because he lost 10 m.p.h out of his action, but to be fair, he now has a pure fast bowlers action, everything in straight lines good strong braced back and front legs, good drive through with his back leg, but the secret to all this is strength, no point running into the coil, big bound, and your back leg cant take the weight/strain and it collapses, anyway to see what i mean there are some videos on kirk before and after durham had him, the first ones are a bit grainy, but you can see how poor his action is but how fast he bowls, and later i think its called kirk rowland bowling at norton nets, he is only doing a 1/4 pace session, but check the action out, good feet, braced legs pole vault legs, good head position, but one thing kirk cant lose, is his shoulder rotation, very similar to stuart broad actually, not pure, anyway young man, get into the gym and strength strength strenght, and believe it or not hours of walk throughs too