you start to tuck more over time as your technique progresses, i would advice doing much more stretches before and after riding and this should help decrease the stiffness. also try to relax as your riding as this will also help as your not so tense/stiff... when i used to box/weight train my coach always adviced me to do stretches before and after, something to do with making the muscles shorter if you don't well... just try it and let us know how you get on its a gradual thing and you have to stretch more and more each time otherwise its pointless hope iv helped in some way cheers Jonny