[btam]

Would you please elaborate on this? Why are curled-in shoulders especially bad?

[oostevo]

You might find this Google talk useful:

http://www.youtube.com/watch?v=kfg_e6YG37U

[georgieporgie]

It leads to terrible neck problems, like the pinched nerve I have. It causes significant discomfort, then intense pain and, eventually, numbness in one or both arms. Recovery is on the order of several years (and counting).