I just read the weekly T-Nation newsletter in my email, and was puzzled by the following bit of advice:
"Well, what happens if you contract the hams as you stretch? Easy, you stretch the tendon, and you should never attempt to stretch either tendons (you'll decrease speed and power because they aren't supposed to lengthen) or ligaments (they can tear when stretched beyond 6% of their normal length)."
I thought PNF stretching was essentially contracting the muscle being stretched? Have I been doing it wrongly all this time? It seems to have worked great in improving my flexibility though, and I have never been injured from stretching (yet).
Btw a quick search shows that the advice came from this article: