I want to become more flexible. Unfortunately, there's lots of conflicting and unsourced material on stretching and flexibility training out there.
Could anyone point me to a reputable source for such information? For example a book or website.
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Try Pavel Tsatsouline "Relax Into Stretching" and Beyond Stretching. These are good books that will get you flexible in no time.
These books goes through the basics of flexibility. "Relax Into Stretching" goes through the relaxed stretching method which I think Kurz also promotes. It is a safe way of stretching.
"Beyond Stretching" goes through some more advanced techniques like different variations of PNF and some variations of more force-full stretching to get through plateaus.
The first book is a good start to get into stretching for a beginner, and it is good to then get into the other book to increase the results.
Also Brad Appleton's Stretching FAQ is a good resource for stretching.