For some people the extent of the hyperextension of the knee is problematic and can cause damage. For other people the degree of their hyperextension of the knee is mild and will never become a problem. In fact, virtually all humans hyperextend the knee while standing relaxed. The average amount of hyperextension can vary greatly but one study showed 99% of women hyperextend, with the average being 6.7° (sd of 2.7° and range from 0 − 17°), while 95% of men hyperextend, with their average being 5.5° (sd of 2.5° and range from 0 − 17.5°.)* So, hyperextension by itself is not a problem, we all do it. Can it be too much? Yes, but this doesn’t mean that all hyperextension is bad. Each student needs to be judged on his or her individual situation.
Regarding your point about Mr Iyengar and how his body may have changed over the years: you may be right. This book was first published in the 1960s when Mr Iyengar was in his 40’s (not so young!) but the book has been republished many times. The latest version was republished in 1995 and it remains the #1 selling book on Yoga on Amazon. By then, My Iyengar would have been in his 70’s. If he believed later in his life that the hyperextension he showed in his book to be dangerous, why did he continue to allow the book to be republished? He created many other books over that time, so it was not that he was uninterested in writing books and sharing his opinions and experiences.
You said that we have a responsibility to keep our students as safe as possible. I would agree. But you also said that we should do so “even if it means we have to err on the side of caution.” With that I don’t agree because that belief may actually harm the student, negating your first and most important responsibility. See my article on
Antifragility - in our attempts to avoid over-stressing our joints, we may end up under-stressing them and causing more harm through atrophy and fragilizing the joint. This is an important discussion, and it does point how challenging it is to be a good yoga teacher. It would be nice if there was one prescription we could offer to every student, but each student is unique and we need to figure out what works for each one. Better yet - we need to help the students figure out what works for her.
Cheers
Bernie
* See Normative data for range of motion and single-leg hop in high school athletes by De Carlo MS, Sell KE. in J Sport Rehab. 1997;6:246–55.