Question

What is the universally accepted standard of flexibility that you should compare yourself against?

Answer 1

We can not define the universally accepted standard of flexibility which we can compare ourselves against.

Explanation:

Accordingly, there is not certain or any specific measure of flexibility.

We can not refer to the flexibility of the back of humans even we can not relate it to the flexibility of the rubber or any other material because we can not measure its flexibility.

Answer 2

There is not such thing as a universally accepted standard of flexibility because there's no material that is taken as a reference point. You can't say it's twice the flexibilty of rubber or twice the flexibilty of bamboo or anything similar because no official reference points have been established in the SI system.