Kneeling training

Getting old is compulsory - getting frail is optional

Many people say they cannot kneel at all let alone on a surfboard.

This can be fixed.What you need to be able to do is a kneeling tibialis interior stretch. This is covered in the 12th minute of the video.This video from Unity Gym tells you how to work up to this. Do all the foot and ankle exercises. Before you go surfing, you may have to build up with these exercises over weeks or months, but it is worth it.
Foot And Ankle Strengthening
[FULL WORKOUT & STRATEGY]

Exercises - kneeling

Can you kneel up and roll back onto your toes?

If you can't kneel - fix that first.  

Check these searches: for the information you need.

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Ankles:
youtube.com/results?search_query=unity+gym+ankle
Knees:
youtube.com/results?search_query=unity+gym+knee
Unity Gym think that the fitness industry is backward.
Your health should be measured by how your body performs and feels, not how it looks.
Don't think: diet and exercise, instead learn to nourish and move.

Looking after the knees and ankles

So you want to put it all together and do the stuff covered on the other site:
https://kneelupps.blogspot.com