So Jo is obviously usually hitting a physical wall. In most other cases - and this is not limited to temporarily able-bodies human beings, even when you think you hit a physical wall, it is mental wall. Probably it is the mental wall (buffer-zone) you create to avoid crashing into your physical limit, but very often it is a mental barrier you just create to simplify your world.
After all the years I still have problems throwing strong and heavy fellow-aikidoka, although I know from a few "accidents", that they fall as easy as the others, when the technique is applied correctly.
And when ever I am totally exhausted, it is mostly just because I blocked myself from staying calm and breath. I see this very often at other aikidoka so it not just me, but I guess most aikidoka run against mental walls.
Jo Adell is an exception
Best regards Dirk