I am continuing to do abdominal and back (and pull-ups) exercises with the above device.

It is generally accepted that a "slightly painful" (abdominal or back) numbers are good, but in my experience this is not correct.

"Stop at the number of times I don't feel any pain at all"

is correct.

I know very well that even the slightest amount of 'suffering' will not 'keep me going'.

"To continue to train without any pain at all" is much better than

"To skip the occasional workout that's a little painful"

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I have never read a self-help book that states this fact.

Frankly, I am wondering.