It's not going to land me in trouble for a few reasons: first, I don't particularly plan to implement those in a lifetime. Second, and it's something I apologize for, there's a point I overlooked: I realized that we were talking about two completely different applications. He was talking about replacing an user's mobility in medical fields. I was talking about augmenting an user's already existing mobility - this idea was mainly to amplify the strength of an user that already had full mobility.

As for that stretch - that's 10 degrees only if you use a very basic driver. The wonderful thing about electronics is that you can account for that stretch. Or you could use materials science and use, as I mentioned, pre-stressed cable that does not stretch as much, or even design your system so that the cable is already under a tension high enough that any further stretch is even less of an issue (though I do acknowledge that this is not a fail-proof execution either).

And as far as your example go - I also have issues with it. The one percent stretch I mentioned was in the case of an extreme load on a thick cable at a very high weight - something that would be found only in very overweight people or military applications. The aforementioned finger would, in all likeliness, never even reach 0.5% of that load. Using that same linked calculator, with a 6-inch 0.11 inch cable under 30 pounds-force of load, it's 0.02% of cable length - an accuracy of about 0.018 degrees, which I do believe is on par, if not finer than human motor control.

For the motor issue, I'll give you that - but with a stepper motor (or an array of stepper motor) you could have enough control to make it useful. And I used the term spool to refer to a circular thing that turns and affects the "length" of a cable - or, as I mentioned, a bar.

As for the "lever principle" thing, it was not sarcasm. I have honestly seen people with higher education, like software engineer, that forget about such basic principles. And I did not disavow math - math is the only thing that matters in engineering - I just consider all the options, like actually designing things to avoid the bigger math problems like stretching. But yeah - the lever principle comment was not sarcasm - it was merely to ensure that we had a similar knowledge base. It's not because he's clever that he didn't forget about things. Sorry if I came across as a smartass.