This vid is worth reposting to this thread:
https://www.youtube.com/watch?v=y_7DgCrziI8
One thing that I'm still curious about is the statement about there being "no such thing as muscle memory". Obviously, since muscles don't have brains, we all knew that was technically true; but anybody who's ever learned to play an instrument or a sport or a martial art knows that there's a kind of physical micro calibration that must be maintained to keep that skill viable, even after a few days of not actively using that skill. The longer the interval, the longer the re-calibration. Elite performers and athletes have incredibly detailed rituals for maintaining their skill sets from day to day to perform at the highest level. It would be interesting to learn more about the biomechanics going on there.
I'm guessing that the bulk of the movements is generally there but the ability to modulate them and move in a refined way starts to degrade. In other instances, some skills are ingrained so much that it's going to be very very hard to shake. For instance, I can play Maple Leaf rag the same way I did when I was 16 - even after not playing piano for over a year. It just takes a little nudging for the synapses to refire. For me, it's gone beyond muscle memory. I feel like it's burned into my psyche. I learned it to the point where automaticity took over a long time ago. The bad part is that all of the mistakes associated with my rendition are still there - and they're sometimes kinda severe and very very hard to undo (I've tried in the past). It's partly why I dropped piano altogether just to get a fresh start.
OTOH, the ability to finger chords are still there (so long as I know what keys that need to be hit). The motor skills for that are generally sharp for basic chords - it takes no brushing up for me to execute them, ergo it's beyond "muscle memory". Where I'm slow now is knowing which keys to hit to get a jazz voicing, and getting accustomed to hearing those soound.
So IMO, it depends on the activity. In martial arts, you're often learning a whole repetoire of moves, and the moves change from night to night. It's not like when I way playing piano in high school. The same song, several times a day, every day for a month. I can't imagine the automaticity that classical players get because they're doing it for hours on end every single day. You don't get the same amount of reps. In MA, if you were to do that, you'd prob end up being that 10,000 kicks guy that someone like Bruce Lee would fear.
So methinks, yeah there's no muscle memory. It just depends on how deeply the skill is learnt.
On the subject of iding a bike: once you understand the feel of how centrifugal force works with two wheels, you won't forget it. The sensation is unique. IMO, riding a bike is more about sensing and feeling than doing. Pedaling is easy compare to understanding how to balance a moving bike. I consider it as a different skill and more of an experience.
TL;DR version: Maybe it's just quicker an easier to say that there are varying degrees of automaticity. Things are either automatic or they're not. And if they're not, some brushing up is needed so get it back to where it's closer to being automatic. And that things are truly automatic when it can't be forgotten, or need brushing up.