I've trained in nine different martial arts since 1973. I currently train under the last of Bruce Lee's students to enter his school before he died----my instructor is the top Jeet Kune Do expert in the world. I am no genius, far from it, but over the decades, I've learned a little bit about what muscles can do, and what the brain can do.
I have to say that if muscle memory worked well, I could take a devastating shin penetrating 24 inches through the side of my head at just under 100 miles per hour, and still stay on my feet and win the fight.
Sadly, that's just not the way it works. When the brain shuts down, so does everything else---instantly. I've shut down many brains, and the body attached to every one of them hit the ground, hard. The brain controls it all.
Just think back on the video footage associated with the phrase "DOWN GOES FRAZIER!" Did he get punched in a muscle? No, he got punched in the head, and his brain shut down.
Train the brain, and you are also training the muscles to respond to the trained commands. Stop the brain, and the muscles no longer know what to do.
I shoot a Hill bow, and it's my brain doing the aiming. My back muscles simply draw the bow when told to, and my fingers let go of the string when they are told to.
Anything else is a miss.
. . . Now, if you have, weak, untrained muscles, we need to discuss a whole different problem.
Train the brain and the muscles will follow. . .