Muscle memory does exist and it is helpful. Be thankful for muscle memory every time you try to walk across the room and don't have to think about which foot comes after the left foot! Muscle memory is part of kinesthetic intelligence and is not related to starting a piece only at the beginning, any more than it is related to starting your walk across the room only at one doorway.
Of course this is for normal, everyday activities that need little thought, but a tightrope walker would soon be dead were he to rely on muscle memory. That is why it is not a good idea to rely on it while playing, because any blackout would be fatal. I cannot think of a more uncomfortable feeling than having the fingers go right and left while I have forgotten how the music goes. It is like seeing that I am going to fall and yet not be able to stop it.
Monica maybe remembers that when I joined I had a broken ankle. It took me about three months to walk again and then... I had to learn again how to do it and it took several weeks. And this is for something I had done every day for over 40 years. Imagine if it is a piece that you have played for a week!
I must say I am now trying to understand the structure of the music: the scale, time signature, rhythm and all that and try to remember the music, not just let my fingers do it. An interesting esercise I have at times tried is to play something in a different octave or even in a different scale. That way you fool the muscle memory and have to rely on the more solid one to get the job done.
I forgot to mention: i asked about jumpimg to see how much you concentrate.