New muscle memory movement is very easy to wire into the brain and it can be immediate however, the brain never forgets the old patterns so, as a musician, if you get nervous or your body is cold, or you go into autopilot, it is very easy for the old movements to reassert themselves and take over despite new and more efficient neural pathways having been created since. This is especially true for musicians and also, how and what we play is very important. This is why musicians often claim they can play perfectly in their living room but on stage it all falls apart. What is happening is the old muscle memory takes over because of environmental factors such as the presence of an audience, different bench height, temperature, nerves, etcetera.
There is another danger here. Many teachers instruct the student to build strength and endurance to overcome technical deficiencies. This works to a certain extent but also puts the musician on the path to injury. If the musician then learns new and proper movements, the improper muscles used previously will immediately atrophy. This is why improperly trained musicians feel rusty or stiff after missing a few days of practice because the wrongly built muscles will get weak, quickly. Proper playing utilizes fulcrums, alignment, gravity, ergonomics and the laws of physics, not muscle. This is counter intuitive to most musicians and to many teachers who are ignorant of anatomy and physics. Mediocrity is the result of using the wrong muscles, not lack of talent. This is because most teachers have no idea what they are doing. They only know what they know but what they don’t know is what creates injury, tension, fatigue and sloppy playing.
A beginning student may learn a piece of music and there may be flaws in his movement. Over time he gets better and learns new songs and rewires some of the improper movements in his brain. He progresses further and his technique improves and his brain learns newer and even more proper movement. THE DANGER is playing old repertoire because even though his technique improves and he now has proper movements, the brain remembers the lesser or improper movements of previous repertoire from a time when he moved less properly. It is important for musicians to either never play old repertoire or, re-learn each piece with the newer, more proper motions.
The greatest danger is, as I previously said, the improper muscles atrophy if not used. If a musician built improper muscles to play a piece well, then as he progresses and loses that muscle because it is no longer needed since he is more ergonomic now, then he plays that old repertoire, the brain expects that the former muscle is there and tries to play the work “normally.” Since the muscle is no longer present, this is when the musician runs the risk of greatly injuring themselves. This is why a well trained musician can one day, out of nowhere, injure themselves. Most injuries are actually cumulative and it is one of those "muscle memory" moments that serves as the proverbial "straw that breaks the camel's back."
In addition, rewiring your brain on your instrument isn’t sufficient. You must simultaneously do the same with how you ring a doorbell, tie your shoes, brush your teeth, pick up a piece of paper, type, swipe, wipe . . .
There is no such thing as repetitive strain, only improper movement. If you move improperly, all movement can become repetitive strain and as I said, it is cumulative. That is why a forty year old might get out of bed with stiffness, aches and pains while a 70 who has moved properly all their lives can rise with elan and alacrity. You can take that to the movement bank.