How long does it take a pianist to retrain muscle memory to play a new motion?

This is a wonderful question. There is no such thing, literally, as muscle memory. Movement is hardwired into the brain, not the muscles.

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.

Point Of Sound

One of the causes for strain, stress, injury and other maladies pianists, organists and typists experience is simply that they press too hard into the keybed of their device or instrument. Let's first take a look at sports and then physics (that HS subject you think you don't use in real life). 

As a baseball player stands at home plate and the ball is hurtling toward him, he back-swings, forward swings, hits the ball, then all the energy left over from the swing dissipates into the follow through.  The same action occurs when an athlete swings a tennis racquet, kicks a ball, swings a golf club, punches someone, throws a ball, etcetera.  That is Newton's third law of physics that every action has an equal and opposite action.  In order to forward swing, one must first back swing.  Even when we walk forward, as one leg is extending up and forward, the other leg is pushing backward.  So according to the laws of physics, in order to type or play the piano down, one must first lift up.  Many of us were trained or taught to play or type from a resting and relaxed position which actually creates tension because holding a position requires effort.  So we know that everything requires an opposite motion and a follow through.  I bet all you smart kids out there know exactly where I'm going with this.

Now imagine that our baseball, tennis, soccer, golf, football players or boxers are standing before a concrete wall and they backswing then forward swing but instead of hitting an object and following through, they strike the immovable wall.  All the energy of the swing, instead of following through and dissipating, ricochets back into the athlete.  That can hurt. 

A piano is much the same.  Many pianists press into the keybed of a piano and not only does that fail to produce any more of a tone but, all the energy of pressing down is being transferred back up into their finger joints and tendons.  We often don't notice this until after an hour or so of practice or the next morning when we wake up with stiff fingers.  We are taught by bad teachers "no pain, no gain."  In this case, it is very much a lie.  No pain, no gain is fine when building muscle but not for bones, joints, tendons, ligaments and certainly not for our technique.  Often poor technique, strain or missed notes are a result of what we are not doing rather than what we are doing and often we are using the wrong muscles.

Sit at any acoustic piano and very slowly, depress a key so you don't play a sound.  At some point you will hit a little bump in the action, then press through it and you will hit the keybed.  That little bump is the point of sound once you actually play. 

As a pianist drops the controlled weight of their arm onto a key, they must use Newton's third law.  As they hit the point of sound, much like an athlete striking a ball, they must then follow through without hitting the keybed.  When they press into the keybed, not only is their energy backfiring but, they are pressing down and according to Newton, we can't set up for the up motion if we are pressing down.  This hinders technique.  You can't play down if you don't play up and you can't play up if you are pressing down.

So, the key and cure to playing without tension or pain is learning to play to the point of sound and simply following through and not pressing into the keybed.  Unbeknownst to most teachers, their students press into the keybed.  It is a motion that is often invisible.  When a student experiences problems with technique or pain, the teacher often says practice more or run exercises to build strength and endurance and the teacher is often oblivious that the way the student practices is what needs to be addressed, not a clock.

Danger Will Robinson. Before anyone tries to learn to play to the point of sound, there are other components of technique which must first be in place.  This includes controlling arm weight, controlling up/down, controlling in/out from the shoulder and elbow, using the fulcrum of the elbow, controlling rotation of the forearm through the use of the pronator and supinator muscles.  Likewise, there are movements to avoid such as abduction, curling the thumb under the palm, isolating a finger, equalizing fingers, radial and ulnar deviation, and trying to play too relaxed, still and quiet.

A virtuoso technique looks like it is effortless and relaxed.  That is true, the fingers are relaxed because the arm does all the work.  Observers are often looking at the pianist's hands and fail to notice the elbow and arm is actually doing most of the work.  Pianists who attempt to play from the fingers and have fatigue, are told to relax so, they relax the same muscles they are continuing to use and they achieve nothing.

Once all of the proper motions are achieved and the improper ones eradicated, point of sound will just happen. Some "techniques" such as the Russian Technique, surreptitiously imbue the pianist with these movements but personally, I would rather learn the physics and ergonomics of movement rather than being tricked through mindless imitation.  Although, it works to some degree. Where it fails is when a pianist encounters a passage they can't execute and if they knew the mechanics of the arm, would be able to figure out what sort of adjustment is required to play that passage.

I once studied with a leading concert pianist in my area who didn't know what he was doing but had a phenomenal natural technique.  His instruction to me was to watch him play then imitate his motion.  That would have been fine but I already had bad habits hardwired into my brain which were getting in the way.  Since he didn't know anything about ergonomics nor physics, he had no idea how to fix me other than prescribing "practice more."

I once gave a lecture on this topic and a pianist disagreed with me about Newtons third law citing that the piano is down, not up.  The finger must come straight down onto a key.  If the pianist is playing with a "still and quiet hand" and they must also play black keys,   note that the black keys are higher than the white keys.  This results in the still and quiet pianist to stretch or twist to reach those keys which in turn create vector forces or, two muscles pulling one bone in two directions simultaneously.  This creates tremendous imbalance in the arm which controls the hand and fingers and this leads to an incoordinate technique.  Keep in mind your fingers have no muscles.  They are moved by the flexor muscles in your forearm so that is where the pianist must first play from. 

If you were to walk up stairs, your ascending leg would lift HIGHER than the next stair, then come straight down onto it.  If you tried to walk upstairs without lifting your foot higher than the step, you'd trip.  Playing the piano is the same.  We must use the larger muscles of the arm to get the fingers higher than the notes we are desirous to play.  Of course as we become more efficient, we minimize the height but make no mistake, although it may appear invisible, it is still there. Hanon knew this and prescribed the pianist to isolate one finger and lift it high but, this isolation engages the flexors and extensors at the same time resulting in strain to the long flexor tendons which leads to median nerve entrapment (AKA carpal tunnel syndrome). Remember the arm, hand and fingers can only move in one direction at a time.  By abducting, for instance, the hand gets pulled in two or four directions despite the pianist trying to play a passage in a specific direction.

I have no conclusion to this post other than don't try this at home.  Find a teacher who knows what a pronator and abductor is and work from there.