Technique is in your brain, not in your muscles. You brain hard wires movement which we call muscle memory but, it originates in your brain. That is why you never forget how to ride a bike or swim because the act of balance and using all those ancillary muscles gets mapped directly into the brain through the initial trial and error.
There are two ways to move: properly and improperly. Virtuosos move properly thus, they have virtuoso techniques. People without virtuosity simply move improperly and those improper movements get in the way of proper movement. Much like two people engaged in a three legged race. Both of you must perfectly sync your movements together in order to move freely and efficiently. If one doesn’t, anarchy reigns. Likewise, within our arms are muscles which must work in sync. If they don’t, we are mediocre at best.
When you first touch your instrument you brain makes those neural pathways and if you move improperly that improper movement becomes your permanent technique. Quite simply, you are using the wrong muscles to play and you will forever battle those bad habits. It is possible to brow beat strength and endurance into those wrong muscles and make progress but the day you skip practice, your body immediately begins to atrophy what it doesn’t need and you will feel rusty.
Proper technique uses the laws of physics and ergonomics and never atrophies because gravity requires no muscle. Pianists, for instance, think they depress the keys with their fingers but the fingers have no muscles. They are moved by the flexor muscles in the forearm and they are all interconnected with each other and also with the extensors. Each muscle moves one bone in one direction and there is an opposing muscle to move it back. If you use two muscles at the same time to move one bone, you will create tension, cramps, fatigue, injury or if you are lucky, play poorly. Even the slightest imbalance will create tension. This often happens when a musician isolates a finger. You can not extend and flex at the same time. Even though you can, you should not because they are still interconnected and they will be pulling on one another creating tension.
Proper playing is actually the result of several movements coming together so no single muscle is taxed through repetition. For pianists, this is called shaping. These multiple muscles include the pronator, supinator, shoulder, elbow and most importantly, gravity. Gravity never fatigues, it is always there. I don’t play guitar but I am going to guess that you never have to strum down because gravity or the weight of the arm will play down. Your only job is to play up. If you remember from HS physics, every motion has an equal and opposite motion. As a pianist, I can’t play down unless I first play up. If you sit at a piano, play a chord. Notice that you must play up, first. Up is the only work because down is effortless. Many pianists go wrong there because they press into the keybed. Since they can’t go any further and it creates no additional tone, then why press? It only creates tension and worse, prevents them from playing up because they are pressing down. You can’t move in two directions at the same time.
There are also muscles we should never use such as the abductors. Again, you can’t use two muscles at the same time. If you flex and abduct simultaneously, you are pulling one bone in two directions and the tendons are what will bear the brunt of this vector force and that is where cramps, fatigue or uneven playing comes from.
Go with gravity. Don’t fight it or try to force it. Remember from HS physics, if you push on a wall, it is pushing back with equal force. The wall will always win. Relaxation only comes from effort but it must be from the equal and opposite direction. Like walking. In order to propel your body forward, one foot is behind you pushing backward. In order to walk up stairs you must lift UP one foot higher than the step then come down on to it, using gravity. Although, I have heard people stomping up stairs. Swing a bat, throw a ball, kick a can . . . they all start with equal and opposite motions. Even your car rolls forward because its tires are pushing backward. Physics is not a useless class in HS that you don’t use in real life. It is everywhere in everything. Use it.
Get the laws of physics into your playing and you will never be rusty. Your technique will be there every day, even with several consecutive off days. Practice doesn’t make perfect. Perfect practice makes perfect.