A common error among musicians is playing with undue physical tension. This can actually lead to physiological problems such as tendonitis, carpal tunnel syndrome, subluxation, etc.
A well-known concert pianist, a graduate of one of the finest music conservatories in the world, almost terminated his career due to tension in his hands and arms. Fortunately, his playing method was corrected, and the condition ceased.
Another example is another well-known classical pianist who holds both of her pinkies (fifth fingers) up while playing. Eventually, this will catch up with her, health-wise. Let’s do a simple test. Put tennis balls under the palms of your hands, with your hands facing down and the finger tips touching the table. If you do not have tennis balls to hand, simply keep your hands in that cupped shape. Lift each of your pinkies up and hold them for a few seconds. Now put them back. Which hand position felt more comfortable and natural?
A musician has to play with balance, support and coordination, without unnecessary or additive actions, gestures or strains, and with his/her weight evenly distributed.
Try this: Sit down onto a chair, but upon sitting, rock your torso back and kick your legs slightly up in the air. That is unsupported and unbalanced sitting. It is also an additive action. Now, get up and sit back down again. But this time, just sit. Imagine your head to be central to the weight of your body. From your head to your sitz bones (in your pelvis), make it just one action. Notice the ease.
Try this as well: Take your instrument and play a few notes. While doing so, deliberately contract the muscles of your arms. Besides causing unnecessary physical tension, the produced tones will have no real resonance. Now, play a few notes again. This time, relax not just your arms, but the muscles of your entire body, including your legs, neck and head. Listen to the difference of the quality of the tone. Again, notice the ease.
It is a common mistake for musicians to lock certain body parts while playing, such as the knees, jaw and neck. A way to prevent and handle this is to give yourself fluid, not rigid, motions with your head that are somewhat “mini-choreographed” according to your musical phrases. For the knees, you can simply move them gracefully. This helps prevent locking and also relieves tension.
There are certain progressive non-musical practices that have been adopted by renowned schools of music and musicians to address this very subject.
As a side note, it is proven very highly beneficial, and I would say even mandatory, for musicians to learn certain key non-musical techniques and practices.
Incidentally, there is a common falsehood that long, slender fingers are a distinct advantage for pianists. There are many accomplished world-class pianists who have small hands. And interestingly enough, many European scholars believe that plump finger pads are more pianistic.