If you're into movement, you've probably heard of the concept of closed chain exercises vs. open chain exercises. If not, here's a starter definition- the "chain" is your kinetic chain, which according to NASM "comprises the muscular, skeletal, and nervous system," which I realize is not the most helpful definition. Let me continue though- movements can be open chain, meaning that appendages are free to move, or closed meaning hands/feet are in a fixed position. A closed chain move might be a push up or down dog, in which hands and feet are stabilized. An open chain movement would be a kettlebell swing or getup, or a simple chest press/overhead press, in which arms/hands are moving and not stabilized. Let's broaden this definition and look at what parts of the body are contacting the floor, apparatus, or are free floating. (Think pilates apparatus work, which limits range of motion and gives lots of proprioceptive feedback vs. pilates mat work in which hands and feet are often free). Open chain movements can be more challenging in terms of knowing where your body is in space, because there is less feedback from the environment or apparatus.
Let's apply this to musical teaching and performance now- in regards to your instrument, what parts are stable or contacting the floor (or chair) and which parts are free to move? As a violist, my feet contact the earth, and my hands contact my instrument, but it's an open chain practice in many ways because my spine, hips, knees, elbows, fingers, and shoulders are free to move, as well as my hands and fingers. A pianist however will have the contact of the feet, chair (underneath ischial tuberosities) and the contact of fingers and keyboard. This leaves the spine, head, neck, and shoulders free. A singer (or french horn player) will have significantly less environmental feedback from their instrument- they will be relying heavily on proprioception and interoception to assess their playing, breath, embouchure, etc. Some instruments are more "open chain" than others, demanding more heightened external and internal bodily awareness. What can we do to challenge this notion? Close the chain when possible and give yourself new feedback opportunities.
Sit instead of stand (if possible) or vice versay to get an idea of the relationship between your pelvis, instrument, and chair. Lean against a wall or the back of a chair to focus on the movement of the ribcage while breathing or to notice the relationship of the scapula while playing. On violin, viola, cello, and bass, certain left hand positions are easier than others because of their relationship to the body of the instrument- for me, 5th position is strangely comfortable because of the feedback I get from the instrument. For many students, third position is stable because of the proximity to the neck and body of the instrument. The most obvious manifestation of these ideas is the usage of different tactile sensations in studying an instrument- many string students learn vibrato using a tictac container which creates a different kinesthetic experience away from the violin. You can also hear the shake of the container as an aural cue, although this is still very much an open chain exercise in many ways.
When teaching or practicing, look for where you can open or close the chain to change sensation. Different chairs offer opportunities to introvert your attention, different standing surfaces can tell you what your lower limb is doing (I sometimes stand on a wobble board to get a sense of if my quads are gripping and how my feet are interfacing with the environment), and there are now a multitude of different devices out there to "close the chain" for musicians, offering opportunities for new kinesthetic experiences. If we think of the bow and instrument as appendages, making string playing an open chain experience, howcan we stabilize and then transition away from that? And what can you or your student learn from that? How can you apply this to other instruments or other movement practices? Just my early morning thoughts for a springtime morning.