I think you raise a good point when indicating that we can improve physical ability through repetition, and that these are ultimately due to changes in the nervous system. However, I feel that but saying that proprioceptive input is filtered through the I-function makes it too concrete and distinct, and would like to propose another theory. Our nervous system is infinitely complex and plastic; it is capable of modifying or eliminating pathways or parts of pathways. These modifications are based on how much the pathways are used, sort of a neural evolutionary process. Pathways that are infrequently used are reduced to maintain efficiency, while those that are used more often are modified to allow frequent use. In the case of your dedicated dancer, proprioceptors detect the usage of muscles contracted in order to perform a certain motion. When the pathway is continually activated it undergoes a process, as I imagine it, similar to the evolution of a species through natural Darwinism. The result is a more efficient pathway that caused the motion to appear more accurate, precise and seemingly free from conscious thought.