Muscular and Nervous Energy
"The manifestation of nervous energy in this manner is quite different from the manifestation of muscular energy, although both are, of course, intimately connected. Muscular energy begins at its maximum and gradually diminishes to the point of exhaustion, whereas nervous energy rises in an inconceivably short space of time to its climax, and then drops immediately to nothing. Nervous energy may be said to be represented by an increased rapidity of emission. It is what the athlete would call a 'spurt.'
"What I have said about the voice applies equally to all other instruments, the piano and the organ alone excepted. It is obvious that the playing of the wind instruments must be subjected to the limitations of the breath, and in the case of the violin and the other stringed instruments, where the bow supplies the motive power, it is impossible for two notes played in succession to sound absolutely alike. If the first note of a phrase is attacked with the weight of the whole bow behind it, the second note will follow with just so much less weight, and if the violinist desires to intensify any of the succeeding tones, he must do so by the employment of the nervous energy I have mentioned, when a difference in the quality of tone is bound to result. The pianist should closely observe and endeavor to imitate these characteristics, which so vividly convey the idea of organic life in all its infinite variety, and which are inherent in every medium for artistic expression.