[1.III.7.1.1] Fast versus Slow Muscles
Understanding the difference between (1) control and speed, and (2) finger strength for technique, is important. All muscle bundles consist mainly of fast and slow muscles. The slow muscles provide strength and endurance. The fast muscles are necessary for control and speed. Depending on how you practice, one set grows at the expense of the other. Obviously, when practicing for technique, we want to grow the fast muscles and reduce the slow ones. Therefore, avoid isometric or strength type exercises. Practice quick movements, and as soon as the work is done, rapidly relax those muscles. This is why any pianist can outrun a sumo wrestler on the keyboard, even though the wrestler has more muscle. The fast muscles control the basic rapid finger stroke, down or up, that is needed for practicing every exercise discussed here; see "speed stroke" in section III.7.9.3.
Most of the muscles that move the fingers are in the forearm (Prokop). There are some reports claiming that the most important piano playing muscles are the lumbricals (Jaynes) and the interossi, but these are minority views that must await further research to carry any weight. It is clear, however, that "finger strengthening exercises" such as squeezing spring devices sold for this purpose, are bad for technique, especially speed.
The research on "piano muscles" is woefully inadequate. Because those who designed exercises in the past had little idea or research results on what the exercises need to achieve, most of those exercises are only marginally helpful, and how useful they were depended more on how you used them than their original design. For example, the main objective behind most exercises was to develop finger strength, which is wrong. Another concept was that the more difficult the exercise, the more advanced technique you learned. This is obviously not true; the only truth is that if you are advanced, you can play difficult material. Some of the simplest exercises (as we shall see) can teach the most advanced techniques, and that is the kind of exercise that is most useful.