Tremolos (Beethoven's Pathetique, 1st Movement)
Tremolos are practiced in exactly the same way as trills. Let's apply this to the sometimes dreaded long octave tremolos of Beethoven's Pathetique Sonata (Opus 13). For some students, these tremolos seem impossible, and many have injured their hands, some permanently, practicing them. Others have little difficulty. If you know how to practice them, they are actually quite simple. The last thing you want to do is to practice this tremolo for hours in the hopes of building endurance -- that is the surest way to acquire bad habits and suffer injury.
Since you need the octave tremolos in both hands, we will practice the LH and alternate with practicing the RH; if the RH catches on faster, you can use it to teach the LH. I will suggest a sequence of practice methods; if you have any imagination, you should be able to create your own sequence that may be better for you -- my suggestion is just that: a suggestion for illustration purposes. For completeness, I have made it too detailed and too long. Depending on your specific needs and weaknesses, you should shorten the practice sequence.
In order to practice the C2-C3 tremolo, first, practice the C2-C3 octave (LH). Bounce the hand up and down, comfortably, repeating the octave, with emphasis on relaxation -- can you keep bouncing it without fatigue or stress, especially as you speed it up? If you get tired, find ways of repeating the octave without developing fatigue by changing your hand position, motion, etc. If you still get tired, stop and change hands; practice the RH Ab4-Ab5 octave that you will need later on. Once you can bounce the octave, 4 times per beat (i.e., include the correct rhythm) without fatigue, try speeding it up. At sufficient speed, you will develop fatigue again, so either slow down or try to find different ways of playing that reduces fatigue. Change hands as soon as you feel tired. Do not play loud; one of the tricks for reducing fatigue is to practice softly. You can add dynamics later, once you have the technique. It is extremely important to practice softly so that you can concentrate on technique and relaxation. In the beginning, as you exert yourself to play faster, fatigue will develop. But when you find the right motions, hand positions, etc., you will actually feel the fatigue draining out of the hand and you should be able to rest and even rejuvenate the hand while playing rapidly. You have learned to relax.
Now add the parallel set exercises. First the 5,1 parallel set. Start with the repeated octaves, then gradually replace each octave with a parallel set. For example, if you are playing groups of 4 octaves (4/4 time), start by replacing the 4th octave with a parallel set. Soon, you should be practicing all parallel sets. If the parallel sets become uneven or the hand starts to tire, go back to the octave to relax. Or change hands. Work the parallel sets until you can play the 2 notes in the parallel set almost "infinitely fast" and reproducibly, and eventually, with good control and complete relaxation. At the fastest parallel set speeds, you should have difficulty distinguishing between parallel sets and octaves. Then slow down the parallel sets so that you can play at all speeds with control. Note that in this case, the 5 note should be slightly louder than the 1. However, you should practice it both ways: with the beat on the 5 and with it on the 1, so that you develop a balanced, controllable technique. Now repeat the whole procedure with the 1,5 parallel set. Again, although this parallel set is not required to play this tremolo (only the previous one is necessary), it is useful for developing a balanced control. Once both the 5,1 and 1,5 are satisfactory, move on to the 5,1,5 or 5,1,5,1 (played like a short octave trill). If you can do the 5,1,5,1 right away, there is no need to do the 5,1,5. The objective here is both speed and endurance, so you should practice speeds that are much faster than the final tremolo speed, at least for these short tremolos. Then work on the 1,5,1,5.
Once the parallel sets are satisfactory, start playing groups of 2 tremolos, perhaps with a momentary pause between groups. Then increase to groups of 3 and then 4 tremolos. The best way to speed up the tremolos is to alternate between tremolos and octaves. Speed up the octave and try to switch to the tremolo at this faster speed. Now all you have to do is alternate hands and build up endurance. Again, building endurance is not so much building muscle, as knowing how to relax and how to use the correct motions. De-couple the hands from your body; do not tie the hand-arm-body system into one stiff knot, but let the hands and fingers operate independently from the body. You should breathe freely, unaffected by what the fingers are doing.
In the end, you will be playing the tremolo mostly with hand rotation, which means that the hand will be rotating back and forth around an axis through the forearm. That axis is a straight line extending through the hand towards the fingertips. Therefore, the palm should extend straight out from the forearm and the three free fingers (2,3,4) should stretch out like the spokes of a wheel and almost touch the keys; do not lift them up, except to the extent that they move with the hand rotation. For those with small hands, the hand rotation should be accompanied by a small sideways motion so that you can reach the keys without having to fully stretch the fingers. This should help reduce stress.
For the RH, the 1 should be louder than the 5, but for both hands, the softer notes should be clearly audible, and their obvious purpose is to double the speed compared to playing the octaves. Remember to practice softly; you can play louder whenever you want later, once you have acquired the technique and endurance. It is important to be able to play softly, and yet be able to hear every note, at the fastest speeds. Practice until, at the final speed, you can play the tremolos longer than you need in the piece. The best way to develop this endurance is to practice at even faster speeds. The final effect is a constant roar that you can modulate in loudness up and down. The lower note provides the rhythm and the upper note doubles the speed. Then play the different successive tremolos as indicated on the music. That's it! You are done!