Very few classical guitarists have a native ability to alternate i and m VERY fast, as is required in much virtuoso level music. The signal scales passages in the major concerti, those scale bursts in the works of Piazzolla and Rodrigo and others, the simple, raw ability to play scales REALLY fast: this style of playing is commonly regarded as out of reach for many students.
I had a vivid experience when I was an undergraduate with respect to fast playing. I was scheduled to play Rodrigo's Tonadilla for two guitars (with my roommate and duo partner at the time, Nicholas Goluses) in a masterclass for Oscar Ghiglia. I was playing the first guitar part. The fast scales in the third movement were simply beyond my level. There was no way I could play that fast. And certainly not in the one week I had left to prepare. So I invented the excersise I'll describe here and did it every day. In one week, I had the scales moving fast enough, and reliably enough to pull off the piece. I've been using this method ever since with great success.
In my experience, the barrier we hit when trying to play fast is often of our own making. The most common method of teaching students to play fast is doomed to fail. Using scales as an example, it is common to play a given scale slowly, with a metronome, let's say 16th notes at q=80. Then "notch it up" little by little, trying over and over again, at increasingly faster tempi. Inevitably, the student reaches a break-point. The hand just tightens up too much and further increases in speed become impossible. Some hit this wall at q=126, for others it might be at q=92. This, of course, is not fast enough for the type of playing referenced above.
The method I "invented" turns that approach on its head. In this method, you start too fast and gradually slow it down. It goes like this.
The musical gesture I begin with is a simple five-repeated-note burst. I think of it as "four-e-and-a-one."
I always start with a high note on the first string. This guarantees a tight, quickly responsive string (the vibrations make a smaller arc, making accurate nail contact more reliable), and eliminates the potential distraction of bumping into the adjacent upper string. For instance:
I encourage using rest strokes, or a low free stroke very similar, and a close thumb placement, for instance, resting on the third string.
Set the metronome to 200. Conduct this tempo in a simple down-up pattern and try to vocally produce the five 16th-notes in time. I use "tick-a-tick-a-tick," but use whatever pneumonic device your mouth can produce clearly.
Then play the five-note gesture, i-m-i-m-i. Take a few beats off and try again. And again. And again. Focus on starting exactly with the pulse. You most likely won't be able to do it, but try anyway. The sound may be crummy and undisciplined. This is ok. It's about loose movement and hearing the speed.
Now turn the metronome down to 194 and try again, always conducting and singing the rhythm first. Do the little speed burst several times. Take two beats off between tries as in the example above, or pick up the pace, taking one beat off instead, and feel it in 3/4.
Change notes occasionally to keep it interesting, but stay high on the 1st string.
Relax for a moment. Turn the metronome down to 188. Do the routine again, always starting with conducting and "singing" the rhythm. At each iteration of the routine, lower the speed by 4-6 clicks. By the time you get down into the 170's it is likely you will be able to "lock in" the 16th notes consistently. Repeat the process into the 160's and the 5-note gesture is reliably in time. (Now you're playing 16th notes at 160!!).
But of course, there is more to it than this. Once you have found the tempo at which the five-note burst can be played reliably in time, it's time to add the left hand. One of the reasons scale playing is difficult is that it requires perfect coordination of the movements of both hands. Address this little-by-little. Change the fifth note in the group. Do it in every combination to exercise this coordination thoroughly. It will sound like this:
If it doesn't work cleanly at your starting tempo (around q=160), move the metronome a little slower until you can do this reliably.
Next, play one-string scale fragments in different combinations. This will be a bit harder, and may require the metronome be moved lower, perhaps into the 140's. Try each figure several times.
After that, it's time to introduce string crossing. Start with a descending 5-note scale fragment set up so that there are two notes on the first string and three on the second, and be sure to start with i, as in this example:
Try ascending figures, similarly set up, to avoid crossing backwards. If it is impossible to do it at the tempo you've got set, slow it down some more. But just a few notches. This will generally be comfortable in the 140's-130's (give or take!) Try each combination a few times:
The next level is to play an entire octave, moving for the first time to nine-note bursts (yes, an octave plus one note). These figures will inevitably include one backwards right hand fingering cross. Here are some examples:
By the end of this sequence of activities, you should be playing these scales comfortably and reliably, perfectly in time, at a tempo much faster than you could before. The reason is simple: when starting at 80, 140 seems hopelessly fast and out of reach, whereas when starting at 200, 140 seems slow and lazy. This is so obvious it seems silly to describe it, and yet very few musicians are practicing it this way.
There are both psychological and physiological components of this phenomenon, and they are intertwined. When we start fast, we are compelled to relax our muscles or the alternation simply isn't possible. You may find it exceptionally difficult to relax sufficiently to get the fingers to move quickly like that, but with repeated efforts, it will eventually happen. It is a mysterious sensation: after struggling with many unsuccessful attempts, suddenly, on one random effort, you get it. It may be difficult to replicate it in that sitting. The next day you may get it again, perhaps twice. Eventually, you learn what it feels like to "let go" and can switch on this curious combination of highly controlled fine motor control and total looseness at will.
Further, slowing the tempo down gradually permits us to relax our muscles a bit more with each slower tempo. As noted, the relaxing of the muscles is what permits the fast, easy alternation of the fingers. When we begin at 200, our perceptions become finely tuned to that high velocity, making 140 seem slow by contrast. We know empirically that it is "fast," but it seems slow. It is this perceptual shift that enables us to relax enough to play the excercise. This is the same perceptual shift that causes us to regard 40 degrees as cold in the Fall but warm in the Spring. It is relative.
Of course there are other important components of reliable high-velocity playing: the length and shape of the nails, the angle and position of the right hand, the angle and position of the left hand, the firmness or looseness of the right tip joints, the staccato preparation of each note, the size of the strokes and general efficiency of the movements, and so on, but those issues will have to wait for another post.
In the meantime, consider this excersise an invitation to try getting your alternation to another level. It worked for me. It just might work for you!