S.U. asked:  I began learning to swim at a TI Workshop in November 2008 and can now swim 19 strokes for 25 meters. Now that my stroke is reasonably efficient, I’ve begun using a Tempo Trainer in whole-stroke practice to build speed.  At what  tempo should I set it ?   I read somewhere to start at 1:40,  and experience has shown that the slower I go the better my stroke.

Your instincts about how to improve are right on. After five months of letting your newly efficient habits "register" in your muscle memory, this would be a good time to begin honing a sense of rhythm for your new stroke. As you, do practicing at a range of tempos, rather than just one, will bring two benefits: (1) train your nervous system for more adaptability — this will be useful for finding the most efficient way to swim different distances, different speeds, and even different conditions — smooth or choppy water for instance if you will swim outside the box (in open water; and (2) give you more information about te optimal combination of SL and SR for any speed. 

Let’s consider first your experience that stroking more slowly leads naturally to greater efficiency. I’ve seen the same thing duriing a few clinics for swimmers with little or no TI experience, using the Tempo Trainer. I have them start by counting strokes for a 25. Most take 20-25 SPL in a 25 yd pool. Then I give them 2 to 3 rudimentary focal points to start, and one drill (swimming with only the index finger ) to smoothe out the most glaring inefficiency. Then I start them with the TT, set at 1.40. Within minutes the group (who are at ) has increased Stroke Length (reduced SPL) by 10% to 20% from their first lap, which is a really striking improvement to be achieved so quickly and simply.

Since you’re already more efficient than they are — taking 19spl in a 25m pool – you could probably start at 1.20 to 1.30. Here’s an interesting exercise that will get you started on developing a more adaptable nervous system and help you find the Sweet Spot in stroke length and rhythm.

Gradually slow your tempo from the starting point and see what happens to SPL. For instance, do 2-3 x 25 at 1:20, then repeat several 25s at say 1.22, 1.24, etc. Stay at each tempo until you feel you’ve optimized your SPL at that tempo.
After you get to 1.30, reverse the process, going 1.28, 1.26, etc until back to 1.20. How does your SPL — and sense of efficiency and ease — compare with when you began? Each time I’ve done this my SPL has been better, sometimes strikingly better, at the end of the set. It’s a real insight-producer.