Improving swim pace is no mystery. The number of yards you train, or the heart rate at which you swim them, are only peripheral factors. The determining factor is what we call the 'math of speed.'
I.E. Stroke Length x Stroke Rate = Velocity, or SL X SR = V
. Velocity over time equals pace.
We can calculate all of these quite simply by plugging in Strokes Per Length (SPL) for SL and determine or adjust SR with great precision with a Tempo Trainer.
When you say you can't break 1:43 pace for 100y are you referring to practice pace? I.E. Please tell me a typical set. E.G. If you swim a series of 100y repeats in the pool,
1) How many repeats in a typical set?
2) On what interval do you swim them?
3) What pace times do you swim? And how consistent are your pace times on the set?
4) What is your stroke count on such a set? And how consistent is your stroke count?
E.G. Might you swim 10 x 100 on an interval of 2:00, averaging 1:43 at an average SPL of 20 strokes per 25 yards.
In that set how great a range would you have from slowest to fastest 100. Can you keep them within say 2 to 3 seconds. Or is the range closer to 10 seconds?
What about stroke count? Can you keep your SPL range within 3-4 stroke counts, say 18 to 21? Or would the range be greater than that?
And finally, how tall are you? There should be a relationship between your SPL and height. Check here
for our Green Zone height-indexed
charts of efficient stroke counts in 25y or 25m pools.
If you provide answers to these questions, we can far more specifically and accurately assess what is keeping you from swimming faster and suggest prescriptions.
I think you will learn you have far more potential to coach yourself effectively and improve steadily, possibly supplementing your self-coaching with a periodic technique checkup with a TI coach--which will be available on-line shortly.