Our speed is a result of stroke length x stroke rate minus drag. Two of these factors (stroke rate and length) we have control over and one, drag, is just an unbiased party in the agreement that is determined by the forces of nature. The other two, stroke length and rate are simple enough to explain, yet despairingly difficult to put into practise.
The length of our stroke is determined by the length of our arms. Theoretically the maximum length of one stroke is the length of our outstretched arms. However, in practise the stroke length is shorter than that due to factors like hand entry, rotation and a shorter push. Logically, of course, shorter people have shorter strokes and taller people have longer strokes. The shorter our arms are the faster we have to turn them to cover the same distance so inadvertently creating a higher stroke rate and vice versa. Our stroke rate is how many times our hands enter the water in one minute. So, if you see 60 spm, this is referring to 60 strokes per minute, and is basically how quickly our arms turn.
If we put this into the equation, our optimum speed is then in the sweet spot where we can hold the length of our stroke at the highest possible stroke rate. So, it sounds easy! Right? Well it’s not. Holding the length in our stroke and pushing out completely is the technical aspect and hard to do as the stroke rate goes up. In fact, there is an inverse correlation between stroke rate and stroke length. As the one increases the other decreases.
This is the reason why as a beginner or mid-level swimmer your steady pace is marginally faster, if at all faster, than your easy pace. The trick is to keep the one without losing the other.
Stroke rates of professional swimmers (of course this is in competition and sprinting):
Michael Phelps, 72 spm, world record 200m Free 2008
Rebecca Adlington, 110 spm, 800m Olympic gold Free 2008
Try and swim at these stroke rates and you will see how very difficult it is to keep one's composure and technique. Ask your coach what their competitive stroke rate was and compare it to their height. You might be surprised.