We know that quite a few of you like a bit of science behind your swimming, so lets take a look at an important piece of research conducted by Southwestern University, Texas . Even if you're not a numbers person bear with this - it's not too complicated!
Scott McLean and his team asked ten college swimmers of a range of abilities to swim in a flume tank (an endless pool). The flume was fixed to a speed of 1:40 /100m so no matter what, the swimmers had to maintain that speed, they couldn't slow down or speed up.
As a starting point, the study recorded the swimmer's natural stroke rate - i.e. how many strokes they take per minute (SPM) at 1:40 /100m. If you own a Tempo Trainer Pro or Wetronome you will have a good idea of what yours is, most age group swimmers are in the range 50 to 65 SPM.
Each swimmer was then asked to swim at 10% below their natural stroke rate and 20% below it, controlled by a Tempo Trainer beeping the timing to them. Since the actual swimming speed was fixed at 1:40/100m, as they slowed their stroke cadence they had to lengthen their stroke to maintain their speed.
For each swimmer they also sped up their stroke rate to 10% and 20% above their natural stroke rate. To keep the same speed, the swimmer had to shorten their stroke.
For each test oxygen uptake, heart rate and perceived exertion (how hard it felt to the swimmer) where recorded to give an indication of economy. They also recorded the kick rate (kicks per stroke cycle) and to keep the study fair the order of the tests was randomised.
OK that's the technicalities, what did the results say? First lets look at what happened as the swimmers lengthened out their strokes at a lower stroke rate. If you believe that a longer stroke is more efficient, then we'd expect the swimmers to become more economical:
The fascinating result was that as the stroke lengthened, oxygen uptake, heart rate and perceived exertion all rose significantly. The kick rate also increased significantly, suggesting the swimmer had to start kicking harder to maintain speed in the dead-spot created between strokes. All of these things are strongly suggesting that trying to maximise stroke length makes you less efficient, not more efficient.
Now let's add the data onto the right side of the graph as stroke rate increases:
Heart rate, oxygen uptake and perceived exertion all dropped slightly at a 10% increase in stroke rate and then rose a little at a 20% increase. When you get into the maths, the increases at 20% above natural stroke rate are not statistically significant but the mean does rise.
1) Swimmers tend to naturally select the slowest stroke rate from the range that is economical for them.
2) Don't overly lengthen your stroke below that point by trying to over-glide, it actually makes you less efficient, not more.
3) You are likely to be able to lift your stroke rate by 10% without losing any efficiency and for some swimmers as much as 20%. In open water the ability to swim at a higher stroke rate is a huge advantage as it helps you punch through wake and chop created by other swimmers. Try and overly lengthen your stroke in open water and you can literally be stopped dead in the gap between your strokes and you will slip to the back of the field despite working hard.
4) The increase in kick rate with longer strokes correlates well with what we see in the elite swimming world where Smooth swim types with a long stroke style (e.g. Ian Thorpe, Michael Phelps, Ross Davenport) use very powerful kicks to help power them through any gaps in propulsion. The thing you don't want to attempt is a long over-gliding stroke with a two beat kick as you simply decelerate far too much between strokes.
 McLean SP, Palmer D, Ice G, Truijens M, Smith JC. (2010). Oxygen uptake response to stroke rate manipulation in freestyle swimming. Med Sci Sports Exerc., 42(10):1909-13.