Underwater Kicking: Some Numbers

These days everybody knows about that underwater kicking is a fast and necessary aspect of swimming. And we all know that the fastest swimmers generally have incredible underwater kick speeds and distances. But what surprises me over and over is the general lack of numbers involved with this knowledge.

How fast is it? How long can top underwater speeds be maintained. What’s the optimal breakout point? What’s the speed difference between underwater kicking and swimming.

I’ve done some tests on my own swimmers, viewed a ridiculous number of videos, and have done some research to try to come up with some of these answers. So far, just for backstroke.

Underwater Speeds

The key to underwater kicking is that the swimmer is not generating speed, but merely trying to maintain speed. This is an important distinction, as it affects how the body should move. (For more on this, see the second half of my post, More Than You Want to Know About Underwater Kicking).

There are two other things to keep in mind with underwater kicking:

• underwater kicking at its fastest is slightly faster than freestyle swimming
• underwater kicking speed cannot be maintained for long

Put these two things together and we can start to understand the underwater strategy of elite swimmers. Ideally, you want to breakout of the underwater phase when the underwater kick speed slows down to swimming speed, or 15 metres, whichever comes first. There is no advantage in staying under if you’re kicking slower than you can swim. However, speed, fatigue and a need for oxygen all pay a role in the underwater phase. We’ll try to find out how much of a role they play.

First, how fast is underwater kicking?

Here are the results for average speed to 15m for the fastest underwater swimmers in the first length of the 100 backstroke at the Olympics, going back to the start of the underwater era. Unfortunately, I just couldn’t find adequate videos for two of those women’s races. Keep in mind that this average speed includes the start phase which is much faster than underwater, but doesn’t last very long.

What surprised me about this graph is how relatively consistent the average speeds have been since 1992, with the one exception of 2008 when the now-illegal shiny suits were used. Our speeds haven’t really improved all that much, generally topping out at 2.54 m/s for men and 2.21 m/s for women (ignoring 2008).

But that number gives us a combined push-off and underwater phase. So I looked further into Matt Grevers 2012 100 backstroke win, and separated out the first 5 metres of each length from the next 10 metres.

Grevers averages 5.6 m/s for the first 5 metres including the start, and then dropped to 1.88 m/s for the rest of his underwater. Then he averaged 4.0 m/s for the 5 metres including the pushoff, and then dropped off to 1.89 m/s.

So it seems we’ve found that 1.9 m/s is a reasonable underwater speed for elite backstrokers.

Underwater Past 15m

The above results however, are capped at 15m as that’s all that the rules allow. But one swimmer is famous for his 50m long underwater kicking, and that’s Hill Taylor of the US. Here’s the video of Taylor’s 2008 50 Backstroke LC race that he did completely underwater. 23.10 seconds! Keep in mind that he was wearing a now-illegal full body suit.

Let’s analyze this race in terms of speed. I’ve watched and timed this video dozens of times and averaged the results, so hopefully my timing is somewhat accurate.

We can see that his initial speed up to 7.9 metres is impressively just over 3 m/s, but this also includes the start. From there to the 15 metre mark he is maintaining about 2.4 m/s: a speed that is truly astonishing compared with Grevers, although he does have on a full body suit.

But notice the significant reduction in speed over the 50 metres. By 35 metres he had dropped to 1.8 m/s, and finished at 1.7 m/s. Even a phenomenal underwater kicker like Hill Taylor can’t maintain his speed over the 50, or even over 35m. In fact, if he had popped up shortly after the 25m mark and swam backstroke, he might have gone faster than his 23.10 seconds.

So even a remarkable underwater kicker has significant speed loss over time.

Short Course Underwater Kicking

Short Course presents enormous opportunities for underwater kicking. So here the question is whether an elite backstroker can hold that high initial speed off every turn. To look at this, I viewed Ryan Murphy’s incredible 1:36.77 for 200 yard backstroke. But what interested me was whether he was able to sustain his underwater off all of the walls.

I measured the time to the 7.9 metre mark (which is 15m from the other end) for each length, as well as each 25 time. This gave me 2 splits for each length: a pushoff/underwater time, and a time for the rest of the length which was mainly swimming. I adjusted that second time to account for the flip turn (took 0.9 seconds and 0.7 m off that split) and then calculated what is basically an pushoff + underwater speed, and a swim speed. Here are the graphs.

We can see that while his swim speed was relatively steady from the second through eighth lengths, his underwater speed did drop slightly more. Even more important is that his distance off the wall drop considerably over the race. In other words, even one of the best underwater kickers in the world was unable to keep up the underwater distance and/or speed over 8 lengths.

Unfortunately, I couldn’t find a video of Missy Franklin’s NCAA 2015 200 Backstroke win.

Summary

There is no doubt that underwater kicking has caused a revolution in swimming. And no doubt that we will continue to get better and better at it. But the numbers would suggest that we are already starting to reach a plateau with those speeds, probably around 1.9 m/s for pure underwater kicking.

We can also say that underwater speeds cannot be maintained over 50 metres (possibly due to the use of big leg muscles plus the impact of oxygen debt). And that the underwater speed during the many lengths of a 200 short course appear to drop off faster than swim speeds.