A press release from Simon Fraser University in Vancouver reported a few days ago that a pair of biomedical physiologists have invented a “cruise control” device for runners. As far as I can tell, it’s basically a metronome that provides a beat for you to synchronize your strides with; it measures your speed (presumably via GPS) and increases the cadence if you’re going slower than your desired speed, and slows it down if you’re going too fast. Right now, it’s basically a clunky backpack prototype, but future versions might be, say, an iPhone app that provides music with a “sliding tempo” to keep you on pace.
Okay, so not a device I’d feel much need for, but I can see a potential market. One hesitation, though. The entire device is predicated on the following assumption:
“We know that for higher running speeds humans prefer higher step frequencies,” says Snaterse. “This relationship can be inverted – for higher step frequencies, humans prefer higher speeds. The cruise control for runners uses this principle.”
Is that really true? There’s a lot of dogma floating around the running world that running speed is essentially independent of stride rate — if you go for a jog and gradually pick up speed until you’re nearly sprinting, your stride length will get longer and longer but your stride rate will stay essentially unchanged. For example, check out this recent post from Amby Burfoot’s blog about the potential benefits of shortening your stride:
Most of us, when we increase pace, increase stride length much more than stride rate. So our stride rate stays roughly the same at different paces, slow and fast.
Now, that doesn’t necessarily mean the reverse is true. It’s possible that when you increase speed, your stride rate stays the same, but when you increase stride rate, you speed up. And there’s some evidence that other effects might crop up when people exercise while listening to music — for example, I wrote about a study where British researchers secretly sped up and slowed down workout music by 10% and people on exercise bikes sped up and slowed down without realizing what was happening. If we’re dealing with a cruise control that, by design, is intended to make only small corrections to your pace, maybe a small effect like that is sufficient.
What does the actual research say? It’s harder to dig up than I expected, partly because it’s such an “old” question that some of the relevant studies aren’t online. Here a description of an older (1974) study from a 2009 paper:
Saito et al.  showed that trained runners increased their speed to 7 m/s [2:22/km, 3:50/mile] by lengthening their stride, whereas untrained runners increased stride length only up to 5.5 m/s [3:02/km, 4:53/mile]; any further increase in running speed was achieved primarily by increasing stride rate.
In other words, you have to be sprinting pretty darn fast before you start increasing stride rate instead of stride length. Still there must be some better and more recent data out that show the typical relationship between speed and stride rate — if anyone knows where I should be looking, please let me know!