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One quick graph from a new study by Robert Chapman and his collaborators at the University of Indiana, just published online in Medicine & Science in Sports & Exercise:
This is data from 18 elite runners (12 male, 6 female), showing their stride frequency as a function of speed. For reference, 3.00 Hz corresponds to 180 steps per minute; 3.3 Hz corresponds to about 200. On the speed axis, 4.0 m/s is 4:10 per km, and 7.0 m/s is 2:23 per km. In other words, these are FAST paces. The key point: they get faster, in part, by quickening their cadence. There’s no magic cadence that they stay at while lengthening their stride to accelerate.
Interesting wrinkle: the women have faster cadence than the men at any given speed. Chapman assumes this is partly due to the fact that the men are taller — but even normalizing by height doesn’t quite erase the difference. (And that even ignores the argument that, as I blogged about here, cadence should be proportional to the square root of leg length, not leg length itself.) The remaining difference, Chapman hypothesizes, could be due to “application of greater ground forces by the men or differences in muscle fiber type distribution.” This makes sense: if you’re stronger (as the men, on average, will be), you’ll have stronger push-off, longer stride, and thus shorter cadence at any given speed. But it seems pretty clear that height plays at least some role.