It's a well-accepted approach in sports that the fastest way to a finish line is with negative splits—i.e. to get faster as the race progresses. In the ultrarunning community, this approach more resembles a sacred mantra than a mere guideline.

You see, in ultramarathons, the penalties for going too hard are just so much worse than in other running races. After all, even in a marathon, if you hit the wall at 20 miles, you have 6.2 miles to tough it out. But in a 50 miler? Now we're talking about 30 miles. That's a lot of toughing it out. (Trust me.)

A couple of days ago, I found myself asking, How good are ultrarunners at following this advice?


To answer this question, I looked at the splits from last year's Western States Endurance Run. A 100-mile race from Squaw Valley to Auburn, CA, this is one of the premier events in the ultracalendar and often features a fairly stacked field of elite runners. From the perspective of answering my question, this race has several benefits. A runner has to qualify with at least a 50 miler, and so this isn't anyone's first rodeo. The race has also been around for 40 years (!!), which means many different datasets, spanning generations of ultrarunners and a wide variety of race conditions. Western States is also thought to have a faster back half1, thus providing the perfect opportunity for negative splitting. And, finally, I was at this year's race, crewing for my coach, Ian Sharman, and I'd be lying if I said that I weren't still riding that wave.

First, I calculated the speed for (roughly) each quarter of the race for all the runners who finished:


What was immediately clear is that all of the red points (representing the speeds in the first 24 miles) are pretty well separated from the other three colors, especially for the later finishers. In other words, for nearly all runners, they ran faster in the opening quarter than in the rest of the race.

So here's the good news. For every ultra runner who can relate to that feeling of hemorrhaging time in the second half, of just barely hanging on: You're not imagining things, and you are most definitely not the only one.

And here's the bad news: Only ten runners (of 277 who finished) were able to negative split Western States.

This is a game of minimizing losses.

To see the slow-down a bit more clearly, I next separated the runners by their finishing position, and then looked at their pace in each quarter of the race.

(In case you're not familiar with box-and-whisker plots, here's the quick run-down of how to read them: The bars span the range of values; the line in the box is the median value; the lower edge of the box is the 25th percentile, and the upper edge of the box is the 75th percentile. These plots are useful for looking distributions, especially when comparing a lot of groups like we are here.)

It's hard for me to look at this plot and not think that pretty much every single runner (with the exception of the top 10, maybe 20) is going out too hard . I mean, way too hard.For instance, take the 151-200 group: The slowest pace in the first quarter for this group was faster than the quickest pace in the second quarter of the race. The median first quarter pace for those runners (13.3 min/mile) was closer to what the elites were doing at the start (9.1 min/mile) than what those same mid-packers did at the end of the race (19 min/mile).2 3

And it's not just the mid-packers that do this—pretty much every runner slowed down in the last quarter:

As an ultrarunner, I've got to say that this is a rough graph. People are going minutes per mile slower at the end of the race. And there are several runners whose difference in speeds is larger than the speed they had in the first quarter. Yeah, "hemorrhaging time" sounds about right.

So let's call a spade 'a spade' here. Most runners just have no business throwing down 13 min/mile splits in the beginning of Western States. And yet more than 50 percent did.

Not only do people go slower in that last quarter, but the further down the pack a runner is, the larger this difference is, both in absolute time and as a percentage of their original pace:

As I said, some pretty rough numbers. Let's take that same 151-200 group: Their pace was, on average, 50 percent slower in the last quarter as it was in the first quarter. The only group that seems to escape relatively unscathed is the elite, top 10 group. Although even these, as a group, get slower through the race—they just slow down a lot less.

On average, runners went 20 percent slower in the second half of the race. So much for negative splits. In a sense, though, these results aren't news: Pretty much every ultrarunner knows that paces that felt easy in the beginning can be impossible to hold onto at the end.

Yet, what I find most surprising is that, despite knowing this, experienced runners still went out too hard. By a lot.

So for my next ultramarathon I'm going to go out slow. Slower than I think I should, even after writing this post. 4 Because there is one bright side. If I pace myself right and manage to slow down by only 10 percent in the second half, I'll be beating the house, and I'll pass a lot of runners.

And that sounds like a lot of fun.

1The runners are greeted with 2000 ft of ascending and their maximum elevation in the first four miles.

2 Yeah, you really do slow down that much. The numbers don't lie.

3 For the non-ultrarunner. I know, these paces seem pretty darn slow. But take a look at the race profile: Runners climb more than 18,000 ft, and descend around 23,000 ft. And, although it may sound easy, holding a 15-minute-per-mile pace through the mountains for over 24 straight hours is hard.

4 In fact, I should probably apply a variant of Hofstadter's law quite liberally here.

Olivia Rissland writes about science on her blog, How To Be A Scientist. She heads a molecular biology lab at The Hospital for Sick Children Research Institute in Toronto and runs ultramarathons (always starting them at a sensible pace).

Originally published on How To Be A Scientist, republished here with permission.