Each year, since the advent of GPS watches, a massive debate ensues at my local Memorial Day 10K. Why, everyone wants to know, does one person’s watch say 6.31 miles and another’s says 6.42 miles?
“This is an outrage!” they cry. “A travesty! The course is inaccurate!”
Inevitably, if you’ve had a version of this debate before—and you probably have—someone has argued the problem is that you aren’t running the tangents. It’s possible, but what the heck are they talking about?
In geometry, a tangent is a straight line that touches a point on a curve or a circle, but does not cross it. For road racing purposes, a tangent is the line that touches the inside of a curve.
“You want to follow the line that’s closest to that corner,” says Dave Munger, the author of the blog, Science-Based Running. “If you run the tangent, you’re actually running the shortest distance possible.”
Some estimates suggest that running the outside of a turn, as opposed to the inside, can turn out to be up to 40 feet farther. That can add up to nearly a half-mile of extra running over a marathon—but it also depends on the course. If there are more turns or wider roads, then the difference between the inside and outside of each of those turns adds up.
This is all fairly straightforward and logical: you want to run the shortest distance possible around a curve. In fact, it’s so logical that in most races you rarely see anyone running around a corner along the far outer edge of the street. Instead, everyone bunches up at the curb.
But that doesn’t mean they’re really running the tangents.
If you’re running down the street in a race and you make a right turn followed by a left turn, then the shortest route from one corner to the next would be a diagonal straight line. However, that’s not the route most people take. Most will run on one side of the road and then cut in at the last second. Or, they’ll weave all over and still come in for the turn.
The other problem is that in big races, with so many people going different speeds or stopping at aid stations, it’s hard to run anything resembling a straight line. That’s why mid-pack runners often end up running farther than those at the front—they have to dodge and weave the whole time. It’s also because they’re following the crowd.
“Even at fairly large races, I can usually run the tangents fairly easily,” says Munger, simply because not many people really do so. “I’ll end up being out by myself on the left side of the road.”
In the New York City Marathon, there is a blue line painted down the middle of the road to mark the race course—but that’s not the line that’s used to measure it. Many of the elites follow the blue line from start to finish, even though no one is getting in the way of them running a shorter route.
Any USATF-certified course, says Steve Vaitones, the managing director for USATF’s New England region and a long-time course measurer, has to follow a standard measuring procedure that can be duplicated. Understanding how that course measurement works helps explain why you want to run the shortest route possible in a race.
When he measures courses for USATF certification, Vaitones rides a bike with a counter clicking one number for every wheel rotation. Before he starts, he calibrates how many clicks or rotations it counts over a known distance—typically over 1,000 feet. He does that four times to establish a baseline average. He then calculates from that average how many rotations it would be for one mile and adds 1/10 of 1 percent to that number as a “short course prevention factor.” Adding a buffer accounts for human error, he explains.
He then rides the shortest route he can along the race course with his calibrated counter, marking when he reaches each mile marker. He does that twice and uses whichever version is slightly longer (as long as the two attempts are within a given variance). He also calibrates again when he’s done.
The reason USATF requires bike measurements for its certification instead of walking with a measuring wheel or running with a GPS watch—besides the fact that commercial GPS watches are not always accurate to that high a degree—is to ensure that the shortest possible route is still the full advertised race distance. “You can ride a far straighter line than you can walk or run,” says Valtones. And the more experienced the measurer is, the tighter they can ride the shortest route.
It also means that unless you actually run the tangents, you’ll probably be running farther than you think.
There’s the added issue of losing speed around sharp corners or 180-degree turns, in which case it can be beneficial to take a slightly wider route to maintain momentum. But, in most instances, running the true tangents is an easy way to get a faster time, says Munger, “without even exercising.” And then you can argue with all your friends after the race about why they ran farther.
This article originally appeared on Competitor.com.