The Broad Street Run is mostly downhill, but how much does that really help?

Perhaps it’s a joyous blast from the Temple University marching band that gets your motor running, or the sight of William Penn’s statue rising from the skyline as you plod through the heart of Philadelphia.

At this year’s Broad Street Run on Oct. 10, there’s the added appeal of making up for the pandemic-induced cancellation of 2020. But year in and year out, a primary draw of the 10-mile race is that it’s (mostly) downhill.

Yes, there’s that quarter-mile climb starting at West Courtland Street, and a few other stretches of uphill unpleasantness. But for the most part, the course is smooth sailing, declining in elevation by an average of 15 feet with each mile of pavement.

Is that enough to make a difference in racers’ performance?

The answer, according to biomechanics experts, is yes. While the gentle downhill slope is barely enough to notice in some spots, racers will use slightly less energy than if they were running on a flat surface, and can — if they so choose — “spend” that energy savings to run a bit faster.

Exactly how much faster requires some explanation. To get up to speed, we spoke with researchers Paul DeVita at East Carolina University, Max Paquette at the University of Memphis, and Jinger Gottschall, a science director at two fitness companies: Wahoo Fitness and Les Mills International.

As with anything else involved in moving an object from point A to point B, the answer involves physics. For the science-averse, we promise that the explanation will be fairly painless. Or at the very least, less pain than you feel upon rounding the turn at City Hall.

And a reminder: However much the roadway is inclined, the race is still 10 miles. Do not try it without proper training and a clean bill of health.

A simple way to approach the downhill question is to think of running as pushing a big wooden block along the street, like those classic physics problems that teenagers tackle in high school.

No mystery there: The job is easier on a downhill slope.

The steeper the angle, the more that gravity is pulling the block (or the runner) in a downhill direction, said DeVita, who runs a biomechanics lab and is a professor in East Carolina’s kinesiology department.

Except the human body is not a block of wood, but a convoluted assortment of bones and joints exerting forces every which way.

“The reason why people like me have jobs is because we’re not blocks of wood,” he said. “The mechanics is the simple part. It’s the bio part that’s tricky.”

» READ MORE: Broad Street Run course changes for 2021

For runners (and pedestrians), it turns out that gravity also helps in a second way. To understand it, imagine the human body as a series of stiff, interconnected springs, said Gottschall, formerly an associate professor of kinesiology at Pennsylvania State University.

With each step, a runner is compressing these springs — the tendons, bones, and other joints — deriving a slight boost when they rebound. On a downhill slope, when the runner’s foot lands at a lower elevation with each step, the springs are compressed even more, Gottschall said. And as a result, the rebound effect is greater.

“You can think of that distance it springs back as an energy savings,” she said. “It’s saving the muscle from having to do the work.”

Up to a point, that is. On steeper slopes, the muscles compress so much that the runner must spend a fair bit of energy to put on the brakes, eventually wiping out the energy “savings” from the rebound effect, she said.

In addition to the energy cost, running down a steeper slope puts more mechanical wear and tear on the body than running uphill, said Paquette, a former collegiate running star who is an associate professor at Memphis. The calves, quadriceps, glutes, and hamstrings must contract in a way that biomechanics researchers refer to as “eccentric.”

“Despite producing tension and force, muscle fibers are lengthening to prevent our joints from collapsing like cheap lawn chairs,” he said.

But the decline on Broad Street is gentle enough that neither of these issues — the energy required to slam on the brakes and the added muscular wear — is likely to be a factor for most runners, Paquette and Gottschall agreed.

Given the complexity of the human body, and the fact that runners have different gaits and fitness levels, calculating the net impact of all this at a specific gradient is not straightforward.

Instead, researchers have resorted to measuring runners’ performance on a treadmill.

In the best-known study of the topic, published in 2002 in the Journal of Applied Physiology, a team of European scientists tracked the energy usage of runners at gradients ranging from 45% downhill to 45% uphill. Those extremes are pretty steep, corresponding to a 45-foot change in elevation for every 100 feet of roadway.

The average slope on Broad Street is far shallower — just 0.28% downhill, meaning a 0.28-foot decline in elevation (about 3 inches) with every 100 feet of roadway. (The numbers are based on satellite data and other measurements from Strava, the fitness-tracking company.) While that specific gradient was not among those measured by the scientists, the study results nevertheless can be used to get a rough idea of the downhill boost on the 10-mile course.

That’s because the scientists plotted their measurements on a graph, then came up with an equation to fit the data.

It’s a doozy: a six-term polynomial that we can’t fully reproduce with our publishing system.

But the end result is easy to grasp. If you plug in Broad Street’s average gradient, the equation suggests that a race entrant uses about 1.5% less energy than when running on a flat surface. (Props to Guillaume Millet, a professor of exercise physiology at Jean Monnet University in Saint-Étienne, France, for checking our math.)

So again, runners could opt to finish the race feeling a bit less fatigued. Or they could use the energy savings to run that much faster.

In practice, the magnitude of the Broad Street boost likely is not exactly 1.5%, as the gradient varies over the 10-mile course — including those occasional uphill spots.

Psychology also likely plays a role, Paquette said. Inexperienced runners may hit an uphill patch and lose heart, for example. Or upon reaching a downhill stretch, they may feel extra motivation.

But on average, the science supports what runners have been saying for decades: that the Broad Street Run’s course allows participants to go slightly faster than usual.

So lace up, racers. It’s time for those “springs” in your legs to get to work.