The Science of Racing Flats

Dusting off a pair of racing shoes is a common ritual for serious road runners at the start of the spring.  If you did most of your races last year in your regular training shoes, you might be wondering about the advantages and disadvantages of wearing lightweight racing flats (or simply “flats”) for races or workouts.  And, with the proliferation of high-tech lightweight foams and the explosion of the minimalist movement, thin, lightweight running shoes are easier than ever to find.

Today, we’re going to look at what some of the latest science has to say about how lightweight racing flats affect the way you run and whether buying a pair would be a good investment for you.

The Science of Racing Flats

Is there a performance advantage wearing racing flats?

The first issue at hand is the performance advantage they confer.  The logic is pretty simple: lighter shoes lead to better performance since extra weight requires your legs to do unnecessary work.  But, one might reasonably worry that there would be a point at which a shoe that was too thin would decrease performance because it’d affect your running stride.

One very recent study which tested both of these propositions was published by Jason Franz, Corbyn Wierzbinski, and Rodger Kram at the University of Colorado earlier this month.¹  In their study, Franz et al. asked 12 runners who had done substantial amounts of training both barefoot and in shoes to run on a treadmill barefoot and with racing shoes.

Then, while measuring their oxygen consumption, the researchers added lead weights either to the shoe or the top of the foot and measured the response.  As predicted, an increase in weight on the foot was associated with an increase in oxygen consumption—a bad thing if you are trying to run fast.  Every 100g of increased shoe (or foot) mass was associated with about a 1% increase in oxygen consumption.

Interestingly, even though the subjects were well-adapted to barefoot running, wearing 150g shoes created a 2.1% decrease in oxygen consumption, representing a performance advantage.  Why would this be?

Softer surfaces lead to greater performance

One factor may be that wearing a shoe decreases the stiffness of the surface you’re running on.  Research by Amy Kerdok and her coworkers at Harvard University highlights how surface stiffness can modify performance.

In a paper published in 2002, Kerdok et al.² measured the oxygen consumption and biomechanics of eight men who ran on a custom-built treadmill which was able to adjust its stiffness.  Over a wide range of surface stiffnesses, the runners’ biomechanics were surprisingly stable—the only difference was the stiffness of their legs, which inversely tracked the surface stiffness: the softer the surface got, the stiffer their legs became.  Softer surfaces were also associated with lower oxygen consumption. 

In this case, the researchers pointed to the “rebound” of energy from the more compliant surface setting for increasing performance.  While the analogy with running shoes is not perfect, keep in mind that during a marathon (or any road race), you have no way of changing the stiffness of the road, but you can alter the overall stiffness beneath your feet via your shoes.  So perhaps one reason wearing a cushioned shoe increases performance over no shoes at all is that it allows you to get more energy back from the ground.

Racing flats and shoe stiffness

We usually think of shoes only in terms of how they modify the impact force from the ground which travels straight up your leg, but they can also moderate the forces that act on the joints of the foot.  During the “toe-off” phase of the stride, when you are pushing off from the ground, there is a spring-like mechanism that acts on your ankle, foot, and toe joints to “fling” your body forward.

Research published in 2006 by Jean-Pierre Roy and Darren Stefanyshyn³ indicates that a stiffer shoe midsole can increase the “rebound” you get from this springing motion, increasing your efficiency.

Using similar conditions to the previous two studies, Roy and Stefanyshyn measured oxygen consumption during treadmill running among thirteen runners wearing either a normal running shoe or a shoe stiffened in the forefoot by a carbon fiber plate.  While increasing the stiffness of the shoe moderately decreased oxygen consumption, increasing the stiffness too much brought oxygen consumption back up, leading the authors to propose a “u-shaped” curve of forefoot stiffness and oxygen consumption.

While carbon-fiber inserts have yet to be seen in retail shoes, this analogy is easily extended to racing flats: one with a very thin or very flexible forefoot might decrease performance by robbing your forefoot of that “springing” action.

Next time you’re at your local running store, take a look at the sprinting spikes on display: they all have rigid plastic reinforcing the forefoot, reducing the energy wasted when pushing off the ground.

How to choose a racing flat

We’re beginning to form a picture of what you should be looking for in a racing flat.  It should be lightweight but still allow you to feel a “rebound” from the surface you’ll be racing on—probably pavement.  And it should also not be too thin or floppy in the forefoot, lest you waste energy during toe-off.

But there’s one more thing to be aware of when considering a change in footwear for a big race: adaptation over time.  Jogging a few times around the parking lot at the running store may not give you a complete idea of the effects of a lighter, flatter shoe on your feet, as demonstrated by a 2004 study by Hardin et al.⁴

Their study involved having twelve men run on a treadmill for 30 minutes in a variety of surface and footwear conditions.  While many findings were investigated in this study, the most salient finding for us is that joint mechanics changed significantly over the 30 minute runs in all footwear and surface conditions.  Unfortunately, we don’t yet have a complete theory of what constitutes “good” joint mechanics, so there’s no way to say whether any of these changes were for the better or worse.  Until then, just keep in mind that your shoes may feel differently on your feet after five or ten miles of running.

Limitations to be aware of

Now, all four of these studies have two of the most common limitations of biomechanics research: small sample sizes and treadmill use.  Running studies with lots of subjects are expensive, and it’s a lot easier to do analysis of gait, joint motion, and oxygen consumption on a treadmill.  But, treadmill running does have some differences from overground running, and one of the most important may be how the surface reacts to an impact.

For example, Hardin et al.’s study actually found an increase in oxygen consumption with decreased surface stiffness, in contrast to Kerdok et al.  Hardin explain that their subjects reported that their custom built treadmill felt “mushy” on the lower stiffness settings, perhaps preventing the “rebound” from the surface—more akin to running on a pillow than a synthetic track.  The good news is that the mass effect of wearing a lighter shoe is unlikely to change in overground running.

Our advice after looking at the research

So, perhaps the best advice we can glean from all of this research is that a lightweight racing flat can boost your performance significantly on race day, provided it doesn’t change the “feel” of how you run too much.

If, like most runners, you usually wear “traditional” running shoes for your training, try to avoid the ultra-thin minimalist shoes that are popular with some runners, since the decrease in weight over a “regular” racing flat probably won’t make up for the effectively “harder” surface you’re running on and the decrease in forefoot stiffness in the shoe.

definitely go for runs in your new racing flats for about the duration of the race you plan to wear them in.  This means you’ll probably have to wear your flats for a long run or two if you plan to wear them in a marathon, since you don’t know yet how your body will react to your new flats after an hour or more of running.

Keeping these things in mind can buy you a few seconds in your next race without having to suffer the effects of sacrificing too much comfort in the name of shoe weight.