Heavy shoes slow you down by forcing you burn more energy at a given pace. But by how much, exactly?

Studies dating back to the 1980s have shown that for every additional 100 grams (3.5 ounces) per shoe, you burn about 1 percent more energy. In theory, running speed and energy cost are proportional, so a 1-percent increase in mass should roughly correspond to a time that’s 1 percent slower.

But that claim has seldom been tested in practice, so that’s what a team in Rodger Kram’s Locomotion Laboratory at the University of Colorado Boulder, led by post-doctoral researcher Wouter Hoogkamer, decided to explore in a new study published in Medicine & Science in Sports & Exercise.

The study used specially modified pairs of Nike Zoom Streak 5 racing flats, with a base weight of just over 200 grams (depending on the size). In some of the shoes, tiny lead beads were sewn into the tongue to add 100 grams, while in others, beads were sewn into the tongue and sides to add 300 grams.

Previous research has found that people notice shoe weight if they handle the shoes themselves, but don’t notice it if it’s on their feet. So the researchers added superlight (4.6 grams) fake accelerometers to the shoes, and told the volunteers they were really delicate, so the researchers had to put on and take off the shoes for them. As a result, only one of the 18 volunteers in the study (all sub-20:00 5K runners) noticed any difference in the shoes—because he had wide feet and noticed that the shoelaces were shorter in the heaviest version.

Each subject did a series of treadmill tests to assess running economy (i.e., how much energy they burned at a standard pace), and then a series of 3,000-meter time trials on separate days. Unbeknownst to them, the shoes were different each time.

As expected, the results showed that running economy worsened by 1.11 percent for every 100 grams added to the shoes.

Running time in the 3,000-meter trials, meanwhile, increased by 0.78 percent for every 100 grams. That supports the main contention that changes in running economy are reflected proportionally in changes in race time. From a research perspective, it means that if you do something in the lab that changes running economy, you can reasonably conclude that it would also change race performance.

Still, 1.11 and 0.78 percent aren’t identical. One possible reason is that running economy was measured at a pace of 7:40 per mile, while the average 3,000-meter race speed was 5:36 per mile. Is it possible that the effects of extra shoe weight depend on how fast you’re running?

In a word, yes. A previous study that investigated the effects of shoe mass on running economy looked at various speeds found lesser effects at higher speeds. In fact, at 5:30 per mile, they found a change of 0.8 percent per 100 grams—strikingly close, the researchers note, to the 0.78 percent change in 3,000 time in the new study.

An intriguing postscript: The researchers point out that the current marathon world record of 2:02:57 was set in shoes weighing 230 grams. Subtract 100 grams, and in theory (at 0.78 percent per 100 grams) you would gain 57.5 seconds. Subtract the whole weight, and you’d gain 2:12.

There’s a problem, though. Without shoes, you lose the benefits of cushioning, which Kram’s previous research has shown can improve running economy by as much as 3-4 percent. In fact, as another of Kram’s studies showed, you can get the same benefit without shoes, by cushioning the ground with 10 millimeters of EVA foam.

So if we’re contemplating what it would take to get a little closer to a two-hour marathon... how about a loop course covered entirely in EVA foam, to allow the runners to save on shoe weight? It might be simpler than hosting a race by the Dead Sea!

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