If you’re gunning for a new 5K PR, you may want to check the scale—for the weight of your shoes, that is. A new study shows that people run slower when wearing heavier sneakers, even if the difference is just a few ounces.

It’s long been known that wearing heavier shoes makes runners work harder, and experts have theorized that that would slow them down. (One oft-cited estimate is that for every ounce a runner shaves off her sneakers, she'll run one second faster per mile.) But researchers at the University of Colorado Boulder wanted to find out for sure if this was true.]]

The researchers recruited 18 competitive runners to complete 3,000-meter time trials (about two miles) on an indoor track, once a week for three weeks. Unbeknownst to the runners, small lead pellets were sewn into two of the three pairs of racing flats they wore.

By themselves, the shoes weighed 7 to 8 ounces each, depending on size. The pellets added about 3.5 ounces (or the weight of a deck of cards) per shoe for one pair, and about 10.6 ounces per shoe for the second pair.

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To help prevent the runners from detecting extra weight, the researchers put their shoes on for them. Still, the runners noticed the difference: In their time trials, they paced themselves differently—and ran about 1% slower—for every 3.5 ounces of lead added per shoe. The researchers calculated that elite runners wearing shoes 3.5 ounces lighter than normal could potentially run a marathon about 57 seconds faster.

The study also measured how much energy the runners expended by testing their oxygen consumption and carbon dioxide production as they ran on a treadmill wearing each pair of shoes. The results compared well with previous studies—and matched the results of the indoor-track time trial—showing that energy costs rose by about 1% with each 3.5 ounces of extra shoe weight.

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Don't go out and buy a new pair of running shoes just yet, though. The researchers note that lighter shoes won’t necessarily make a runner faster. The team’s previous research has shown that proper cushioning also reduces the energy cost of running—so swapping out foam or other padding for a super-streamlined design could potentially backfire. (Studies have also found that switching to barefoot-style minimalist shoes can raise some runners’ injury risk.)

Lighter is not always better,” said lead author Wouter Hoogkamer, PhD, a researcher in CU’s Locomotion Laboratory, in a press release. He recommends that shoppers keep this trade-off in mind when choosing a running shoe that feels good—lightweight still but adequately cushioned—on their feet.

The study was funded by Nike and published in the journal Medicine & Science in Sports & Exercise.