Washington, Oct 29 (PTI): Wearing heavy shoes while running on the treadmill may slow down an athlete in real races, a new study has found.

Running with heavier shoes on will cause you to exert more energy than running with lighter shoes.

However, using more energy while running with heavier shoes may translate into slower running time, according to University of Colorado Boulder researchers.

In a study, they show that running times slow when running shoe weight is increased, even if only by a few ounces.

Researchers measured running economy in 18 runners, who ran on a treadmill using three pairs of nearly identical shoes, with subtle differences.

Unbeknownst to the runners, they added small lead pellets inside the tongues of two of the three pairs of shoes to be used by each runner.

While one pair was normal, each shoe of another pair was made 100 grammes heavier and a third pair was loaded with 300 grammes of lead pellets per shoe.

Each of the runners ran treadmill tests in which oxygen consumption and carbon dioxide production were measured with all three differently weighted shoe pairs.

The treadmill tests compared well with previous treadmill evaluations showing energy costs of the runners rose by about one per cent with each extra 100 grams of shoe weight.

Later, the runners ran 3,000-metre time trials on an indoor track in each of the three shoe pairs once a week for three weeks.

Unaware of the differences in shoe weight (the researchers insisted on putting on and taking off the shoes for the test subjects), the runners ran roughly one per cent slower for each 100 grams of lead added to the shoes in the 3,000-metre race.

"Our results indicate that to evaluate the effects of equipment or technique changes, athletes don't need to run several races at maximum intensity - we can predict performance based on just a few five-minute bouts of less than maximum running effort in the lab on a treadmill," said Wouter Hoogkamer, postdoctoral researcher at CU Boulder who led the study.

One interesting implication, said the researchers, is that elite marathon runners wearing shoes 100 grams lighter than normal could potentially run about 57 seconds faster.

The current men's world record is 2:02:57 set by Dennis Kimetto of Kenya in 2014 while wearing shoes that weighed about 230 grams - just over eight ounces.

However, when shoe mass is reduced, by compromising cushioning for example, it doesn't mean you will run faster, said Hoogkamer.

Previous studies have shown that proper cushioning also reduces the energetic cost of running. So when selecting footwear, be aware of this trade-off between shoe mass and cushioning, he said.

The research was published in the journal Medicine and Science in Sports and Exercise.