Runners, if you are worried about your stride, relax. It is almost certainly fine, according to a comforting new study.

Researchers found that both experienced and novice runners tend to settle into the stride that is most efficient for them. Tinkering with how you run is unlikely to be beneficial for performance and could make running more difficult, the study found.

As a species, humans are innately capable of running. Unless we have a disability, most of us start running as toddlers and continue, sporadically, throughout our lives, racing through airports or after the bus we just missed. But because we can run, does this mean that we naturally run well?

Many experts, including running coaches and exercise physiologists, have debated that question in recent years, pondering whether there is an ideal running form that everyone should adopt.

In particular, they have argued about stride length and cadence, or the number of steps runners take a minute.

Stride length and cadence are intimately connected, and experts and runners have wondered whether altering these variables might make someone a better, faster runner. Many of us who run have been told at various times that we should shorten our strides to run faster, or maybe lengthen them, and perhaps aim for a cadence of more than 160 steps a minute.

But there has been surprisingly little scientific evidence that either reinforces or refutes the idea that modifying how we run is advisable.

So for the new study, which was published in May in the International Journal of Exercise Science, scientists at Brigham Young University (BYU) in Provo, Utah, set out to closely examine the strides of both expert and inexperienced runners and see what would happen if they tweaked them.

They began by recruiting 19 skilled, competitive runners, including 10 members of the university's Division I women's cross-country team. The researchers also gathered an additional 14 active people from other sports, including cycling and swimming, all of whom were fit but none of whom had done much if any running in the past two years.

They then had each volunteer run on a treadmill in the university's human performance lab at whatever speed they felt to be most comfortable. For the experienced runners, this was their typical training speed. For the novice runners, it was the fastest pace that they felt they could maintain for at least 20 minutes.

The researchers manually counted each volunteer's steps, a number that they verified with videotape, and then arithmetically determined the length of each person's stride when they ran at their favourite speed.

Finally, during a second day at the lab, they fitted everyone with masks that determined their oxygen intake, in order to track their running economy.

Running economy is important. In physiological terms, economy is a measure of physical demand. If one form of moving requires less oxygen than another, it is more economical, less strenuous and will be easier to maintain.

So, if one way of running is more economical than another, a runner employing that style will run longer and with greater ease.

So, on this second lab visit, the researchers began by having their volunteers run at the same pace and stride length that they had originally chosen as their personal favourite.

Then the researchers used a metronome to sneakily alter people's strides by asking them to match their footfalls to the metronome's tone - landing with their right foot every time the metronome sounded. The treadmill's speed remained unchanged, but the researchers sped up or slowed the metronome's cadence by 8 per cent and then 16 per cent.

In order to keep pace, the runners had to shorten or lengthen their strides accordingly.

The researchers had them maintain these strides for two minutes, while tracking their breathing throughout.

Then they examined the data to see how the volunteers' running economy had been affected.

They found that the effects had been considerable. When the runners modified their preferred running strides, whether lengthening or shortening them, their economy generally declined. The running became physically more difficult.

Interestingly, this finding held true for both the experienced and inexperienced runners, and to about the same extent. Despite their inexperience, the novices instinctively chose their most efficient pace and stride at the start of the study. Lengthening or shortening their strides subsequently did not make them more economical; instead, it made them less efficient.

These findings indicate that "our bodies know what they are doing" when it comes to choosing running form, even without the benefit of instructions, says Iain Hunter, a professor of exercise science at BYU who oversaw the study.

Of course, this was a one-time look at a small group of runners and focused on a single, sustainable running speed for each. It cannot tell us whether there are other situations in which runners might benefit from altering their strides, including to prevent injuries, or sprint madly toward a finish line in order to set a personal best.

But the general message is encouraging and empowering, Hunter says. For most of us who run, our most efficient stride is not something we have to learn from coaches or other experts.

"It's built-in," he says.

NYTIMES