IT was an afternoon in February 2012 and Elaine Wyllie was shocked as she watched the class of 10- and 11-year-olds doubled over in pain as they tried to complete a warm-up run across the school field.

At the time, Mrs Wyllie was the headteacher of St Ninians Primary in Stirling but that day she was covering the P6 class while their teacher attended a funeral.

She had decided to take the pupils outside for a run after one of the school's local volunteers, an 80-year-old local landowner who helped out with music, storytelling and teaching the children chess, warned her out of the blue: "the children at this school are not fit".

"I knew the children weren't generally fit," said Mrs Wyllie, who retired in 2015 after 38 years in teaching. "I asked the PE teacher Marion and she said 'they're exhausted by the warm up in PE, most of them - most aren't fit at all'.

"So that afternoon I was covering a P6 class for a teacher who was at a funeral and I took them out for PE in the afternoon. I thought 'they can do their warm up running round the field and I'll see who's fit'.

"Most of them, by the far end of the field, were exhausted - doubled up in pain with a stitch."

Read more: Daily Mile does improve children's fitness, major study finds 

It was a watershed moment for the school. From that day on, the class went out for 15 minutes of running, jogging and walking each day to build up their speed and stamina.

As the weeks went on, other classes and eventually the entire school joined in and the 'Daily Mile' was born.

What started life nearly seven years ago as one P6 class in a Stirling primary school trying to get active has given birth to a global phenomenon credited not only with improving fitness - but increasingly also classroom concentration and attainment.

Today the Daily Mile has been adopted by more than 6400 primary and nursery schools worldwide, including nearly half in Scotland, and is a fixture as far afield as Australia and South Africa.

Mrs Wyllie, 63, says the key to its success is its simplicity.

"There is something about the ownership, the motivation, the autonomy - and there's no failure. Everyone succeeds at their own Daily Mile. There's no kit.

"The issues around PE kit are eliminated: 'cool' and 'not cool' kit; unwashed, missing, no kit. There's no revealing of your body because you're not changing in front of people.

"You're not running in skimpy shorts in front of people. And also, time spent changing is gone. The children just down tools and head straight outside.

"It's simple, it's free and it's sustainable. It's not one of these piecemeal initiatives that come and go."

Read more: Childhood obesity rates 'would double' if body fat was measured instead of BMI 

Since the Daily Mile first took hold, it has drawn attention from researchers - with some surprising findings. There are signs, for example, that it may be reversing a previously steady rise in dyspraxia rates.

Dyspraxia is a developmental condition affecting physical coordination and motor skills, as well as memory and information processing. The cause is unknown, although active play has been shown to improve motor function.

Incidence has been increasing across the UK but researchers involved in a 15-year study, currently at the halfway point, were surprised when results from Scotland appeared to reveal the trend reversing.

When they delved into it, the decline was being driven by schools participating in the Daily Mile.

It comes after the results of a major study by the Universities of Edinburgh and Stirling, published in May, found that children who did the Daily Mile for seven months reduced their body fat and improved their fitness and overall activity levels.

The benefits are not only physical, however. A study of maths performance among boys in a Morecambe Bay school is reporting an "acute improvement" in the numeracy of boys doing the Daily Mile.

Read more: Attention span and memory improves after short burst of exercise, study finds

That concurs with the 'Coppermile' report which followed 76 children at Coppermill Primary in London. Over 12 weeks in 2016, the pupils ran, jogged or walked for 15 minutes at least three times a week.

By the end of the study period, the average distance run was up 19%, but more striking was their academic performance.

The Year 6 pupils were were sitting England's SATs tests did the Daily Mile every day for a week prior to sitting the tests, which measure reading, writing, maths and grammar.

The school had been predicted to achieve average results in the high 60s-low 70s. In reality, it scored between 88% in writing to 96% for maths and grammar - significantly above the local and national average.

"Their attainment was up 20-25% compared to the previous year's results, and we know that's been replicated in Morecambe Bay," said Mrs Wylie. "Any headteacher would be happy with a 5% improvement.

"And what's more, when they did the measurements of those 76 children's fitness at the beginning, 36% of the children had predicted poor health in later life based on their readings. That dropped to 12% after 12 weeks.

"The Daily Mile is not the only answer, but it's a huge part of the answer and it's a simple, free, impartial solution."