About 55,000 people will be running the Great North Run on Sunday and their reasons for doing so are as varied as their kit and costumes.

The BBC has been finding out what motivates ordinary men and women to attempt to run, jog or walk just over 13 miles.

Nicola and Rob Crawshaw thought their son was a late developer.

Olly was showing no sign of being able to walk.

And then, just as he turned two years old, the doctors had to tell his parents it was nothing to do with his development and that Olly, in fact, would never be able to walk.

He was diagnosed with Type 2 Spinal Muscular Atrophy.

A rare disease, it is caused by a loss of specialised nerve cells in the brain and spine which, in turn, leads to muscle wastage.

“It turned out we carried the genes that led to Olly’s condition,” Mrs Crawshaw said. “A condition we hadn’t even heard of.

‘Highly intelligent’

“It’s a neuromuscular disease that affects the nerves, and the signals don’t get from the brain down through his spine to his muscles, so he has no muscles in his legs,” she added.

“He can’t stand or walk or hold himself up at all.

“He’s just beginning to have an understanding that he can’t do things for himself, like stand or walk or run around like his friends.”

Nobody knows why, but children with the condition can often be highly intelligent. Olly is certainly that.

His parents are acutely aware that he is clever enough to manipulate them to get what he wants. They are also acutely aware he needs his independence.

So Mrs Crawshaw, an animal welfare inspector with Northumberland County Council, is running the Great North Run to try to raise money towards the £20,000 wheelchair that will help Olly live as full a life as possible.

‘Real livewire’

“Because of his intelligence, it’s really important he gets to stay in a mainstream school and have no different an education,” she said. “It’s just the mobility that he’s going to need a lot of help with.”

Mr Crawshaw, a police officer, thinks his son is “a real livewire”.

“He really enjoys his sport,” he said. “We want to make sure that continues.”

The couple, who live in Whitley Bay, are also raising money for the English Federation of Disability Sport to help other children with disabilities.

“That’s really important”, said Mrs Crawshaw. “Olly loves his sport – his golf, his tennis – and other children do, too.

“There’s no reason they can’t excel at it just because they happen to use a wheelchair or whatever. The Paralympics has just taught us all that.”