The "roller coaster years" usually allude to adolescence, but in Mr James Tang's case, is an apt descriptor of his life until three years ago.

His academic journey - from primary school to university - is a series of peaks and troughs: he flunked and aced exams in equal measure.

His career, too, had many ups and downs, traversing both the public and private sectors, chasing at times after money, and at others, after meaning.

His personal life lurched between confidence and insecurity, anxiety and nonchalance. Even his weight vacillated: now a trim 84kg, the 1.8m-tall man once tipped the scales at 105kg.

The turbulence, he believes, is driven in large part by his ADHD (Attention Deficit Hyperactivity Disorder). The 42-year-old has never been formally diagnosed with the neurodevelopmental disorder - marked by an ongoing pattern of inattention and/or hyperactivity-impulsivity - but is convinced he has it after reading tomes on the subject.

It probably explains, the former teacher adds, why he has a soft spot for students with behaviourial issues or special needs.

A passionate pursuit ensued when Mr Tang - who has a post-graduate diploma in education and sports science - chanced upon research which shows there is a relationship between exercise and cognitive abilities.

Today, he is the founder of Brainy Moves, which provides personalised training to improve fitness as well as psychomotor and cognitive skills.

About 70 per cent of his clients are children with conditions ranging from autism to ADHD to dyslexia. He also conducts programmes for schools and institutions, including the Calvary Baptist Kindergarten, East Spring Secondary School and Institution of Mental Health.

The elder of two children of a couple who ran a handicrafts business, he was a handful as a child. In primary school, he was extremely talkative and hyperactive and got up to all manner of mischief like slashing the arms of classmates with metal rulers and leaving mounds of staples on the chairs of his teachers.

"To cope, my parents sometimes caned and scolded me," he says with no hint of resentment, adding that the thought of testing him for ADHD probably never crossed their minds. "They just thought I was lazy, talkative and not focused on what I was doing," he says.

That he was intelligent, however, there was no doubt. When he applied himself, he often excelled in his studies. Restlessness and playfulness, however, often tripped up the former St Andrew's student. He flunked quite a few exams but always pulled through when it mattered.

"Fundamentally, people with ADHD are not dumb, we just have issues," he says with a laugh.

Until his teens, he led a fairly cushy life. The family lived in a three-storey corner terrace house in Bartley and could afford holidays.

But when he was 18, his mother told him she had to cash out the insurance policies she had bought for him and his brother to save their troubled business. The family also downgraded to a four-room HDB flat in Lakeview.

"I had to find my own means to go to university," says Mr Tang, who took a five-figure loan to study mechanical engineering at Nanyang Technological University (NTU) in 1995.

A six-month attachment with an ammunition design firm in his final year convinced him that he wanted to work with people, not machines.

He ended up selling insurance but quit after eight months to become an untrained teacher at Hua Yi Secondary. It was not his first teaching stint. While waiting for his A-level results, he did relief teaching at Teck Whye Secondary and enjoyed it immensely.

"I was then teaching Maths to Sec 5 students. I was 18, they were 17," says Mr Tang, whose students included many who came from dysfunctional families.

Teaching at Hua Yi Secondary offered fulfilment, he says, but something gnawed at him. "I felt that I was not equipped with work experience and I was not adding value to my students," he says.

Although he was about to enter the National Institute of Education (NIE) to get his post-grad diploma in education, he quit and became a management trainee with Borneo Motors in 2002. "But I left with the intention to come back to teaching," he says.

The car distributor gave him the corporate experience he craved. Over four years, he worked in various departments, including workshop, marketing and sales.

The training and the insights he got from first, marketing, and later, selling Lexus - the luxurious marque of Toyota - were more invaluable than the years he spent at NTU, he says.

He reels off anecdotes of the people he met while selling cars - humble judges, arrogant ambassadors - and how they taught him precious lessons in humility and humanity.

His four years in Borneo Motors were prosperous, in more ways than one. He drew an ample salary which helped him to clear his student loan and get married to a marcoms executive; he also piled on the kilos because of the drinking and entertaining.

Despite the good life, he nursed a deep desire to help children. He went on several humanitarian trips to Cambodia, organising summer camps and distributing food and other essential items to poor communities.

His next gig was a regional one - as country manager in Indonesia - with a German equipment company. "I learnt a lot; I also saw a lot of the dark side of the corporate sector," he says.

That and health reasons - his weight had ballooned to 105kg - prompted him to call it a day.

"A colleague just collapsed and died. I developed hypertension and couldn't even finish running 1km even though I was a track and field athlete in school. It was time to leave," he says.

True to his word, he returned to teaching even though it meant a substantial pay cut. "I felt I had experienced enough to share with students. I don't know why but I have a lot of heart for kids who need help."

He got himself admitted to NIE in 2008 to get his post-grad diploma in education and sports science. But before that, he overhauled his diet and started exercising to lose 25kg in three months.

After graduation, he was posted to Ang Mo Kio Secondary to teach PE and Maths. He told his department head he wanted the most challenging class.

"He laughed and said, 'You don't have a choice. PE teachers are always sent to the most challenging classes'," he says, adding that some of his charges wrestled with conditions such as dyslexia and ADHD.

The turning point came when, over time, Mr Tang noticed changes in his class of 20 students which had their PE lesson just before Maths. "I saw changes in their motivation, in the way that they engaged with me in class. I was curious and started to read up more."

Through a trainee teacher, he met a Korean professor researching the relationship between exercise and the brain. The latter introduced him to the works of Dr John Ratey, an associate clinical professor of psychiatry at Harvard Medical School who has published several books on ADHD as well as the relationship between exercise and the brain. They include Spark, Driven To Distraction and A User's Guide To The Brain.

"The books gave me a lot of insights into my work as a PE teacher," he says, adding that he also ploughed through scientific papers on neurological conditions. "When I read about ADHD, I realised how many symptoms I had."

He began to incorporate what he had learnt into his class. Except for one student who did not sit the Maths examination, all his students passed their N-level Maths paper.

Three years later, in 2013, with encouragement from his wife, he decided to strike out on his own.

In the first few months, the father of three children - aged between four and 10 - earned just $800 doing fitness coaching with a few clients.

One of his first clients, referred to him by his wife, was Dr Jia Jia, the Singapore youngster best known for his series of YouTube videos on Singlish.

Then eight years old, Dr Jia Jia - whose real name is Chua Jin Sen - is dyslexic and has ADHD. His parents wanted him to be more focused.

Mr Tang got the boy to take a psychometric test before putting him on his programme. "After eight weeks, I administered the test again. There was a huge jump in the score. It gave me the confidence to know that my programme works," he says, before launching into an animated discourse on how different exercises can stimulate different parts of the brain to help it become more malleable and supple.

After seeing the improvements his son made, one client got Mr Tang to write a business proposal to formally set up Brainy Moves. He became Mr Tang's business partner and Brainy Moves was officially incorporated in November 2013 with a paid-up capital of $160,000.

The centre in Joo Chiat opened in March 2014 and Mr Tang beavered away, calling on schools and speaking at roadshows and educational fairs to promote his programme. He recruited more than 20 students in its first month of operations; today, the number averages 170.

In November 2014, his wife urged him to write to Dr Ratey to come to Singapore to train him and to give a talk. The public workshop that the Harvard clinical psychiatry professor gave at the SportsHub in November that year was well attended not just by the public but also folks from NIE and the Ministry of Education.

Dr Ratey was so impressed by what Mr Tang had done with Brainy Moves that he agreed to become an ongoing consultant for free.

Word of mouth spread. Last year, Dr Zachary Walker, an associate professor in early childhood and special needs education at NIE, invited Mr Tang to give a couple of guest lectures to his students.

Dr Walker says Mr Tang does three things really well.

"He follows the research which has always shown that movement is critical to learning. He relates very well to children and adults so he establishes a connection with them quickly. And he is always willing to learn more and modify his own practice for the children he works with in his centre," he tells The Sunday Times.

Mr Tang - who has secured a couple of licensees for his programme - says he takes great pride in what he does.

"I interview every single candidate and I tell the parents I will not give them a schedule unless I meet their children and understand their competency and personality," says the entrepreneur, who now has three trainers on his payroll.

Although 70 per cent of his clients have a condition, he has no objections from parents of children who are neurotypical.

"Parents know I have a mix of kids in my class. But I think it's a good thing to let children know from a young age that there are different people out there. They need to learn to accept differences; they should accept differences," says Mr Tang who intends to get his master's and doctorate in special needs education.

He believes more needs to be done to raise awareness of children with special needs who are often misunderstood.

"I really hope that our education system will become more inclusive. The number of people with special needs is huge and many fall through the cracks. I hope the Government will allocate more funding for early intervention."