Image copyright Studio Canal/A Ticklish Tale Image caption The film featured young men in sportswear tickling each other

Of all sports that are unlikely to be considered for the next Olympics in Tokyo, "competitive endurance tickling" might be at the top of the list.

However, a documentary called Tickled shines a spotlight on those who take part in it, when a TV reporter from New Zealand, David Farrier, uncovers not just a quirky sport, but a whole industry, and an underworld with allegations of cyber bullying.

Two years ago, Farrier, known for his "and finally" news pieces at his local TV station, discovered what was described as "competitive endurance tickling" videos online.

They featured young men in professional sportswear tickling each other.

A US-based company, Jane O'Brien Media, was producing the videos and offering substantial fees for anyone selected to take part in the shoots in Los Angeles.

Image copyright Studio Canal/A Ticklish Tale Image caption David Farrier said he was "intrigued" by the sport

"Right in the beginning I thought it was entirely innocent, perhaps with a subtext," explains Farrier. "I thought it was someone's idea of a funny strange sport, as it was in a photography studio, a professional space. All the men were wearing sportswear so I thought it was someone's odd idea of a tickling league."

"I was intrigued and thought about doing a two-minute feature on it for my show, "he adds, "so I got in touch with them. Really, a short feature was all I was aiming for."

In response however, Farrier, who is bisexual, says he received emails stating that the company did not want to deal with "a homosexual journalist".

"I was a little upset but part of me thought it was funny," he adds. "I didn't understand why a company that makes men-only tickling videos would say that."

Not necessarily for pleasure

"Instinct, intrigue and fascination" drove him and his co-director Dylan Reeve on to find out more about the videos, despite representatives of the company flying to Auckland to threaten legal action. Farrier successfully raised the money for a documentary using the crowd-funding platform, Kickstarter.

"We probably did go into it quite naively," he believes.

"We never expected to find what we did. If you watch the film you'll find it's not really about tickling - it's about power.

"We were interested in the psychology of what made these men take part in the videos, and it was mainly guys who were athletes or military, and from poorer states."

Although being tickled usually provokes laughter, science has long been interested in whether it's involuntary. Ten years ago, a study at the University of California at San Diego concluded that tickling did not necessarily produce pleasure, just the outward appearance of it.

Alan J Fridlund, a social and clinical psychologist, points out that whether it's enjoyable or not "depends upon the social context of the tickle. Charles Darwin thought that infant tickling was the basis of adult humour and I agree".

Image caption Being tickled can cause a weakening of the body's muscles

He added: "But the insidious side of tickling is that if the tickler has a perverse motive, they will interpret the reflexive responses to tickling as a sign of approval, even when the social context turns dark.

"As the tickler proceeds, he or she is abetted by the nasty fact that strong tickling begins to cause cataplexy, a generalised weakening of the body's striate muscles, and one that many of us have known if we've been 'tickle tortured'. "

That phrase is repeated by a participant of the tickling videos called TJ, who tells Farrier on camera that he was told taking part "was for a project, as tickling was being considered as a military tactic for the army, tickle torture. I thought that was untrue".

TJ says the reason he did it was "because I was a little desperate for money. I was young at the time and I didn't think anything of it. I would get $2,000.

"On the day, I noticed it was all guys, and I didn't know I would get tied down, but there were athletes there, bodybuilders, a couple of actors I had seen on TV commercials - completely normal people. I still thought, 'I hope no one finds out about this'."

'Pain in tickling'

In the documentary, TJ alleges that after the videos surfaced online, he asked them to be taken down, and was subjected to a campaign of abuse and harassment - including emails being sent to his employer at a high school.

The film claims others involved with making tickling videos have suffered controlling and abusive behaviour. However, Jane O'Brien Media strongly denies any wrongdoing.

The director confesses that his own viewpoint on tickling has shifted since making the film - and getting tickled himself for the purpose of making the documentary.

"There is pain in tickling, it's not always enjoyable. You're not laughing in the way you laugh as a joke. It can be used as torture and power play. There's also always one who is dominant, and one who is submissive. It's a metaphor for the power and control we talk about in the film.

"The innocence of it has gone for me - I don't tickle my nieces any more."

The one word to sum up his experience, Farrier adds, is "weird".

"I was completely bemused a lot of the time about what I was discovered. You couldn't actually make it up. Some people though think we have - they think it's so unbelievable that it's a spoof mockumentary."

Tickled is on release in the UK now

Follow us on Twitter @BBCNewsEnts, on Instagram, or if you have a story suggestion email entertainment.news@bbc.co.uk.