I’m sitting in a bank of empty seats at Half Moon Young People’s Theatre, watching two actors leap through lines of dialogue more like performance poetry than speech. Their rhythms and tempos are pulsing and thudding through the phrases, conjuring up the slap and thwack of palms and soles on the urban skyline as they play out the journeys of two obsessive parkour athletes – chasing one another whilst both running from the forces they can’t control, hearts beating with the desire to transform and transcend.

Three years ago I was watching some free-runners on the South Bank leaping up and down the concrete steps around the National Theatre, then up on to the promenade searching for other platforms and levels. Their movements were controlled, repetitive, fluid – and mesmerising to watch.

When I started to speak with them, I realised many were half my age and that this wasn’t a showing-off pastime designed to impress: it was a serious discipline, more like a martial art, bringing their psyche and physicality into balance with their environment so they could learn how to navigate it better.

It went beyond the self-improvement of fitness and into a much deeper realm that was about harmonising with your environment, trying to attain a Buddhist-like momentum as you traversed the cityscape, reading the landscape with your body “a bit like that bit with Neo at the end of the first Matrix film”, as one of the coaches I later interviewed described it.

Research is a big part of my process as a playwright, and this was the beginning of an on-off year-long period of watching parkour documentaries, DVD instruction videos, You Tube channels of the parkour legends Sebastien Foucan and David Belle, interviewing coaches and athletes from Parkour Generations in London and even attempting a parkour training session myself (hundreds of sit-ups or ‘conditioning’ and absolutely no leaping off buildings).

Writing for teenage audiences often involves finding an incredibly strong heartbeat in the narrative, something that encapsulates that period of your life when you can become instantly consumed by a fascination: so much so that it defines your world, your values, your friends and your philosophy on how to live. Parkour was a natural candidate for a story, and Half Moon invited me to explore the idea further through its fantastic Careers in Theatre scheme, where very early excerpts of the writing are road-tested by schoolchildren who respond with a play-in-a-day inspired by the content.

I set myself one limitation and one ambition: to write a play where the act of parkour was held entirely in the language and no dance or physical theatre was required, and to create the city as a character itself. Watching rehearsals today, I feel so excited by how the performers, design and music are developing this concept even further: this is a huge idea in a small space, and it’s now bursting into life with its own heartbeat.

David Lane’s FREE runs at Half Moon from Wed 15 to Sat 18 October, before embarking on a national tour. Visit Half Moon’s website for further information.