FIRSTBORN has become an aficionado of a fast-growing but still little-known sport called slacklining.

It has yet to be adopted by the Olympics but it may be appearing in a park near you any day now. We saw someone slacklining in our park only last weekend – and it wasn’t Firstborn.

The sport involves stringing a length of webbing between two trees/rocks/posts and then walking along it. It’s not tight-rope walking because the line, while taut, can move about.

When our son first told us about slacklining we weren’t too alarmed because he practised on lines only a couple of feet from the ground.

But, of course, now that he’s got the hang of it he’s out and about looking for ever-more challenging places to slackline: rivers, ravines and quarries seem to be his locations of choice. He does, however, wear a safety line, so I guess I can almost sleep soundly in my bed at night.

As this fantastically atmospheric picture of him shows this is not a sport for the faint hearted – or one that mothers enjoy watching.

Slacklining has been around for about 30 years and is said to have been invented by a pair of rock climbers in America. It builds a high level of fitness as it uses a wide range of muscles – imagine the strength needed to keep your balance while standing on a wobbling line.

It would, I suspect, be a good sport to introduce into schools because it would fire the imaginations of even the least-sporty kids and improve their fitness without them even realising it.