The sign on the tennis courts read 'no bikes and no skateboards'. How does that work, wondered young American Brad Parks, when his front wheels were like a skateboard's and the back ones like a bike's, and he wanted to play tennis? And could he even get on to the court in the first place as he would need to dismantle his wheelchair to get through the gate?

But he had invented a sport and would do whatever it took to play it. Parks was on a family picnic when he had his lightbulb moment 50 years ago. His relatives were playing tennis in a park in Indiana and 18-year-old Parks was watching in a wheelchair having been paralysed in a freestyle skiing accident in Utah a few months earlier.

His dad called him over to join in. "So I started hitting tennis balls," Parks tells BBC Sport. "I was in literally a hospital wheelchair which is what they gave you back in 1976, so you couldn't really move very well but I could hit balls.

And right then I decided 'I'm going to give it a shot.'" He didn't just mean in that moment. Eventually he was going to devise a whole new sport of wheelchair tennis, introduce it to the world and see it played at Grand Slams and Paralympics. But first, he needed to find some people to play with.

Perhaps even a Hollywood actor (more on that later). In those days, basketball was the go-to sport for wheelchair users and had been suggested to Parks while he recuperated in hospital but he had other ideas. Before his spinal cord injury - sustained when he had over-rotated on a backflip and landed heavily on to packed snow at a freestyle skiing competition - the Californian had been wanting to improve his tennis and now he had even more reason to.

"If I'm going to be in a wheelchair the rest of my life I've got to be the best that I could be and I want to still live my life, I didn't want people to feel sorry for me so I wanted to play," he says. "So I thought well, maybe I could play tennis and then I could play with my able-bodied friends so that was my first thought." After that picnic, Parks started playing tennis nearly every day with friends, family and opponents that his parents found him to play. None of them used wheelchairs.

Everything changed when he went back for his next hospital check-up and met physiotherapist Jeff Minnebraker, who had also been exploring the possibilities of tennis in a wheelchair "Instantly we bonded because we figured we were the only two guys who were playing wheelchair tennis in the whole world," says Parks. They started experimenting with rules. Should the court be smaller, should the balls be different, should the net height change?

Because they started out playing against able-bodied players, they played some early versions where the non-wheelchair player would only get one serve or have to serve underarm or have more court to cover. But by the time they had decided to hold their first wheelchair tennis event in 1977 in Irvine, California, they had settled on keeping it very simple. The only difference between tennis and wheelchair tennis would be that the ball could bounce twice – and this remains today.

Parks won that inaugural event and dominated the nascent years of the sport, which were often played on sub-standard public courts, with dust and dirt preventing wheel-grip.