Hime Romero/Tracy Press

Members of the Stanislaus United Open mens’ futsal team practice Saturday at Gretchen Talley Park.

Whether with a stick and a rock or with a shortening of the field, athletes have long found ways around the structure of the traditional sport.

For as long as there have been rules to sports, there have been limitations. When kids cannot meet the demands of soccer, such as 22 players on a giant field of smooth grass, they improvise.

It is with this necessary imagination that futsal was born. Growing up in cities of concrete and without the patience required to form a full soccer game, youth organizers invented futsal in Uruguay in the 1930s as a way for kids in the inner city to learn a version of soccer.

To the seeming few in America who know of futsal, it is an underground game. But to those who play it, it is a game known around the world by professionals and amateurs, and it is taking hold in the United States.

Antonio Reis is part of the foundation. He helped form Stanislaus United in 1990, and Saturday, 15 years later, he was schooling a group of kids 20 years his younger in preparation for the U.S. National Futsal Championship in two weeks. Reis will take four teams from four different divisions — each with several players from Tracy — to the Anaheim Convention Center, where the winning team will earn a trip to compete in Europe.

“Go, go, go — come on, you’ve got 50 seconds,” Reis shouted as two of his teams sprinted on the hot asphalt court. “We gotta put speed, we gotta put speed.”

Speed is the name of the game. From a scrimmage at Gretchen Talley Park on Saturday, futsal looks a lot like indoor soccer — played, in this case, on an outdoor basketball court. There are five players on each side, including the goalkeeper, but there are no walls. The goal at each end is roughly three meters wide and two meters tall, and the ball is smaller than a soccer ball but heavier, with virtually no bounce.

Watching the game, it is easy to see why some credit it with training the Brazilians in the masterful footwork they have become famous for on the larger, more open traditional field. Futsal demands quick feet and precise passing. Reis described the game as “five times faster than indoor soccer.”

Sanctioned by soccer’s international governing body, FIFA, futsal strictly penalizes contact. In a 20-minute half — where substitutions go on the fly like in hockey, and there is virtually no stopping like in soccer — if a team accumulates more than five fouls, each additional one results in a penalty shot.

“That’s what we play in the streets in Europe and Brazil,” Reis, a native of Angola, said. “Because we started in the streets, we don’t want to push someone because they could get hurt.”

That isn’t to suggest that there are no injuries. In a short 15-minute scrimmage Saturday, three different players were forced to sit out at one point with injuries. But considering the speed at which they run and the force with which the heavy ball is struck, it is surprising there are so few injuries.

Isaac Robledo, a former West High soccer player, is a member of the men’s team preparing for nationals.

With six players at practice, he was the odd man out for the first half.

“There’s never a moment to stand — it’s always fast,” Robledo said. “You don’t see (the ball) coming. It’s just ‘Bam!’ and it’s there. It can be 4-0 with two minutes left and you can always catch up. That’s how fast it is.”

And that’s how fast the game is spreading. This will be the 20th national championship, and, according to Reis, there are more than 4,500 registered teams nationwide.

Reis is happy and excited to compete and is focused on making the team better. But he is more relaxed than most coaches just two weeks from a national competition. He still carries with him his motivation for starting Stanislaus United 15 years ago.

“The other part, too, is that most of the kids who play are inner-city kids,” Reis said. “I would run midnight basketball if it was a situation where the kids could get out of the swamp and be together.”