Martin said the idea for a preschool program arose in the early 2000s, when he noticed that the younger siblings of his elementary-, middle- and high-school-age students were frustrated that they had no activity in which to participate. In 2003, he and his wife, Mikki, started CrossFit Kids, and four years later they began consulting with pediatric physical therapists on developing a program for recently potty-trained children.

In recent years it has ballooned to about 700 active preschool CrossFit Kids classes across the country.

Preschool-age CrossFit participants do not use weights. Class time is short, typically 30 minutes or less. There are basic lessons on nutrition, such as where tomatoes come from. There are no weigh-ins or flexing of muscles before a mirror. Instead, activities are done for only a few minutes at a time in short bursts followed by rests.

In preschool CrossFit, dangling off hanging bars is likened to being a monkey. Squats are frog-inspired. Box jumps, plyometric leaps long beloved by elite athletes, are smaller and rebranded for children as superhero leaps.

In Long Island City, a tunnel constructed from red tumbling mats inspired comparisons to snakes and worms. Games and exercises were punctuated by water breaks and doodling. CrossFit Kids instructors are discouraged from telling children to move faster, Martin said. High-fives for effort are prevalent.

As long as that remains the case, some pediatricians said, CrossFit preschool classes can be suitable for youngsters. But some cautioned that the same criticisms leveled at CrossFit’s grown-up counterpart — that there can be great variation among the thousands of CrossFit outposts in the style and quality of trainers and regimens — may be true for preschoolers. The doctors suggested that parents would benefit from observing a class beforehand.

“CrossFit has the image of pushing people beyond their limits,” said Dr. Gregory D. Myer, director of the Human Performance Laboratory at Cincinnati Children’s Hospital Medical Center. “You want to make sure people are trained in understanding a child. Kids are likely going to have a disconnect with their ability and what they want to do.”