You won’t find dumbbells or weight machines in the gym Sean Keogh runs. At Calisthenics Club Houston, it’s all about training with body weight.

Calisthenics is proving popular enough to keep new members coming through the doors, excited to learn moves like handstands and pullups.

"Calisthenics, basically, is just body weight, strength training. But there's different types of calisthenics, or people will do calisthenics in different ways," says Keogh.

Keogh and his members have plenty of company.

These days, content creators, independent gyms and megachains alike are promoting calisthenics, an age-old form of fitness that uses little or no equipment and instead relies on body weight for resistance.

In the United States in July, President Donald Trump even reestablished the Presidential Fitness Test, intending that youth across the country will again practice old-school exercises like situps, pushups and pullups.

Melissa M. Markofski, Health and Human Performance, Associate Professor at the University of Houston explains the difference between calisthenics and weight training.

"So exercise with weights and calisthenics have two separate goals. They're both a type of exercise called resistance exercise, but one is more loaded, the ones with weights, than calisthenics."

With no equipment required, calisthenics-based programs are affordable for exercisers and profitable for gyms that offer them.

Plus, people may have gotten used to exercising with few accoutrements during the pandemic.

There’s plenty of research to suggest that calisthenics can improve everything from muscle strength to aerobic conditioning.

Gary Rodriguez is a regular at Keogh's gym. He says he has found it to be the perfect exercise for him:

"It's just a good way to keep muscle tone as you grow older without risking a lot of injury, I think. So, it's a perfect fit for me.”

Lifting weights also damages muscle tissue in a way that can be productive, as muscles grow larger through the body’s repair process.

Over time, though, it may take larger amounts of weight to keep seeing gains.

Progress plateaus as the body gets used to exercises it’s done before.

It’s not impossible to grow muscle through calisthenics, it’s just harder to continuously level up exercises for sustained progress without increasing external weight.

“With calisthenics, you can definitely build a lot of muscle, you can build a lot of strength. The key to doing that is understanding the progressions and regressions, rep ranges and intensity of the exercises as well,” says Keogh.

Gym regular Sid Tulsiani combines calisthenics with other forms of exercise including weight training.

"I've been doing calisthenics for the better part of five years, and I found it to be incredibly beneficial in that it's helped me with injury prevention," he says.

If you’re after bulging biceps, you may need more than calisthenics to get there.

But if you’re just looking to get moving and improve your health, your body is probably enough.

Particularly for the roughly 75% of Americans who aren’t meeting federal physical activity guidelines — which call for at least 75 minutes of vigorous or 150 minutes of moderate aerobic activity a week, plus two strength-training sessions — so calisthenics could be a good option for some.

AP video by Lekan Oyekanmi