Nathaniel Nolan faced a dilemma ... he wanted to stay fit, but high-intensity workouts caused intense joint pain. So, what's a 31-year-old personal trainer to do ... run like a dog on all 4's.

That's what the Indianapolis dude calls it ... "all fours training." He scurries around on his hands and legs for hours a day ... and it's done wonders for him. Turns out its really good strength training and it made him super flexible. And, you know what flexible dogs can do, so congrats!!!

Nathaniel acknowledges he gets scoffed at by folks who think he's crazy, but he doesn't give a damn. In fact, it's catching on like wildfire ... millions have viewed his doggy-style workouts on TikTok.

He's been doing it for a year and swears by it.

Nathaniel also does exercises of the human variety ... breakdancing, handstands, push-ups, and martial arts ... so exercise doggy-style is not the only game in Indianapolis.