The middle Kardashian sister -- who's in the middle of a divorce with Kanye West -- just posted footage of her breaking a sweat with fitness guru Melissa Alcantara ... who was showing her some exercises to really get the blood pumping -- and the booty popping.

Kim had another way to describe it with some clips she threw up of Mel doing squats with a kettlebell ... you can watch for yourself -- another sort of "poppin" workout, it seems.

Then came some HIIT reps of her own ... after showing Mel doing deep squat jumps with the help of some suspension straps, Kim gave it a go herself -- and offered up a very up close and personal shot of the action. She even put part of her video in slow-mo!!!

The caption she added that was superimposed on her extraordinary derrière ... I Got You Back. Yes, yes she does. Not that she'd ever really lost the "back" -- but we get it.

Maybe it's just us, but Kim certainly appears to be embracing the fact she's unattached with gusto -- not shying away from going out and looking fabulous, as she did just a few short weeks ago for a girls' night out, or getting in tip-top shape and showing off for the 'Gram.

As perhaps the most eligible (and coveted) bachelorette in town at the moment, we can't really blame her for getting a move on in her next chapter.