Three days later, my butt cheeks were still throbbing.

A little over a week ago, SLT — strengthen, lengthen, tone — opened its 16th boutique fitness studio, in Rittenhouse Square: Philly SLT-NYC.

The 50-minute workout, a combination of Pilates, weights, yoga, and pain created by New York City wellness expert Amanda Freeman, has quite the lithe following. Model Chrissy Teigen is an SLT-er, as is actress Sofia Vergara. Word has it several Victoria's Secret models routinely engage in SLT's mindful brand of core-building movements, which work the slow-twitch muscle fibers, resulting in the ultimate bikini-ready, tube-top-chic bod.

And when I read that former first lady Michelle Obama was a fan of the MegaFormer — a part Pilates reformer, part treadmill apparatus invented by Los Angeles trainer and Lagree fitness founder Sebastien Lagree — I straight had to give it a shot. Were Mrs. Obama's perfectly buffed arms possibly within my reach?

Maybe.

I knew I could hang. After all, I've taken my fair share of boutique fitness classes, from Barre Focus Fitness to Flywheel. And I'm a regular at Core Power Yoga's way-too-hard sculpt class. So SLT should be a breeze, right?

Wrong.

Everything about SLT  was intimidating, starting with the steep walk-up to the entrance on Moravian Street, the tiny alley tucked between Walnut and Sansom Streets.

And then there were the toeless socks that I struggled to fit my long toes into while my tiny hard-bodied instructor, Emma Lippert, gave a demo on how to adjust the MegaFormer's red and yellow weights. Fiddling with those darned socks, I almost missed the instructions.

There are MegaFormers for 13 people on the hardwood floor of SLT's breezy and sunlit Philadelphia studio. With the small class size, the instructor all but doubles as a personal trainer. That kind of one-on-one attention, and the fact that regular SLT-ers have goddess abs, explains why the classes cost $32 each. (The first class is half off.) SLT-ers who go three to four times a week, Freeman said, see optimal benefits.

So, yes, this is an investment.

Amanda Freeman, founder of SLT-New York. Freeman was looking for a workout that was fun and that resulted in long, lean muscle tone.
SLT-New York
Amanda Freeman, founder of SLT-New York. Freeman was looking for a workout that was fun and that resulted in long, lean muscle tone.

"I was looking for certain results from a workout," said Freeman, a 41, a graduate of Harvard Business School and soon-to-be mom.  She opened SLT-New York's first studio in 2011 in midtown Manhattan, two years after she took her first class on a MegaFormer while on a business trip to California.  "I wanted a workout that was fun, challenging, and resulted in long, lean muscle tone."

Before opening SLT-New York, Freeman was CEO of Vital Juice, an online newsletter geared to the wellness community; Freeman sold the company shortly after SLT-New York took off.  In 2016, Freeman received a sizable, albeit undisclosed, investment from private-equity firm North Castle Partners that funded SLT's expansion to Philadelphia as well as to other cities including Hoboken, N.J., and Greenwich, Conn.

A few minutes into my SLT warm-up — which, quite honestly, left me drenched — it became clear that SLT-New York is all about form.

We started with a series of lunges, our hands lightly placed on a red pole shaped like a plunger with its flange facing up.

The lunges, repeated on each leg, each time for a little more than a minute — were followed with a series of planks; some straight, some side. These already hard movements were even more challenging because my core was fighting to keep me steady on the moving platform.

Clearly, this is how the abs get ripped.

Then came the hard work.

The workout was set to the beat of late' 90s dirty south hip-hop. (Think Mystikal's "Back That Thing Up.") And because of the playlist, the initial pain didn't feel so painful. More leg lifts. More lunges. More planks. Then a few side sit-ups. A little weight. A lot of weight. Too much weight.

Too, too much weight.

Eventually, I got the hang of it. And around the time I felt like I wasn't going to tumble off the platform to my death,  my first SLT class was over.

I was drenched.

Everyone else was just kind of moist.

Afterward, I had drinks with a girlfriend and  even made my way to Eakins Oval, where D.J. Mike Nyce spun a few old-school tunes.

But the next day — and the day after that and the day after that — my booty was on fire.

That, however, doesn't mean I won't go back.

Elizabeth Wellington [10:35 AM]
Philadelphia SLT-New York

1625 Walnut St., Floor 2. (The entrance is on Moravian Street.)
sltnyc.com/philly