Twelve stomach crunches.

That seemed easy enough.

Until I was practically tipped upside down over a gigantic fit ball. After only three crunches, where I struggled to pull my head and torso up from the side of the ball, it felt as if someone were marinating my stomach muscles in acid.

This was after an hour of strength-building lunges and squats, fast-footed drills through an agility ladder, sideways jumps over mini-hurdles, and two-footed jumps in a minefield of BOSU balls (short for “both sides utilized”).

The previous week, when we all blew through one-legged weighted squats too easily, Bill made us do them while balanced atop a BOSU ball.

Who knew there were so many ways to use these squishy half balls?

And no doubt Bill will think up more.

Welcome to iSport, Bill Knowles’ training and sports rehab facility in Killington, Vermont. Despite the fact that my entire body is sore, I feel lucky to be here — part of a five-week training class run by Bill and trainer Tyler White for a few of us has-beens and never-weres to get us ready for ski season.

Bill is a world-renowned athletic trainer who honed his skills while working as the head trainer and performance director at Burke Mountain Academy, a prep school for ski racers in northern Vermont. At Burke, he became so effective at dealing with knee injuries that injured skiers from around the world flocked to him.

Several years ago, he teamed up with the Vermont Orthopaedic Clinic and opened iSport. His mission: to help injured athletes get back to their sports even fitter than they were before.

Now world-class athletes from a variety of sports come to iSport to rehab and train. iSport clients have included Olympic skiers and swimmers, NFL players, and rugby athletes from Europe.

“Bill, Thanks for all your energy and expertise,” wrote 2010 Olympic moguls gold medalist Hannah Kearney on an autographed picture that hangs in iSport’s entry hall.

“Thanks for putting innovation and creativity into my training program, you’re the best,” signed three-time Olympic Chip Knight on his picture.

Athletes don’t come to iSport to just lift weights though. Training at iSport is all about building strength while doing the motions of your chosen sport — and maintaining balance through even the trickiest drills.

This means we aren’t sitting on weight machines or running on treadmills. Instead, the iSport facility is filled with equipment that makes it look, at first glance, as if it’s a day care center.

The flag-draped walls are lined with racks of brightly colored balls — large fit balls, weighted medicine balls, and blue BOSU balls, their bouncy side facing out. Nearby is a box of colorful stretchy fit bands of varying tensions. Next to the box are orange and yellow cones, tall orange “slalom” poles, blue and green balancing poles, racks with webbing and jump ropes draped over them, and the Pacer, a blue cube that’s made of foam.

Fun! Right?

Until Bill and Tyler make us do two-legged jumps (more like knee-ups) as fast as we can on the Pacer. Within seconds, I hate this foam cube.

Then there’s the dreaded blue rope. It’s thick enough and long enough to hold a large boat at anchor. Our job: to hold an end in each hand and keep the whole rope constantly moving for 30 seconds. It takes muscles I didn’t even know I had — and lung-fulls of air — to do the job.

“The NFL guys like it,” says Tyler, when asked why they have this hateful rope.

And probably a few of those rugby players too. But women with feeble arms? Not so much.

It’s not just the intensity with which we work out that Bill and Tyler are watching. It’s the way we perform each drill.

Better to do one perfect lunge — shin forward, heel down, knee stable, thigh parallel to the floor — than 10 wobbly, off-kilter ones.

“Arms straight!” says Tyler demonstrating with his arms perfectly straight over his head as he does a lunge. He gently taps my upper abs to show what muscle group I should tighten.

Who knew my stomach muscles were needed to hold my arms overhead? It’s hard.

“Chest up!” he gently chides as I collapse forward in a deep squat.

Again, hard.

Feet together, back leg straight, arms forward. Hard, hard, hard.

“Quicker!” Tyler says, when he sees my squats.

Ack!

“Legs together!” barks Bill as I’m coming undone on side-to-side Pacer hops.

Just when I think he’s going to go all drill sergeant on me, he breaks into a smile and chides, “You old school ski racer, you.”

(In the old days, we ski raced with our legs working independently.)

They don’t want us thumping through the drills like we’re wearing cement shoes either. “Light on your toes!” says Bill — scolding, yet smiling.

Perhaps this is why I never made it to the Olympics, I think, as I hammer through the Ickey shuffle on the agility ladder. It takes enormous strength to hold oneself lightly over the earth and not come crashing down with each step.

Not like the Olympic skiers and snowboarders with whom I’ve had the honor of taking a run or two. Or the World Champion cyclists whom I’ve pedaled next to for a few miles.

They all seemed to have a different relationship with gravity that I do. As they danced from edge to edge on their skis and snowboard, or accelerated up the road, they had the light feet of Fred Astaire.

And here I was clunking through the BOSU ball “moguls” like Herman Munster.

While I will never have Hannah Kearney’s light touch in the moguls, or Chip Knight’s quick feet in slalom, I have a new appreciation for how hard they train.

And while gravity will probably always have me firmly in its grasp, I hope by the end of this five-week session, that I can do at least one squat without wobbling.