“Take it into a jog! It’s a hard two-minute jog,” barks Katie Honaker.

“Every 15 seconds reverse direction. You want to make these reversals as fast as possible. Knees high! … At the end of this 15 seconds, run it hard to the wall.”

Sounds like a track coach putting her team through a tough practice. Only there’s not much solid ground in sight, let alone a track.

Honaker, a group instructor at the Maryland Athletic Club in Timonium, Md., is pacing the side of a pool, exhorting 18 members to keep themselves moving and their hearts pumping.

This isn’t your basic laid-back aqua aerobics class. It’s called “Deep Water Running.” Think of it, in the lexicon of water sports, as synchronized sweating.

These “runners” may be reminiscent of cartoon characters trying to negotiate a mudslide – legs spin furiously, forward progress incremental – but there’s nothing funny about this hourlong workout.

Ask former Johns Hopkins lacrosse player Brian Kelly. He has not-so-fond memories of his first few classes.

“There were times I was ready to throw up,” he says.

‘An unbelievable workout’

Health club instructor Katie Honaker leads a deep-water running class. Swimmers don weighted vests to run laps in the pool, which is a good workout for the heart and lungs, but much easier on the knees than conventional running.

Kelly, a 33-year-old real estate salesman from Lutherville, Md., has had seven arthroscopic procedures on his knees, which are almost devoid of cartilage. Road running and pickup basketball games are now so painful they’ve become distant memories of his college days.

Deprived of his two favorite forms of exercise, Kelly found himself creeping toward obesity. He came to the Maryland Athletic Club last December intending to take up swimming. Someone suggested he instead try deep-water running.

“I got in the pool, and it was an unbelievable workout,” Kelly said. “Thirty-five pounds later, I’m still at it. There’s no impact. There’s no stress on the joints. It’s a total-resistance, full-body workout.”

That’s the beauty of deep-water running: Put on a flotation belt or foam ankle cuffs, jump into the pool, and it’s “Look, Ma, no gravity.” You can replicate the mechanics of running without suffering any residual wear and tear on the body.

What’s more, since water provides 12 times the resistance of air, the rewards of expended energy are geometric: The faster you move, the harder your muscles work. Imagine jogging in chest-deep molasses, then sprinting through it.

One study by exercise researchers at the University of New Mexico found that deep-water running burns 11.5 calories a minute, about the same rate as a 9-minute-per-mile road run.

Rose Anne Kearns, of Towson, Md., participates in a deep-water running class at the MAC health club in Timonium. Swimmers wear a weighted vest, then jog or perform exercises that simulate the aerobic benefits of running.

Tough switch

Unfortunately, switching mediums doesn’t come naturally. Count on experiencing moments of spasticity during those initial deep-water runs. This is one instance where being out of shape has its advantages.

“The more fat you have, the more buoyant you are,” explains Kay Carney, a MAC aquatic personal trainer. “For people who are leaner, it can be very difficult.”

Carney has been holding private deep-water running sessions for several months with Jennifer and Jack Kent of Cockeysville, Md. They’re sister and brother. Both are in their early 20s, both athletic, and both coping with surgically repaired knees. This gives them a good cross-training option.

A typical workout combines long water runs with interval sprints that can range from 10 to 45 seconds, plus some underwater abdominal crunches.

“I got used to it pretty quickly,” Jennifer says.

“It doesn’t feel like running. It’s weird,” says Jack. “But after doing this a couple of months, I definitely feel stronger and not a lot of knee pain.”

Martha Q. Robinson, another Carney client, says she also felt “weird” at first. She’s preparing to run a marathon this October and turned to deep-water running partly as a preventive measure.

“The main reason I wanted to do it is I’m a little behind in my marathon training,” explains Robinson, 50. “As I up (the distance) I don’t want to get hurt. I also wanted to do more speed work to get my time down.”

Not just for ‘old ladies’

Doug Stern, who teaches deep-water running classes in Manhattan in conjunction with the New York Road Runners Club, guarantees racing success: “This is an intense workout. Every single person who runs in my class and then runs on the road improves (his or her) time.”

Today about 75 percent of the people who take Stern’s classes are female, he said. Men seem to regard deep-water running as patty-cake aerobics.

“They think it’s for old ladies,” he says.

A variation of that prejudice exists at the Maryland Athletic Club, which offered a deep-water running class at night that flopped.

“Young people are going to do step class and spinning,” says Carney. “Till they get hurt.”

Then, suddenly, they’re receptive to deep-water running.

Women over 50, on the other hand, tend to be an easier sell from the start. The club’s early-morning class has been a big hit with that older-demographic crowd – and with Brian Kelly, the only man who shows up every day for the 8 a.m. class.

“It has been a big benefit in my life,” says Kelly, who hasn’t been in this good a shape since he played college lacrosse. “If I can inspire somebody who may not be looking at aquatics as a way to get fit, that’s great.”