LEE, Mass. – Fitter Critters & Aqua Paws might be the prime example of why dog and man are best friends.

Kalia, a 9-year-old greyhound, walked on a treadmill submerged in a tank of water that filled up just under her neckline. She pushed against the water’s resistance to keep up with the moving floor below her. The dog biscuits handed to her at the edge were incentive to keep her paws moving.

For two years, the underwater treadmill has been a staple of Fitter Critters for aging, recovering or injured pooches.

Kalia’s owner, Cindy Rancourt of Ware, noticed Kalia limping after one of her races. The therapy session at the end of last month was the retired racing greyhound’s third time on the underwater treadmill.

“Now the limp is gone,” Rancourt said. “I could tell an immediate difference when we went out for walks and she was able to stay by my side.”

The $54,000 piece of equipment is one of four in Massachusetts, the next closest one being in Springfield. With most of the dog’s body submerged in the tank, the apparatus utilizes the water’s resistance, buoyancy and viscosity to strengthen a dog’s injured, arthritic or aging muscles.

Water levels can be adjusted to accommodate dogs ranging from a majestic Newfoundland, to a petite Chihuahua.

“When they’re standing up to their necks in water, they’re 80 percent weightless, and you don’t get that kind of weightlessness anywhere, except the moon,” said Jody Chiquoine. She opened Fitter Critters, and the facility’s pool portion, Aqua Paws, in 1999 at her home in Lee.

“A dog swimming for 60 seconds is equivalent to a person walking 15 minutes,” Chiquoine said.

Chiquoine, a former nurse practitioner, combined her vocation, her love for dogs and her love of swimming to establish the first pet rehabilitation center in Massachusetts.

“I just happened to be on the cutting edge,” Chiquoine said. “I think dogs were ready for this.”

Chiquoine’s husband, Tim, built the Fitter Critters addition to their house. The facility has hosted interns from as far away as Holland and Canada, and treats clientele from all over the region.

“A lot of people who choose not to do surgery come here,” Chiquoine said.

Although there are the obvious differences in anatomy between humans and dogs, there are plenty similarities, too, Chiquoine said. Though she had some training in pet rehab later on, her training as a nurse practitioner is what Fitter Critters & Aqua Paws was founded on.

“I think holistically, and the impacts the treatment will have on the person,” Chiquoine said. “I do everything as a nurse practitioner.”

The panting pups that took turns in the underwater treadmill looked like they were smiling as they trotted along on the underwater treadmill. Clients like Barney, a 10-year-old Bernese Mountain Dog, walked with finesse in the water — after limping slightly moments before, on dry ground, to get into the water tank.

“This is exercising him without hurting him,” said Barney’s owner, Paul Deres of Canaan, N.Y.

Hydrotherapist Mary van Alstyne stood in the underwater treadmill tank with the various dogs. Afterward, the wet Alstyne dried off the soggy doggies with a towel.

Van Alstyne will either stand in front or behind the animal as it walks on the treadmill. For smaller dogs, a seat is available to attach to the tank for her to sit on.

“We’re trying to get the full range of motion,” she said. “It’s surprising how many dogs just get in and start walking.”

The Aqua Paws portion is the 5,000-gallon pool that the canine clients swim in. Always heated at about 90 degrees and assisted by hydrotherapist Joanne Nunes, the pool is used for weight loss, joint and muscle rehabilitation, and even therapy for dogs with cancer.

Rascal, a 13-year-old black Labrador that’s had a history with Lyme disease and partial paralysis, was being assisted by Nunes as he stood on a ledge on the side of the pool.

Rascal’s owner, Lakeville, Conn., resident Harriet Weiss, has been taking him to Fitter Critters since after the dog had back surgery in 2008.

“It’s helped him maintain muscle mass that’s so easy to lose,” Weiss said. “For a 13-year-old dog his size, he’s pretty active.”

As a hydrotherapist at Fitter Critters, Nunes said she’s seen plenty of dogs, including Rascal, improve because of their time at Fitter Critters.

“That’s the reward — when you see a dog that came in half-crippled start to walk again,” she said.