A runner since his days at the University of Chicago Law School, Paul Toback had been dogged by back pain for years. He noticed that his most challenging and least painful runs were on sand along Lake Michigan.

So when Mr. Toback, 50, left his job as CEO of Chicago-based Bally Total Fitness Holding Corp. in 2006, he began tinkering in his north suburban Bannockburn garage, trying to build a better treadmill that simulated beach running. He hired Steve Lenz, formerly vice president of engineering at Rosemont-based Life Fitness Inc., the world's largest maker of commercial fitness equipment.

The pair built the first machine, which they dubbed Sproing, from an air mattress and wood planks. Four prototypes later, the perfected Sproing was introduced in 2011, selling for around $6,000 to rehab centers and gyms in the U.S. and as far away as India.

“We invented Sproing to have the cardio burn of a treadmill and the low impact of an elliptical,” Mr. Toback says.

The machine replaces a traditional treadmill's hard base with a soft, adjustable air cushion. A user is tethered to the back of the machine with a harness around the waist, which provides resistance while encouraging correct running form. “As a runner, you should fall forward and strike the ground with your forefoot,” Mr. Toback says. “Conventional treadmills encourage runners to strike with their heel, which can lead to injury.”

Two resistance bands anchored to the back of the machine can be used to do strength moves such as bicep curls and overhead presses.

THE WORKOUT

In addition to selling to fitness chains, physical therapy providers and professional sports teams, Mr. Toback opened a boutique gym in Old Town in April. Sproing Sport offers 45-minute high-intensity interval training sessions that combine sprints with strength training — all on the machine.

A two-time marathoner, Mr. Toback had scaled back his runs to once or twice a week due to back and knee pain. Now he completes six Sproing workouts per week — a testament, he says, to the machine's low-impact nature (and the convenience of putting Sproing's corporate offices in the back of the studio). “I'm developing muscles I didn't even know I had,” he says.

THE DIET

Mr. Toback has oatmeal or a protein shake for breakfast and grilled chicken — either in sandwich form or with veggies — for lunch. Dinner depends on his family's schedule, but he avoids red meat. Mr. Toback eats pizza twice a week, but it's made with low-fat cheese and no pepperoni or other processed toppings.

THE COST

A drop-in class at Sproing costs $25. Prices drop as low as $15 per class if you buy a package.

The machines begin at $5,499 for a basic model without resistance bands.