Have a Seat and Start Working Out The founder of the New York City-based custom closet design company Clos-ette, Melanie Charlton, 37, does a lot of yoga and dance, but says she has had some of her best workout results sitting down. Josh Holland, 29, a trainer at the Core Club in Midtown Manhattan, has taught her and her eight employees to use a large rubber ball for "active sitting," a new mantra in the fitness community. Research has addressed the negative effects of excessive sitting. Dr. James Levine, director of obesity solutions at the Mayo Clinic and Arizona State University, for example, has found that too much time in a chair can increase the risk of diabetes, hypertension and cancer. If that doesn't scare you, Michelle Olson, Ph.D., an exercise physiologist at Auburn University in Montgomery, Ala., says that excessive sitting can flatten the buttocks, soften the stomach and round the spine. "Stability balls," as the equipment Mr. Holland uses are called, have been around at least since the 1960s, when they were popular in Europe for physiotherapy with adults. Physical therapists in the United States also began incorporating them in sessions with patients who had orthopedic problems. Now, though, the stability balls - some of them souped-up specifically to address concerns about sitting, some covered in snazzy fabrics - are becoming a coveted office accessory. "One of the hot topics in the fashion and entertainment world these days is how sitting is basically the equivalent of killing yourself," said Hitha Prabhakar, the founder of AitchPe Retail Advisory, a firm that specializes in retail analysis and fashion commentary. And you definitely hear people talking about the latest chair or ball they're buying to help. Mr. Holland, a brand ambassador for Technogym, uses its Wellness Ball ($225), which has a weighted bottom to prevent rolling, a black washable mesh cover and a QR code that allows users to access exercise tutorials on their phone and tablets. He teaches exercisers how to work with the ball at their desks to move while they sit. He and a handful of his trainers lead four sitting classes a week at the Core Club and at Technogym's SoHo showroom. The Wellness Ball has plenty of competitors. Ms. Prabhakar, 35, who says that her work often requires her to spend 10 hours at a stretch in front on her computer, which leaves her feeling sluggish, is remodeling her office. She is buying sitting gear from two other brands: the $100 balance ball chair from Gaiam, and a Muvman from the Munich company Aeris that resembles a chic stool and costs $700. Gaiam's new chair covers include $30 ones in brown, green and purple faux velvet and a $350 silk version handmade by artisans in northern Thailand in muted metallic colors. Lisa Sharkey, a senior vice president and the director of creative development for HarperCollins Publishers Worldwide, has a $299 textured gray terry cover, also from Thailand, for the stability chair in her office, whose design is so attractive, she said, "it reminds me of something I might see at the Whitney." Aesthetics aside, Ms. Sharkey, who is in her 50s, believes the chair gives her a fitness edge. She often takes the ball off its base to do back flies while she's on calls with the authors she works with, including Kardashians and Randi Zuckerberg and gives it part of the credit for her toned abs. "I swear it has helped give me visible transverse abdominal muscles," she said. Others fans include Monika Chiang, 39, a fashion designer, and Gillian Hearst, 32, the society editor of Town & Country. The Muvman, which Ms. Prabhakar is getting, is newer to the market than the Gaiam. Backless, it comes in a half-dozen colors and adjusts up so the feet are elevated off the floor. The seat tilts back and forth which supposedly helps engage the muscles from the abdominals down. Another model from Aeris, the $700 Swopper, which has a cushion with a spring, has been sold in the United States for more than a decade, but over the past few years orders of one at a time have jumped to 100 or more, according to Nora Fenlon, the marketing and sales vice president for Via Seating, its United States distributor. A few months ago, Turnstone an office furniture company based in Grand Rapids, Mich., came out with the Buoy ($200), a modern-looking stool that comes in 25 fabrics, including a handful of plaid patterns from the British men's wear designer Paul Smith for $100 more. Shelly Sable, 43, the design director for WorldStage Inc, an event production firm, ordered the Buoy after reading about it online. She says she likes how it lightly wobbles and tilts, forcing the lower body muscles to fire up. "I actually have a standing desk, but to me this chair is as good as standing because it doesn't let your body go soft," she said. Ms. Sable ordered a half-dozen Buoys for her office in a black base with different-colored tops including orange and magenta and said they were so popular with the staff that she bought six more. But are stability chairs destined to wind up in attics along with previous faddish fitness equipment like stationary bikes and toning sneakers? Dr. Olson, the exercise physiologist, said that although the chairs aren't necessary for active sitting, they can help. "Since you're not sitting in a stiff position, your body is reminded to move since the materials are malleable," she said. She advised, though, that moves should be done every five minutes for best results, not just sporadically throughout the day. Slobodan Randjelovic, 37, an architect, is more relaxed about his Wellness Ball, believing that even though he doesn't do conscious exercises while he is on it (aside from stretching his back), his legs and hamstrings get an automatic workout from balancing. "It might be psychological, but it makes me feel good to know that I'm doing something to reverse the effects of sitting," he said.