What do you make of the BOSU Balance Trainer? It’s a lump, a mushroom. A semicircle that exists somewhere between those big inflatable exercise balls and pancake-flat wobble boards.

BOSU stands for “Both Sides Up,” because you can use either side. Both challenge your balance, leg strength and core muscles. When used as a stair-stepper, it can also be a cardiovascular challenge. It is a relatively new product that costs about $130, but is making headway in health-club fitness classes.

Seattle trainer Linda Ross has incorporated the device into a recent exercise video, “Zen Fusion,” using movements from yoga and tai chi. This fall, she taught her techniques to trainers and teachers at clubs around town. I caught a class taught by Heather Parsons, one of her pupils, at Seattle Fitness.

After a warm-up, Parsons led 13 students of various shapes, sizes and fitness levels through an energetic workout. They did lunges and a series of stepping and jumping movements designed to get heart rates high. (One participant who wore a heart-rate monitor reported an average 150 beats per minute.) They did push-ups on each side of the BOSU. Working on the domed side isolated different muscles than those activated while working the flat but teetering side. Some of the maneuvers involved barbells, hand-held weights and exercise bands. The devices are stable on hardwood floors like the one at Seattle Fitness, but club owner Tija Petrovich said the flat side can slip on carpeting. She also suggests not working out on the BOSU while wearing running shoes.

Another move you can do on the device is simply jumping. Stand with both feet on the domed side, squat and jump, lifting your arms above your head as you reach peak. Land with your feet evenly spaced back on the dome and proceed until you’re back in a squat. Your body must quickly find center when it lands, and the maneuver works the quads, calves and hamstrings.

I’ve also seen people standing with one foot, balancing atop the dome, or using it to do crunches or back strengthening, or lying backward over the dome to stretch the lower back. You can also do push-ups with your feet instead of your hands atop the dome. As versatile as the BOSU is, I could imagine this getting old, which is why a creative instructor is important.

Ross, who usually teaches the class at Seattle Fitness in downtown, also offers a class at Rain Fitness in the Queen Anne neighborhood of Seattle.

The thinking behind the BOSU or any other balance and stabilization tool is that opposing forces equalize one another. Because muscles on each side are working against each other, you build symmetrical strength. The BOSU (www.bosu.com) boasts that simply standing on the dome can engage and develop the central nervous system and that the body intuitively finds its most efficient neurological path. The BOSU is used in some schools and programs that cater to children with sensory-integration issues.