They go by many names and come in all colors and sizes. They seem to be everywhere these days -- in gyms, physical therapy centers, offices, homes, even birthing rooms. But just what are you supposed to do with those big round things?
"You can do just about anything on the ball," says Bob Eckert, a personal trainer who teaches a stability ball class at the O'Fallon, Mo., YMCA. "With the ball you can get a great workout at home, right in front of the TV, even."
The stability ball (which is what we'll call it for the sake of consistency) was invented in the 1960s by an Italian toymaker. It was first used in physical therapy by a Swiss spinal rehab specialist.
The success of that therapy caught on as the ball made its way to physical therapy centers in the United States in the 1980s. Now, the balls are the new craze in the world of fitness.
How do the balls work? Well, they tend to roll, and because of that you have to engage a series of muscles just to keep your balance on them.
"Stability balls are great because they provide an unstable surface," says Becky Vitale, head of physical therapy at Missouri Bone and Joint Center and a certified Pilates instructor. "They force you to use your postural muscles just to keep your body stable. You engage muscles you may not otherwise use. It's really versatile and great for your trunk, for that all-important core stability."
Paul Frediani explains the benefits of core training in his book Powersculpt: The Women's Body Sculpting & Weight Training Workout Using the Exercise Ball.
"Like the steel cables of a bridge, your core is the foundation and support system of your body," he writes. "It stabilizes your spine and connects your upper and lower extremities. It is the central source of power and is essential for efficient movement."
In fact, movements on the ball have been proved more efficient than similar movements on the ground. The National Strength and Conditioning Association recently published a study that found crunches on a stability ball elicited more muscle activities in upper and lower portions of the abdomen than any other ab exercise studied.
Although the basic movement is essentially still a crunch, more muscles are engaged just trying to maintain balance on the ball.
Many people, however, still don't know how to get the benefits of the ball.
That was the case for Kandi Holbrook, 47, and Denise Sorenson, 47, both of Weldon Spring, Mo. They had been going to the O'Fallon YMCA for some time and had watched Eckert do a series of exercises on the ball.
"He had this whole routine," Holbrook says. "And it looked very challenging, so we wanted to try it. We had him show us how to do it."
The women then sent in comment cards to the Y's front office; from there, a stability ball class was born.
Eckert, who had started using the ball after a series of injuries, read and studied how to use the ball.
"The correct form is very important on the ball," he says. "So I watch the class carefully."
That's one of the things Sorenson appreciates. "It's a lot like personal training. He watches you do everything."
Sorenson, who used to lift weights, says she's benefited greatly from the class. "I've lost two dress sizes and gotten a lot more toned," she says.
Though the ball is a great exercise tool, it's also still widely used for rehabilitation.
Vitale says therapists use it for back and knee patients and for stroke and head-injury patients.
"It's a great tool for balance. People who've had certain injuries have trouble knowing where their body is in space, and this is great to help that."
She says some people who can't even walk can use the ball to work muscles that otherwise they'd only be able to work while standing.
In addition, the ball is replacing chairs at workstations all over the world. In Europe, some schools use the balls instead of chairs. Alice Marre, 26, a Web content manager at Washington University's office of public affairs, has become a believer in using the ball day in and day out at her desk.
"I was working with a physical therapist a little while ago because of some unexplained back pain," Marre said. "No one could figure out what was wrong with me. She gave me some basic exercises with the ball and said I could sit on it for a few hours a day to see how that worked."
Marre took the advice and eventually worked up to a full day. Now the ball replaces her office chair at the computer she's parked in front of at least eight hours a day.
"It really strengthens your core and makes you sit stick-straight," she says. "You can't lean and you can't slouch or you'll fall off. ... It's made such a huge difference for me. I'm pain-free. I grab anybody I can get ahold of and recommend it to them."
Marre has been so convincing that several of her co-workers have adopted the stability ball, too.
One of the most interesting uses for the ball has been for pregnant women in labor. It has become standard equipment in some birthing centers and hospitals.
At St. John's Mercy Medical Center in St. Louis, you can find one in every labor and delivery room.
"I'm a big fan of these balls," says Jodi Vuksic, a registered nurse and Lamaze educator at St. John's. "You can use them in early labor for comfort and for progress of labor."
Women can sit on the ball, bounce on it and rock back and forth to help ease the pain of labor and help move labor along.
Doctors usually recommend walking to speed labor, but most women tire after just a little while of that, Vuksic says. "This offers a happy medium. They are still moving around, but without all the weight.
"The ball helps free pelvic mobility, which allows labor to speed up. It opens up the pelvis and allows more room for the baby to come down, which is great if you are having a big baby. Once you have the epidural (after early labor), you're kind of stuck in bed, but if you are doing natural childbirth, you can actually push while sitting on the ball."