This is the second in an occasional series on seemingly small acts of physical activity that can, over time, have a surprising impact on your life. Last week GGNC looked at taking the stairs vs. the elevator. Today: Watching TV on an exo ball rather than hunkered down in a La-Z-Boy.

Over the next 13 days you’ll sit and watch Apolo Ohno go in circles on the short track, Shaun White defy gravity in the halfpipe, and Lindsey Vonn — maybe — appear to break the sound barrier in the Super-Combined. And all the while you could be creating a six-pack with more body and definition than a Imperial Stout.

OK, that could be overstating things. But as you sit there watching others go for gold you could be silently developing the most important muscles in your body. And all it takes is a $25 investment from your local big box store. Rather than watch the games hunkered down in your La-Z-Boy, watch ‘em perched on an exercise ball.

What it is: In the 1960s, Swiss physical therapists began using exercise balls — bigger than a beach ball — ranging from 55 to 85 centimeters — and more durable to help kids with cerebral palsy improve their balance and equilibrium. Gradually, the exo ball — a k a Swiss ball, physio ball, fit ball — was adapted for rehab for other conditions, primarily for people with back troubles. Most recently, it’s infiltrated the workplace as a therapeutic alternative for office workers whose bodies don’t respond well to 8 to 10 hours seated at a computer.

What’s it do: The reason the exo ball has helped people with back pain is that it forces the muscles supporting the back — the abs, the glutes, et al — to constantly work to keep you balanced on this unstable surface. Since worked muscles become stronger muscles, many of the common causes of back pain (related to weak muscles) tend to disappear. Another benefit of stronger core muscles: better posture and those rock-hard, Imperial Stout abs.

Concerns: While the benefits of the exo ball for rehab are well documented, little research has been done on the effects of sitting on an exercise ball for extended periods. What little investigation that has been done suggests prolonged sitting can result in spinal shrinkage, muscle fatigue and discomfort, at least initially. A big reason some workplaces have been slow to swap office chairs for big balls: People occasionally fall off the balls. Reaction from the rank-and-file, the pioneers of proactive perching, seems favorable.

However … Even those dubious of the ball’s benefits over extended periods believe it has benefits.  “The exercise ball might be great for strengthening and toning in the gym or at home, but it can’t complete with a truly ergonomic chair for long-term sitting in an office environment.,” according to the Ergonomics Report, published for folks who make ergonomic equipment.

Tips: Some who sit on an exercise ball report that at first they were uncomfortable atop the ball. After a while though, that discomfort went away. Start slowly, in sitting sessions as short as two minutes, then build up. Several reports suggest you should sit at least 20 minutes to see and feel results from the ball.

Be sure to sit with your feet flat on floor, your hips and knees at 90 degree angles, with your hips slightly higher than knees. Make sure the ball is properly inflated and that you get a ball that’s right for your height. Suite101.com, offers this rough guide for finding the right size ball:

• 55 centimeters: 5’ or under
• 65 centimeters: 5’ — 5’7”
• 75 centimeters: 5’8” — 6’2”
• 85 centimeters: 6’3” +

Sitting may lead to … More aggressive forms of exercise. “Once you have the ball handy it’s easy to just roll back from the desk and do a few ab crunches,” Wendy Bumgardner, author of the About.com Walking guide, writes of the ball she uses as a desk chair. “I love doing crunches on the ball — it doesn’t irritate my hips like doing them on a mat does, and it doesn’t mess up my hair since my head doesn’t touch the floor.” Find some basic ball exercises here.

Sources: “The gym ball as a chair for the back pain patient: A two case report”; “Exercise Ball Moves to the Office: Just Sitting at Your Desk is a Workout”; “10 Reasons to Use an Exercise Ball as Your Chair”; “Core Exercises on the Ball”;
“Opinion: Balls as Office Chairs a Bad Idea,” “Having a Ball at the Office,”