Sitting on a chair requires no muscle activity at all and it actually hurts us for a number of reasons, says exercise physiologist John P. Porcari. “For one, people tend to slouch and use poor posture, and sitting in a chair puts your abs on ‘slack’ and decreases core strength. Using an exercise ball counteracts both of these things.” Although you can still use poor posture on a ball, sitting properly requires subtle muscle contractions of the core, hip and leg muscles to maintain balance, which contributes to muscle tone.

But there are some drawbacks, says exercise science professor Jessica Matthews. “Sitting on a ball creates an increased load on the lumbar (lower back) spine and discomfort over time. Prolonged periods of sitting, whether in a chair or on an exercise ball, can also lead to poor body mechanics and posture.” 

Instead of swapping one for the other, avoid the “all or nothing” mentality so common in health and fitness, says Matthews. “Instead of ditching your desk chair entirely for an exercise ball or skipping out on using the ball altogether, Matthews recommends alternating between sitting on the two throughout the day.” Try 20 to 30 minutes at a time. And be sure to inflate the ball to its proper height; your thighs should be parallel to the floor when you’re seated on the ball. A 45 cm ball works best for women under 5 feet tall; a 55 cm ball fits women between 5’ to 5’ 7” and a 65 cm ball is best for those 5’8” and over.

The bottom line: Sitting on a stability ball doesn’t automatically improve core strength, but when used correctly and with good posture it offers subtle benefits as an alternate desk chair.