Unstable devices like the Bosu ball, Airex pad, Wobble Disc, or balance board are touted as strength builders. Their variability increases the difficulty of exercises, calling into play more muscles and making them work even harder with every rep.

But have you ever seen a record-setting weight lifter, an elite powerlifter, or Mr. Olympia using a Bosu ball while doing lower-body strength exercises?

Probably not—and there are several reasons for that, says Bret Contreras, M.A., C.S.C.S., owner of bretcontreras.com.

For starters, performing heavily loaded squats, lunges, hip thrusts, or deadlifts with your feet on the ground is risky enough, he says. Stepping onto an unstable surface while performing big movements like those only increases your risk of injury.

Secondly, Contreras says your glutes can’t “turn on” all the way if you’re standing on something like a Bosu ball.

“When you’re on an unstable surface, your glutes are working hard to keep you balanced,” he explains. “Because of that, your glutes are distracted from performing the task at hand: lifting and lowering the weight.”

Remove the Bosu ball, though, and suddenly your glutes can put all of their effort behind moving iron.  “The glutes prefer a stable environment in order to achieve their maximum activation potential,” says Contreras.

And when you can fully activate your glutes, you can lift heavier loads, he explains. That leads to a bigger calorie burn per workout and more muscle all over—not just in your backside.

Contreras also points out that every rep you perform on an unstable surface is different, making it difficult to achieve proficiency in an exercise and stalling your results over time.

Keeping your feet on solid ground, however, allows you to perfect your form, he says. You’ll be able to recruit more muscle and do the movement the way it’s meant to be performed, maximizing your muscle and strength gains.

So are there any times you should use a Bosu ball, Airex pad, Wobble Disc, or balance board?

Definitely, says Contreras. His opinion is that you can add them under your hands or forearms for an extra challenge during stability exercises like the plank, side plank, or mountain climber.