When guys lift weights overhead using a shaky bar with weights swinging off it, their core and shoulder muscles have to work harder than when using a regular, steady bar.

When guys lift weights overhead with a shaky bar, their back muscles work harder—but not all wobbly bars have the same effect. One type makes certain muscles fire more, while another type ramps up even more muscles, showing that not all unstable bars are the same.

When men lift weights overhead with a shaky Earthquake bar, they wobble more than when using a regular or slightly shaky bar — their balance gets a tougher workout.

When male athletes lower a kettlebell during a shoulder press, their upper trap muscles work harder than when using a barbell—probably because the kettlebell wobbles more and needs more control.

If you're used to lifting weights and do shoulder presses on a bouncy ball instead of a bench, your lower back muscles have to work harder to keep you stable — even though you're probably lifting less weight.

When guys who lift weights do shoulder presses sitting down with a kettlebell instead of a dumbbell that weighs the same, their shoulder and back muscles seem to work a bit harder — maybe because of how the kettlebell is shaped and balanced.