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 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 male athletes do overhead presses, using a kettlebell makes their lower back muscles work harder than when using a barbell — probably because the kettlebell is less stable and wobblier.

If you're already strong and used to lifting, doing overhead presses with shaky weights or on wobbly surfaces probably won't help you build more muscle or get stronger — and you'll likely end up lifting less weight overall.

When guys who lift weights do a one-arm shoulder press, using heavier weights (up to 70% of their max) makes several key shoulder and back muscles work a lot harder than using just a light 6 kg dumbbell.