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.

If you're used to lifting weights and do shoulder presses sitting down, using an unstable weight or an unstable surface means you won't be able to lift as much weight.

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.

Even if you cheat a little to lift heavier, your muscles still grow just as well—so maybe the key to building muscle isn’t perfect form, but just getting your muscles to work hard enough.

If you're used to lifting weights, your triceps work harder during shoulder presses when you use a barbell on a solid bench than when you use dumbbells on a wobbly ball — the more stable the setup, the more your main muscles have to engage.

Lifting heavy weights regularly is needed to get the most strength gains in exercises you practice, because doing lighter weights until tired gives less strength boost even if muscles grow the same size.