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.

For guys who lift regularly, the top part of their shoulder muscle (trapezius) works differently when doing overhead presses — it doesn’t ramp up much with kettlebells and doesn’t respond as strongly to heavier weights, which might mean it’s more about keeping things stable than powering the movement.

When guys who lift weights do the lowering part of a shoulder press, using a kettlebell makes the front shoulder muscle work harder than using a barbell — probably because the kettlebell is less stable and harder to control.

When guys who lift weights do a shoulder press with a kettlebell instead of a barbell, their butt muscles work way harder to keep them stable.

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.

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.