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 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.

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.

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.

Lifting weights with a barbell over your head works the side and back parts of your shoulders more than using a shoulder press machine, because your muscles have to work harder to stay balanced.