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 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 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 strong male lifters do overhead presses, using a kettlebell makes the forearm muscles work harder than using a barbell — probably because the kettlebell wobbles more and is harder to grip.

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.