Why are dumbbells so much hard than a bar? A barbell locks movement in to a more structured plane of movement, which creates what we call a bilateral movement. Both arms (or legs) are working together to stabilize and move the bar. When using one or two dumbbells, it is called unilateral movement - even when both arms (or legs) are working, as you must stabilize and move interdependently. Bilateral movements are #1 when it comes to getting stronger, but unilateral exercises lay bare instability and strength asymmetries. Do both BB and DB movements and unlock your strength potential!