It's not meant to replace barbell or two-arm bench presses, but the one-arm bench press helps build your chest muscles while using more of your core for stabilization. It's also helpful to fix muscular imbalances you may have. Since you're not distributing the weight across a barbell or mirroring the movement of the other side, each side has to do the same amount of work.
Before you start your presses, carefully pick a dumbbell weight. If you typically perform two-arm dumbbell presses, choose a slightly lighter weight for the one-arm press. The one-arm press can throw you off balance if your core isn't ready to hold your body stable on the bench, so stick with a lighter weight until you're confident with the move. Sit on the bench and rock back slowly into a lying position, lifting your dumbbell arm as you lie back; your elbow should be straight above your shoulder. Use your other hand to stabilize the weight as you move, then move it to hold onto the bench. Have a spotter help you if necessary.
Getting Into the Action
The dumbbell bench press movement is fairly straightforward. Lower the weight toward your shoulder by bending your elbow below the level of the bench, but stop the movement before the weight reaches your shoulder. Raise it straight up again until your elbow is fully extended. It should take about twice as long to lower the weight as to raise it, which will help ensure complete control.
Building Those Muscles
The one-arm dumbbell bench press works the same muscle groups as standard bench presses. Your chest does most of the work, specifically the pectoralis major. The deltoids in the front of your shoulders help stabilize the movement, as do your triceps. Your biceps help to a smaller extent. Because you're only working one side, your abdominal muscles help hold your body steady during the movement.
Get the Most Out of the Press
When performing the one-arm dumbbell bench press, get a fuller range of motion by lying off-center on the weight bench. If you're working your right arm, keep your left shoulder completely on the bench, while your right shoulder and shoulder blade hangs off slightly. Also, hold the dumbbell so it's turned slightly rather than being parallel to the weight bench. Point the thumb side in toward your head at about a 45-degree angle.