Standing dumbbell curls engage several stabilizers.
The biceps can be tough to build because of the repetitions necessary to do so. Unlike other muscle groups that you can strengthen with a variety of exercises, the biceps must be engaged through basically the same motion: the curl. This can get boring, slow your progress and give you an incomplete arm workout. But you can change up the dumbbell curl to provide a more well-rounded experience. For example, performing curls while standing, or in an alternating pattern, provide substitutes for seated, one-arm curls.
The dumbbell arm curl targets the biceps, but it also engages the anterior deltoid and the trapezius muscles for stabilization. The exercise begins with the weight at your side, then you bend your elbow and lift the weight upward. Keep lifting until your elbow is completely bent, then control the dumbbell back down to your side. The curl is a basic isolation exercise that requires the movement of only the elbow joint. Keep the wrist in a neutral position and don't rotate the shoulder.
The standard barbell curl forces you to work both arms in unison. While this can help you to lift a heavy amount of weight, it also allows your dominant arm to compensate for your weaker arm, since the force of the weight is distributed across the plane of the barbell. Dumbbells allow you to alternate arms, letting you focus special attention on one arm at a time. The alternating dumbbell curl also gives you plenty of options. You can perform them while seated in a chair, standing or even while lying flat on a bench.
The standing dumbbell curl forces your body to engage more stabilizing muscles than seated or supine curls. To maintain your posture, stabilizers in your legs and lower back also activate through both phases of the curl. If you perform your standing curls in unison, you increase the level to which your lower body engages to keep you upright, since you essentially double the weight that's actively moving during the exercise.
Ideally, to keep your arm workouts from becoming stale, you'll incorporate both styles of dumbbell curling into your routine. In fact, since you can perform alternating curls while standing, you can efficiently achieve the best of both exercises while limiting the drawbacks of each. For example, the chief benefit of alternating curls is the ability to concentrate on one arm at a time. The primary benefit of standing while curling is a broader engagement of stabilizing muscles. Alternating curls performed while standing gives you both of these benefits simultaneously through every repetition.