ADJUSTABLE DUMBBELL TRAINING
In comparison with any other method of resistance exercise, adjustable dumbbell training is the ultimate form of weight training. It is true for many reasons. As a general rule, dumbbells enjoy the same advantage over machines that barbells do. The main advantage is that they allow for synergistic and stabilizer muscles to come into play far more prominently than most machines are capable of doing. In fact, this is also the major advantage that dumbbells have over barbells. The reason is that stabilizers must act in all directions with dumbbells, whereas the barbell connects your two hands together, thereby offering a measure of greater stability.
Information via International Sports Sciences Association