On some exercises, such as the bent-over lateral raise, bent-over row, upright row, front raise and biceps curl, I'm much stronger when I do them with one arm than when I use both arms. Is this normal?
On many upper-body exercises, using one arm may seem easier than lifting with both arms simultaneously. There are a couple of reasons for this: For one, research has shown that doing upper-body exercises unilaterally (one arm at a time) allows you to use greater force than when doing the same exercise with both arms (bilaterally). This is due to the fact that you use more muscle fibers when you train with one arm at a time.
However, another reason that few people think of is cheating. Doing unilateral exercises doesn't prevent you from cheating - quite the contrary. You probably don't even realize it, but during some unilateral arm exercises you actually cheat more because you're using other muscle groups to assist the muscle you're focusing on.
How can this be? It has to do with torso stability. For example, consider bent-over lateral raises. When you use both arms simultaneously, the equal weight on both sides of your body balances your torso: As you lift the dumbbells out to your sides, it remains straight and balanced.
When you perform the exercise with just your right arm, the lack of counterbalancing weight causes your torso to twist toward your right side in the bottom position, forcing you to resist that twisting by contracting your obliques. As you begin to hoist the dumbbell with your right arm, the momentum generated by the action - along with the pulling of your torso muscles toward your left side to resist the pull of the weight on your right - causes your torso to rotate toward the left. This is cheating, despite the fact that you can't control it. This also happens on one-arm dumbbell rows as well as with standing exercises such as upright rows, lateral raises, front raises and dumbbell curls.
This doesn't mean you should stop performing unilateral arm exercises. After all, they provide the benefit of lifting heavier weight for greater overload. Unilateral exercises have also been found to offer another benefit - core training. Just watch out for cheating on unilateral arm moves. In some workouts it's smart to take advantage of this added momentum to train super heavy. But in others you need to be as strict as possible. Try slowing your rep speed or pausing for a second or two at the start and finish positions.
It's perfectly normal to be stronger on unilateral arm exercises because you stimulate more muscle fibers in the working muscle, and you use more fibers in the assisting muscles. Take advantage of this phenomenon by emphasizing the cheating in some workouts, while putting the brakes on cheating in others. This will lead to exactly what you want - balanced development.