Bicep curls may seem like such a simple exercise, but there are still many ways to do this incorrectly or compromise your posture while doing it. One of the keys I've learned is to only use weights that are manageable for you; ones that you can do at least 8-10 reps with good form and full range of motion. It's great to challenge yourself, but using weights that are too heavy will cause you to do the following:
RIGHT PHOTO: USING MOMENTUM INSTEAD OF BICEPS
I'm using the momentum and swinging of the weights to lift the weights. Not using the biceps totally defeats the purpose of doing this exercise! This is seen usually in people who are lifting weights that are too heavy for them. And while I look a bit ridiculous here with my 8 pounders, I see people all the time with bigger weights struggling like this. Your back should never curve. If you can't lift the weights with just your arms, switch to lighter weights! Duh!
LEFT PHOTO: ROUNDING THE SHOULDERS FORWARD
Everything is too far forward in this photo. Remember that the elbows need to stay by your sides with your shoulders back and down and the wrists shouldn't curl, because it will work the forearm, not the bicep. The back should stay upright, not bending over, another common problem for people with weights which are too heavy! We don't want to hurt our backs, neck or shoulders!
MIDDLE PHOTO: GOOD FORM
Here I'm standing up straight with a dumbbell in each hand, with my arms at my sides and elbows close to my torso. You should start at arm's length. When you raise the weights, the upper arms should stay stationary, raise until they are fully contracted, about shoulder level. Then lower back to the starting position in a controlled manner, resisting using momentum or gravity, but rather using the biceps! I've seen people only lower halfway, but the correct way according to most trainers and bodybuilding sites is to lower to all the way back using the full range of motion. Happy curling!