This exercise is excellent for strengthening, building muscle and toning the arm. The main muscle worked in this exercise is the bicep and this also works the forearm. It is always beneficial to incorporate a different direction when working the biceps since you have two muscles to develop.  Instead of pulling in a forward movement with palms up; try this hammer curl exercise performing the movement with palms facing each other.

• Hold a dumbbell in each hand, stand with feet shoulder width apart with arms hanging at your sides, palms facing each other.
• Keep your elbows locked close into your sides.  Your upper body and elbows should remain in the same place during the whole lift.  
• Without leaning back or swinging the weight; slow and controlled curl the weight in both hands up in a semi-circle toward your shoulders.  Keep your palms facing each other throughout the whole movement.
• Squeeze the biceps hard at the top of the lift and then slowly lower resisting all the way down until your arms are nearly straight.
• Do 2-3 sets of 12-15 repetitions.

Tip:  Do not turn your wrists during the lift.  You can also perform this exercise with one arm at a time and/or alternate.