Muscle Group: Biceps
Machine Preacher Curls work the biceps through the longest range of motion possible. Because a machine is fixed, the movement becomes extremely strict and you are able to feel resistance from a full extension to a full contraction at the top. Consider Machine Curls to be a shaping and finishing exercise instead of one designed to build mass.
Every time you curl with a machine, position yourself with your elbows on the pad and grip the bar with an underhand grip.
Curl both arms as far as possible, get a strong contraction at the top, then let the weight down slowly till fully extended.
For an alternating variation, curl one arm to a point of peak contraction. Let the weight down slowly till fully extended, then do the same with the other arm, and continue to alternate the two arms until you have completed your set.