With your hands shoulder-width apart,grip a barbell with an underhand grip. Stand straight up with your shoulders squared and with your feet shoulder-width apart. Let the bar hang down at arm's length in front of you,with your arms,shoulders and hands in a straight line. WITHOUT leaning back or swinging the weight,curl the bar up toward your chest in an arc. Keep your elbows in the same place and close to your sides. Bring the weight up as high as you can and squeeze the biceps at the top. Lower the weight slowly,resisting all the way down until your arms are nearly straight.