Here’s a quick history lesson: George Zottman was a 19th century strongman who built 19-inch arms and 15-inch forearms using, among other exercises, a unique palms-up/palms-down combination dumbbell curl. Zottman weighed 215 pounds at just under 6-feet tall, and would regularly use a pair of 50-pounders for slow, controlled reps.
Here’s how to follow in his footsteps (or shirtsleeves, as the case may be.) Grab a pair of dumbbells and perform a “regular”, supinated curl (with palms up). When you reach the midpoint of the movement and the dumbbells are at shoulder level, rotate your palms down (pronate) and thenlower the weights. At the bottom, rotate to palms up before starting to curl the next rep.
This is one of the best all-around arm exercises, since it targets all of the elbow flexors in the biceps and forearms together. It does require a bit of flexibility and coordination, so take a few sessions to get comfortable with the movement. Using 4×5-7, as your primary biceps exercise, will be plenty to build old school arm size.