Focus On The Brachialis AND Brachioradialis
The main upper arm flexors include the two heads of the biceps, the brachialis, and the brachioradialis. But when you look at most training programs, rarely will you see any brachialis or brachioradialis-focused exercises.
That's the main reason for focusing on both of those muscle groups. They've been neglected and proportionally undertrained, which makes them an easy target for rapid growth.
Most people can quickly add upper arm size simply by focusing in on the brachialis and brachioradialis for six weeks. How do you do that? First by varying your grip.
To focus on the brachialis, use the hammer (neutral) and pronated (reverse) grips. Also use a slower contraction speed (something like two seconds up and four seconds down – especially on the pronated curls. Adding isometric holds, either with the forearm parallel to the floor or at the peak contraction, will also increase brachialis activity.
To focus on the brachioradialis, your best bet is any form of reverse (palms down) curl. To increase the reliance on that muscle, use a faster contraction speed (without cheating) than what you'd use to target the brachialis.
- Dumbbell Hammer Curl (with a slow eccentric)
- Rope Hammer Curl (holding the peak contraction two seconds on each rep)
- Cable Reverse Curl (hold with forearms parallel to the floor and a slow eccentric)
- Dumbbell Hammer Seated on an Incline Bench (hold the peak contraction for two seconds on each rep)
- Zottman Curl (lift with a palms-up grip, lower with a palms-down grip)
- EZ-Bar Reverse Curl (with a normal tempo)
- Dumbbell Reverse Curl (seated on an incline bench)
Start your biceps workout with a close-grip chin-up (I'd actually go with a pronated grip this time) then use 3-4 other exercises picking from the moves listed above. Keep the chin-up in every one of these workouts and focus on improving performance for 6-8 reps. The other 3-4 exercises can be rotated if you want and the reps should fall between 8-12.
For 4-6 weeks, don't do any biceps-dominant exercises. That means any curl with a supinated or palms-up grip. Instead, specialize on the brachialis and brachioradialis. Don't worry, your arms will still get completely stimulated from your other exercises. And they'll respond even better once you train them directly again.