Preacher curls look simple—you're just curling with your arms on a pad. But the angled pad fundamentally changes which muscles work and how hard they work. Let me explain exactly what's happening when you do preacher curls.
Primary Muscle: Biceps Brachii
The biceps brachii is the main target of preacher curls. This is the large two-headed muscle on the front of your upper arm responsible for flexing your elbow and supinating your forearm.
But here's what makes preacher curls special: the positioning emphasizes the short head of the biceps more than many other curl variations.
Why the short head? The short head attaches to the coracoid process of your scapula (shoulder blade). When your arm is positioned forward—as it is on a preacher bench—the short head starts in a shortened position. This shifts more of the work to this inner portion of the biceps.
The short head contributes to bicep width and thickness when viewed from the front. If you want arms that look big in a tank top, short head development matters.
Secondary Muscle: Brachialis
The brachialis sits underneath your biceps and is a powerful elbow flexor. During preacher curls, it works hard—especially in the lower portion of the movement.
The preacher bench position is actually excellent for brachialis development. The arm-forward position reduces bicep leverage somewhat, forcing the brachialis to contribute more to the curl.
A well-developed brachialis pushes your biceps up from underneath, making your arms look bigger. It also adds width to your arm when viewed from the side.
Secondary Muscle: Brachioradialis
The brachioradialis is the thick muscle on the thumb side of your forearm. It assists in elbow flexion, particularly when your forearm is in a neutral or pronated position.
During preacher curls, the brachioradialis works throughout the movement, though it's less involved than during hammer curls or reverse curls. Still, heavy preacher curls will contribute to forearm development.
Stabilizing Muscles
Several muscles work to stabilize your body and maintain proper position:
Anterior deltoid: Your front shoulder helps stabilize the upper arm against the pad. This is minimal compared to standing curls, which is the point—preacher curls isolate the biceps by reducing shoulder involvement.
Core: Especially with heavy weight, your abs and lower back engage to keep your torso stable.
Wrist flexors: Work to maintain grip and wrist position throughout the curl.
Why Preacher Curls Are So Effective
No cheating possible: With your arms braced against the pad, you can't swing, use momentum, or recruit your shoulders to help. Your biceps have to do the work.
Stretched position overload: The angle of the bench means maximum resistance occurs in the stretched position (bottom of the curl). Some research suggests this stretched-position loading is particularly effective for hypertrophy.
Superior mind-muscle connection: Because you can't cheat, you can focus entirely on feeling your biceps contract. This improved awareness often leads to better muscle recruitment.
Safe failure training: Unlike standing curls where you might drop a barbell on your toes, preacher curls let you train safely to true failure. The weight just rests at the bottom if you can't complete a rep.
Muscles NOT Heavily Worked During Preacher Curls
Long head of biceps: While not completely inactive, the long head is de-emphasized in the preacher position. For long head development (bicep peak), add incline curls to your routine.
Posterior deltoid: No rear shoulder involvement.
Back muscles: Unlike chin-ups or rows, preacher curls don't recruit the back. Pure arm isolation.
Triceps: Your triceps are completely passive during preacher curls—they're the antagonist muscle that stretches as your biceps contract.
Common Preacher Curl Mistakes That Reduce Effectiveness
Partial reps: Stopping short of full extension misses the stretch position—arguably the most valuable part of the exercise. Go all the way down (slight elbow bend to protect joints).
Lifting elbows off pad: The moment your upper arm leaves the pad, your anterior deltoid takes over. Keep arms pressed down throughout.
Going too heavy: Ego weight forces cheating and reduces bicep work. Use a weight you can control for 8-12 quality reps.
Preacher Curl Variations and Their Targets
Dumbbell preacher curl: Each arm works independently. Great for fixing imbalances. Same muscle emphasis as barbell version.
EZ bar preacher curl: More comfortable grip than straight bar. Slightly less supination but easier on wrists.
Single-arm preacher curl: Maximum focus on one arm at a time. Better isolation and concentration.
Reverse preacher curl: Palms down grip. Shifts emphasis to brachioradialis and forearm extensors. Less bicep, more forearm.
How to Maximize Muscle Activation
Full range of motion: All the way down to stretch, all the way up to squeeze.
Slow negatives: 3-4 seconds on the way down. This is where significant muscle damage (the good kind) occurs.
Peak contraction: Squeeze hard at the top for a one-count. Don't just bounce through reps.
Appropriate weight: Heavy enough to be challenging, light enough to maintain perfect form.
The Bottom Line
Preacher curls primarily work your biceps brachii (especially the short head), with significant contribution from the brachialis and some from the brachioradialis. The exercise's effectiveness comes from its strict isolation—your biceps have nowhere to hide.
Include preacher curls in your arm routine for the unique stretch-position loading and short head emphasis they provide. Combine with incline curls (for long head) and hammer curls (for brachialis) for complete bicep development.