Barbell Curl & Skull Crusher: The Complete Barbell Arm Training Guide

The barbell curl and skull crusher (lying triceps extension) are the two foundational barbell exercises for the arms. Together they train both sides of the upper arm — the biceps brachii on the front and the triceps brachii on the back — with the heaviest load any isolation movement can handle. Because the triceps makes up roughly two-thirds of your upper-arm mass, building it is the fastest route to bigger arms, and the barbell skull crusher is one of the most stimulating triceps exercises available.

Why choose a barbell for arms over dumbbells or cables? Two reasons. First, the barbell lets both arms work together to move a single implement, so you can load more weight than any unilateral alternative — and progressive overload is the primary driver of muscle growth. Second, the fixed hand position (full supination on the curl, fixed pronation on the skull crusher) keeps the working muscle under a consistent line of tension, which makes it easier to track progress rep-to-rep and week-to-week.

This guide breaks down both movements with step-by-step form, identifies the muscles each works, and ends with a beginner-friendly barbell arm workout you can run twice per week.

The barbell curl targets the biceps brachii (both the short and long heads), with strong assistance from the brachialis and brachioradialis. The brachialis sits beneath the biceps and, as it grows, pushes the biceps upward — this is what creates visible arm thickness when viewed from the side.

Step 1 — Stance and Grip

Stand with your feet shoulder-width apart, knees soft, torso upright. Grip the bar with a shoulder-width, underhand (supinated) grip — knuckles facing the floor, palms facing the ceiling. A narrower grip emphasizes the long head of the biceps; a wider grip emphasizes the short head. Shoulder-width hits both evenly. Let your arms hang fully straight at the bottom so the biceps starts in a fully lengthened position.

Step 2 — Curl (Concentric)

Keeping your elbows pinned to your sides, curl the bar upward in a smooth arc toward your upper chest. Squeeze your biceps hard at the top — the bar should reach roughly upper-chest height. Do not let your elbows travel forward; if they drift in front of your ribcage, your front deltoids take over and the biceps loses tension. The movement should come entirely from the elbow joint, not the shoulder.

Step 3 — Lower (Eccentric)

Lower the bar back to the starting position under control over 2-3 seconds. The eccentric is where much of the muscle-building stimulus happens, so do not drop the bar. Keep tension on the biceps all the way to the bottom — let your arms fully straighten at the end of each rep so you start the next rep from a stretched position.

Step 4 — Breathing and Bracing

Take a breath and brace your core before the first rep. Exhale as you curl the weight up, inhale as you lower it. Avoid rocking your torso to swing the weight up — if you need momentum, the bar is too heavy. A controlled tempo (2 seconds up, 3 seconds down) produces more biceps growth than a fast, loose tempo at the same load.

The skull crusher — also known as the lying triceps extension or French press — trains all three heads of the triceps brachii (long, lateral, and medial), with particular emphasis on the long head. Because the long head is the largest of the three and crosses both the elbow and shoulder joints, fully developing it is what makes the back of the arm look thick and horseshoe-shaped.

Step 1 — Setup and Bench Position

Lie flat on a bench with your feet planted on the floor. Grip the bar with a shoulder-width overhand (pronated) grip, or slightly narrower. Press the bar straight up so your arms are vertical and directly over your upper chest — this is the starting position. Throughout the entire movement, your upper arms should stay vertical and your elbows should point straight up at the ceiling. If using a straight bar feels uncomfortable on your wrists, an EZ-bar is an acceptable substitute that lets you use a semi-supinated grip.

Step 2 — Lower (Eccentric)

Bending only at the elbows, lower the bar in a smooth arc toward your forehead. Stop the bar just above your forehead or at the top of your head — do not lower it to your chin or chest, which turns the movement into a pullover and shifts load onto the lats. Lower under control over 2-3 seconds. Keep your elbows pointed straight up; if they drift backward toward the floor, the long head of the triceps loses tension and the exercise becomes easier for the wrong reasons.

Step 3 — Extend (Concentric)

Extend your elbows to push the bar back to the starting position directly over your upper chest. Squeeze your triceps hard at the top, fully straightening your arms without snapping the elbows past their natural range. The elbows are the only joint that should move — your shoulders stay locked in place.

Step 4 — Breathing and Setup Safety

Inhale as you lower the bar, exhale as you extend it back up. If you train to failure, use a spotter or a power rack with safety bars — the name "skull crusher" exists for a reason, and the bar is moving toward your face. Never use a thumbless grip on this exercise.

Even experienced lifters make these errors. Fix them and your arms will grow faster — with fewer elbow and wrist problems.

Swinging the torso on the barbell curl: Rocking your hips and lower back to throw the bar up uses momentum, not the biceps. If you need to swing, the bar is too heavy. Drop the weight by 10-20% and keep your torso still.

Letting the elbows drift forward on the curl: When your elbows come forward of your ribcage, the front deltoids take over and the biceps shortens without doing the work. Pin your elbows to your sides for the entire set.

Lowering the bar to the chin on skull crushers: This turns the triceps exercise into a pullover and shifts load onto the lats. Lower the bar toward your forehead or the top of your head, nowhere lower.

Elbows drifting backward on skull crushers: If your elbows drift toward the floor behind your head, the long head of the triceps loses tension. Keep your upper arms vertical and elbows pointed at the ceiling.

Going too heavy, too soon: Both exercises are isolation movements, not ego lifts. Curls and skull crushers build muscle best at 8-15 controlled reps. Loading the bar past what you can lower for 2-3 seconds recruits less muscle and taxes the tendons.