Skull Crushers: How to Do Them, Muscles Worked and Benefits

Skull crushers, also known as lying triceps extensions, are one of the best exercises for building the triceps, the muscle that makes up roughly two thirds of your upper arm. You lie back, hold a weight over your chest with fairly straight arms, then bend only at the elbows to lower it towards your head before extending back up. Because your upper arms stay angled and mostly still, the triceps do almost all the work, which makes this a brilliant way to add size and strength to the back of the arm. Here is how to do them safely, the muscles they work, and how to get the most from them.

How to do skull crushers

An EZ bar or a pair of dumbbells is ideal, along with a flat weight bench. Dumbbells are the most forgiving on the wrists and elbows if you are new to the movement.

1. Set up. Lie flat on a bench with your feet planted on the floor. Press the weight up so your arms are extended over your chest, then tilt your arms back slightly so they point towards the top of the bench rather than straight up. This angle keeps tension on the triceps.
2. Fix your upper arms. Your elbows should stay roughly in place for the whole set. Only your forearms move. Think of your upper arms as fixed posts and your elbows as the hinges.
3. Lower under control. Bend at the elbows and lower the weight towards the top of your forehead or just behind your head, keeping your elbows from flaring out wide. Lower slowly, taking around two seconds.
4. Feel the stretch. At the bottom you should feel a strong stretch along the back of your upper arms. Do not let the weight crash down or touch your head.
5. Extend back up. Straighten your elbows to drive the weight back to the start, squeezing your triceps at the top. Keep your upper arms in the same angled position throughout. That is one rep.

Muscles worked

Skull crushers are about as focused a triceps exercise as you can find, and they hit all three heads of the muscle.

• Triceps long head. The long head runs down the inner back of your upper arm and crosses the shoulder joint, so the arms-back position of a skull crusher stretches it and works it hard. This is a big reason skull crushers are so good for overall arm size.
• Triceps lateral head. The head on the outer part of your arm that gives the triceps its horseshoe shape. Surface EMG work confirms that triceps extension exercises fire all three heads, with load and technique changing how hard each one works (study on the three heads of triceps).
• Triceps medial head. The deeper head underneath that works in every triceps movement and keeps the elbow extending smoothly.
• Forearms and shoulders. Your forearm muscles grip and steady the bar, while your shoulders and upper back hold your arms in position so the elbows can hinge cleanly.

One useful thing to know is that the three heads do not all fire in perfect unison, so varying your exercises and angles over time trains the triceps more completely (research on triceps head activation). Skull crushers pair well with a pushdown for exactly this reason.

Benefits

• They build the biggest arm muscle. The triceps make up around two thirds of your upper arm, so if you want bigger, stronger arms, training them directly matters more than endless curls. Skull crushers load them through a long range.
• They emphasise the long head. The arms-back position stretches the long head more than pushdowns do, which is the part of the triceps that adds thickness to the inner arm.
• They carry over to pressing. Stronger triceps mean a stronger lockout on the bench press and overhead press, so skull crushers support your bigger lifts too.
• They need very little kit. A bench and one set of weights covers it, which suits home training. The NHS recommends muscle-strengthening activity for all the major muscle groups on at least two days a week, and direct arm work rounds out an upper-body routine.

Common mistakes

Flaring the elbows. Letting your elbows splay outward takes tension off the triceps and stresses the joint. Keep them pointing forward and roughly shoulder-width apart the whole time.

Moving the upper arms. If your whole arm swings back and forth, you are pressing, not extending. Lock the upper arms in place and let only the forearms move around the elbow.

Going too heavy. A weight you can just about grind out will wreck your form and your elbows. The triceps respond well to controlled reps, so pick a load you can lower slowly and press without swinging.

Bouncing out of the bottom. Dropping the weight fast and rebounding off the stretch risks the elbow and cheats the muscle. Lower under control and pause briefly before extending.

Using a straight bar if it hurts your wrists. A straight barbell forces the wrists into a fixed position that many people find uncomfortable. Switch to an EZ bar or dumbbells, which let the wrists sit more naturally.

Variations

• Dumbbell skull crushers. The most joint-friendly version. Each arm works independently, which helps even out left-to-right differences, and you can angle your hands for comfort.
• EZ bar skull crushers. The classic gym version. The angled grip is easy on the wrists and lets you load a little heavier than dumbbells.
• Incline skull crushers. Done on an incline bench so the arms point back further, increasing the stretch on the long head. A good progression once the flat version feels easy.
• Behind-the-head skull crushers. Lower the weight past your forehead towards the top of the bench to keep constant tension and stretch the triceps more. Keep it controlled.
• Dumbbell floor press to extension. No bench needed. Lie on the floor and perform the extension there, which limits how far your elbows can drop and protects the shoulder. Handy for home workouts with minimal kit.

Sets and reps

A simple plan that works for most people:

• Muscle and size: 3 to 4 sets of 8 to 12 reps, 2 times a week. Rest 60 to 90 seconds.
• Higher-rep pump work: 2 to 3 sets of 12 to 15 reps with a lighter weight, rest 45 to 60 seconds.
• Learning the move: 3 sets of 10 slow reps with a light weight, focusing on keeping the upper arms still.

Skull crushers work best placed after your heavier pressing on an upper-body or push day, when the triceps are warm but not yet exhausted. Pair them with a tricep pushdown to train the triceps from two different angles. Add a small amount of weight only when every rep of your top set stays controlled and pain-free.

Recommended reads

1. Tricep pushdown: technique and muscles worked
2. The best adjustable dumbbells in the UK
3. The best EZ bar in the UK
4. The best weight bench in the UK
5. Bench press: how to do it