What Muscles Do Skull Crushers Work? Complete Anatomy Guide

Skull crushers (also called lying tricep extensions) are one of the most effective exercises for building bigger, stronger triceps. Here's exactly what muscles skull crushers work and how to maximize your arm development.

Primary Muscles Worked by Skull Crushers

Triceps Brachii (All Three Heads)

Your triceps are the target muscle. Unlike pushdowns that emphasize certain heads, skull crushers work all three:

Long Head

Largest head of the triceps
Creates the "horseshoe" shape
Crosses the shoulder joint (unique among tricep heads)
Most active during skull crushers because the shoulder is slightly flexed

Lateral Head

Outer portion of the tricep
Creates the outer arm sweep
Works significantly during extension

Medial Head

Deep, near the elbow
Active throughout elbow extension
Stabilizes the movement

The long head emphasis is what makes skull crushers special—the arm position stretches it more than pushdowns.

Anconeus

This small muscle at the elbow:

Assists with elbow extension
Stabilizes the elbow joint

Secondary Muscles

Shoulders (Anterior Deltoid)

Your front delts stabilize your shoulder position throughout.

Chest (Pectoralis Major)

Your chest assists slightly with shoulder stabilization, especially at the bottom of the movement.

Forearms

Your forearms stabilize your wrists and grip the bar.

Core

Your core braces to maintain stable body position on the bench.

Why Skull Crushers Are So Effective for Triceps

1. Long Head Emphasis

The arm position (shoulder flexion + elbow extension) maximally stretches the long head—the largest tricep head.

2. Full Range of Motion

Greater stretch than pushdowns, more complete tricep development.

3. Progressive Loading

Easy to add weight over time.

4. Isolation

Minimal cheating potential compared to close-grip bench.

Muscle Activation by Skull Crusher Phase

Starting Position (Arms Extended)

| Muscle | Activation | |--------|------------| | Triceps | Low (shortened) | | Shoulders | Stabilizing |

Lowering (Eccentric)

| Muscle | Activation | |--------|------------| | Triceps (all heads) | High (eccentric stretch) | | Long head | Maximum stretch | | Shoulders | Stabilizing |

This stretched position is where triceps work hardest.

Bottom Position

| Muscle | Activation | |--------|------------| | Triceps (long head) | Maximum stretch | | All tricep heads | Under tension |

Point of maximum muscle length.

Pressing Up (Concentric)

| Muscle | Activation | |--------|------------| | Triceps (all heads) | Very High | | Medial head | High (elbow stability) | | Anconeus | High |

Extension against resistance.

Skull Crusher Variations

EZ Bar Skull Crusher

| Muscle | Activation | |--------|------------| | Triceps | Very High | | Wrist stress | Lower (angled grip) |

Best for: Most people—easier on wrists

Straight Bar Skull Crusher

| Muscle | Activation | |--------|------------| | Triceps | Very High | | Wrist stress | Higher |

Best for: Those with healthy wrists, maximizing ROM

Dumbbell Skull Crusher

| Muscle | Activation | |--------|------------| | Triceps | Very High | | Unilateral work | Yes | | Stability demand | Higher |

Best for: Addressing imbalances, neutral grip option

Cable Skull Crusher

| Muscle | Activation | |--------|------------| | Triceps | High | | Constant tension | Yes | | Peak contraction | Better |

Best for: Time under tension, variety

Decline Skull Crusher

| Muscle | Activation | |--------|------------| | Triceps (long head) | Higher | | ROM | Increased |

Best for: Maximum long head emphasis

Incline Skull Crusher

| Muscle | Activation | |--------|------------| | Triceps | High | | Shoulder angle | Different | | Long head | Different emphasis |

Best for: Variety, hitting muscle from different angle

Single-Arm Skull Crusher

| Muscle | Activation | |--------|------------| | Working tricep | Maximum | | Mind-muscle connection | Better | | Stabilization | Higher |

Best for: Isolation, addressing imbalances

JM Press (Hybrid)

| Muscle | Activation | |--------|------------| | Triceps | Very High | | Chest | Moderate | | Loading potential | Higher |

Best for: Tricep strength, powerlifting accessory

Bar Path: Where Should You Lower To?

To Forehead (Classic "Skull Crusher")

Standard position
Good tricep stretch
Named for obvious reasons

To Top of Head/Behind Head

Maximum long head stretch
Greater ROM
More shoulder involvement

To Nose/Chin

Shorter ROM
Less stretch
More elbow-friendly for some

Recommendation: Lower to forehead or slightly behind for maximum tricep work. Going too far behind adds shoulder stress.

Common Mistakes

Elbows Flaring Out

Reduces tricep isolation, increases shoulder involvement.

Fix: Keep elbows tucked, pointing toward ceiling.

Moving Upper Arms

Upper arms moving forward/backward during the exercise.

Fix: Upper arms stay fixed. Only forearms move.

Going Too Heavy

Form breakdown, shoulder strain.

Fix: Use weight you can control for 8-12 quality reps.

Partial Range of Motion

Not lowering enough to stretch the triceps.

Fix: Lower until you feel a stretch in triceps, then extend fully.

Locking Out Aggressively

Hyperextending elbows, joint stress.

Fix: Control the lockout, don't slam into extension.

Bar Drifting Overhead

Turns into a pullover, reduces tricep work.

Fix: Keep the bar path consistent, over forehead area.

Bouncing at Bottom

Using momentum instead of muscle.

Fix: Control the bottom, no bounce.

Skull Crushers vs. Other Tricep Exercises

Skull Crusher vs. Tricep Pushdown

| Factor | Skull Crusher | Pushdown | |--------|---------------|----------| | Long head emphasis | Higher | Lower | | Stretch | Greater | Less | | Loading | Higher potential | Moderate | | Joint stress | Can be higher | Generally lower |

Skull Crusher vs. Close-Grip Bench

| Factor | Skull Crusher | Close-Grip Bench | |--------|---------------|------------------| | Tricep isolation | Higher | Lower (chest helps) | | Load potential | Moderate | Higher | | Compound benefits | No | Yes | | Tricep focus | Maximum | Shared |

Skull Crusher vs. Overhead Extension

| Factor | Skull Crusher | Overhead Extension | |--------|---------------|-------------------| | Long head stretch | High | Maximum | | Position | Lying | Standing/seated | | Shoulder demand | Moderate | Higher | | Stability | More stable | Less stable |

Programming Skull Crushers

For Size (Hypertrophy)

3-4 sets × 8-12 reps
Controlled tempo (3-0-2-0)
Moderate weight
Rest 60-90 seconds

For Strength

4-5 sets × 6-8 reps
Heavier weight
Rest 2-3 minutes

As Accessory (After Pressing)

2-3 sets × 10-15 reps
Moderate weight
Focus on pump and fatigue

In Superset

Pair with bicep curls
3 sets each
Minimal rest between exercises

Protecting Your Elbows

Skull crushers can stress the elbows. Here's how to minimize risk:

Warm up thoroughly: Light tricep work before heavy sets
Don't go too heavy: Quality over ego
Control the movement: No jerking or bouncing
Use EZ bar: Easier on wrists and elbows
Vary your tricep exercises: Don't do skull crushers exclusively
Stop if painful: Pain is a signal

If Elbows Hurt

Reduce weight
Try different bar (EZ vs straight)
Lower to different position (forehead vs behind)
Switch to pushdowns or dips temporarily
Consider bands for accommodating resistance

Sample Tricep Workouts

Tricep-Focused Session

Close-grip bench press: 4×6-8
Skull crushers: 3×10-12
Tricep pushdown: 3×12-15
Overhead extension: 2×12-15

Arm Day (Bi's and Tri's)

Triceps:

Skull crushers: 3×10
Dips: 3×8-10
Pushdowns: 3×12

Biceps:

Barbell curl: 3×10
Hammer curl: 3×10
Concentration curl: 2×12

Push Day Tricep Work

After chest and shoulders:

Skull crushers: 3×10-12
Tricep pushdown: 2×12-15

Superset Finisher

3 rounds:

Skull crushers: 10 reps
Close-grip push-ups: Max reps
Rest 60 seconds

The Bottom Line

Skull crushers primarily work all three heads of your triceps, with particular emphasis on the long head due to the shoulder position. They're one of the best exercises for building complete tricep development.

Key points:

Keep elbows tucked, pointing up
Upper arms stay fixed—only forearms move
Lower to forehead or slightly behind
Full ROM: stretch at bottom, full extension at top
EZ bar is easier on joints for most people
Don't go too heavy—protect your elbows

Master the skull crusher for impressive tricep development.

Ready to build bigger triceps? Check out our best tricep exercises guide and arm workout guide for complete programming.