How to Do Skull Crushers to Strengthen Your Triceps

This single-joint exercise is the key to making your push-ups much more powerful. Find out how to do skull crushers perfectly with every single rep.

Skull Crushers
Courtesy of Mind Body Green.

You know when you're lying flat in bed on your phone, holding it up over your face, and your arms start to burn? Well, you're kinda doing a skull crusher — a triceps exercise that can give you killer arms and boost your performance in the gym.

Ahead, trainers break down everything you need to know about the strength-building exercise, including how to do skull crushers with proper form, how to modify and progress the move to meet your needs, and how to add it to your workout routine. Trust, skull crushers not only sound badass, but they'll make you feel that way too.

How to Do Skull Crushers

Skull crushers, aka lying triceps extensions, are a move traditionally performed lying down on a bench with one or a pair of dumbbells. You hold the weight over your face (hence, the name "skull crusher") with elbows pointing up, then use your triceps (the muscles on the back of your upper arm) to straighten your elbow and pull the weight toward the ceiling.

But your triceps aren't the only muscles getting in a workout. "By using a bench, you can place your feet on the ground, requiring different engagement in your lower body and core; engaging your glutes, tucking your pelvis, and keeping your core tight and ribs down requires thoughtful effort," says Ash Wilking, C.F.S.C., F.R.C., a Nike master trainer and Tonal coach.

Need help visualizing the exercise? Watch Dannah Bollig, an ISSA-certified personal trainer and the creator of The DE Method, demonstrate how to do skull crushers below. Remember to keep your elbows tucked in and shoulder-width apart, as well as your back flat on the bench, she notes.

A. Hold a dumbbell in each hand and lie faceup on a bench with feet flat on the floor on each side of the bench.

B. Extend arms toward the ceiling above chest, palms facing each other. Engage glutes and core and pull the rib cage down to prevent low back from arching.

C. Tucking elbows in and pressing shoulders down, slowly bend elbows to lower the dumbbells until they hover an inch above head or on either side of head. Avoid moving upper arms and anchor shoulders down to engage the lats, isolating the triceps as the weights lower.

D. With control, straighten elbows to extend arms back over chest.

The Key Skull Crusher Benefits

Building triceps strength isn't the only benefit skull crushers have to offer. Here, all the perks that come with mixing skull crushers into your routine.

Isolates the Triceps

Skull crushers are an isolation exercise, meaning you’re utilizing only one joint (in this case, the elbow) during the movement. In turn, you're relying primarily on the triceps to complete the movement and thus are better able to target these small arm muscles — something that's tough to do with other upper-body moves. "The triceps rarely take the lead, in comparison with biceps for lifting or holding, or glutes for walking or standing," says Wilking. "In other words, they assist larger muscle groups in performing countless movements both in strength training and everyday activity," she adds.

Improves Performance In Upper-Body Exercises

"Triceps help your overall pushing strength and are the key extensor of the elbow joint," explains Riley O'Donnell, a NASM-certified personal trainer and instructor at Fhitting Room in New York City. "So if you're trying to get stronger in your overhead presses, chest/bench presses, or push-ups, strengthening your triceps will help you reach your goals."

Specifically, skull crushers enhance pushing movements because they train your body to load weight with your elbows in a flexed position (a bent arm) and press the weight away into a locked-out arm, says O'Donnell. "When we are pushing things, we not only need to engage our shoulders, chest, and core, but we need to be able to powerfully extend the elbow," she says. So if you've been struggling with push-ups, practicing skull crushers can help make the bodyweight exercise feel a bit easier.

Offers a Low-Impact, Injury-Friendly Workout

By solely utilizing the elbow joint, skull crushers isolate the triceps, which isn't true even for many triceps-dominant exercises, says O'Donnell. "For example, standing triceps extensions and triceps dips require shoulder mobility that not everyone has," she says. Because of this, skull crushers are best suited for those who have a limited range of motion in the shoulders and want to strengthen their triceps.

In addition to building triceps strength, skull crushers are beneficial for those who want a low-impact arm exercise or are working around an injury. "By lying on your back with the weight overhead, you put the primary focus on the triceps and remove pressure from other joints, like your wrists (in push-ups) or lower back (in bent-over kickbacks)," Wilking explains.

Improves Grip Strength

Skull crushers also play a large role in improving grip strength, which is necessary to prevent you from dropping the weight and literally crushing your head. But IRL, grip strength is essential for carrying out everyday movements (think: carrying a bag of groceries, opening a jar of pickles) with ease. "When performing skull crushers, whether it's with a pair of dumbbells, a barbell, or a plate, it's important to keep your wrists straight. It can be tempting during this movement to break the wrist because it feels easier to hold the weight, but focusing on keeping your wrists straight improves your grip strength," says O'Donnell. (Need another lesson in grip strength? Try this battle rope workout.)

Skull Crusher Muscles Worked

As mentioned, the triceps brachii is the primary mover (AKA the agonist muscle group) during the skull crusher isolation exercise. Its main function is to extend the elbow, though it also provides stability when you're using your hands and forearm muscles for small movements (think: writing), research shows. That said, the exercise also targets the muscles in your fingers, hands, and forearms, which are responsible for your grip strength.

Skull Crusher Variations

If you try out the classic skull crusher exercise and it doesn't seem to match your fitness level or individual needs, don't fret. These modification and progression ideas will help you get exactly what you want and need out of the move.

Modification: Skull Crushers On a Mat

The best way to modify a skull crusher? Perform the move on the floor — not on a bench — with your knees bent and holding just one light dumbbell with both hands, says Bollig, who demonstrates the exercise below. "This allows for a shorter range of motion and safety stopping point (the floor)," she says. "It also gives you total control over the movement due to the fact that you’re using a single weight" and holding it with both hands. Yes, your range of motion will be slightly limited, but this tweak will help you learn good form, says Chris Pabon, a NASM-certified personal trainer and fitness manager at Blink Fitness.

Plus, this skull crusher variation comes with its own set of perks: Your knees are much more bent during this modification than they are with the traditional move, allowing you to tilt your pelvis and create a better connection with your rib cage, says Wilking. "This connection will limit shoulder movement and create true isolation of the triceps," she says.

Once the weight you're using no longer feels challenging, you can power through countless reps, or you've perfected your form while doing skull crushers on the floor, consider progressing to a traditional skull crusher, suggests Bollig.

Progression: Incline and Decline Skull Crushers

If you're looking to level up the skull crusher exercise, try changing the grade of your bench. Changing the incline on the bench can engage specific heads (read: parts) of the triceps a bit more than others, says Pabon. For example, using a decline bench (with your head lower than your feet) will recruit more activation from the lateral tricep head, which is toward the outside of your arm, he says. Using an incline (with your head on the higher end), however, will work the long head of your triceps (think: the back inside of your arm, starting just under your armpit and running down toward your elbow), adds Bollig.

With either of these variations, you'll want to make sure your bench is secure and call on your mind-muscle connection, says Bollig. "Understanding exactly what you’re targeting and where you should feel more of a burn will help you maximize the exercises," she explains. And if you're using a heavier weight, consider having a spotter nearby in case you fail the lift, she suggests.

Common Skull Crusher Mistakes

While skull crushers aren't complicated to master, they're a recipe for injury and pain if you're not doing them correctly.

The first major mistake? Flaring out your elbows to make it easier on your triceps. "It’s important to keep your elbows tucked in so that they are about shoulder-width apart and directly behind your wrists," says Bollig. "This not only protects your joints and ligaments, but it also helps you target your muscle properly."

If you need a visual cue, "imagine your elbows are hugging a balloon to keep your elbows from flaring and that your upper arms are against a wall throughout the entire movement," says O'Donnell. This will help hold your upper body in place on the mat or bench. Or, try picturing yourself grabbing a steering wheel, turning your pinky fingers down and in, to help engage the lats, suggests Wilking.

It's just as important to avoid shrugging your shoulders. To truly isolate the triceps and ensure you're not using your shoulders or upper arms, pack your shoulders down, aka engage the lats, says O'Donnell. "When your lats aren't engaged, the tendency is to let your upper arm move during the skull crusher," explains O'Donnell. Engaging your core can also help stabilize the upper body, she says. "Because the skull crusher is performed on your back, your core is working to keep the ribcage knitted during the movement and the low back pressing into the floor or bench," she says. Knitting the rib cage means pulling the down and together, engaging the deep core muscles, to help prevent compressing the low back.

Throughout the movement, make sure to keep your back glued to the bench or ground; arching adds pressure to the low back, which can lead to pain and injury. To avoid the low-back arch, make sure to choose a weight that isn't too heavy, says Bollig. (The weight should be light enough that you can do 10 to 12 reps with good form, but the last few reps should be a bit challenging.) And remember to pull your rib cage down into the floor, Wilking suggests. "Think about pressing your feet into the ground as hard as you possibly can and knitting your rib cage while pressing the back of your ribs into the floor or bench," says O'Donnell.

Finally, move slowly and with control. "Control the weight both ways — during the eccentric [lowering] and concentric [pressing] parts of the movement. Injuries happen during deceleration and/or rotation usually, so really focusing on controlling that weight," says Pabon.

How to Add Skull Crushers to Your Routine

Before picking up a pair of dumbbells and testing out a set of skull crushers, make sure to get the green light from your healthcare provider if you're brand-new to the gym or have any pre-existing injuries or health concerns, says Bollig.

If you're given the all-clear, beginners should typically start with holding just one light weight (think: a 10- or 12-pound dumbbell) with both hands, says Bollig. "It’s much easier to control a single dumbbell because you’re only guiding one weight as opposed to two, which takes a lot more muscle control and coordination," she explains. Once you nail down the exercise, increase the weight or try holding one dumbbell in each hand, she says.

If you're at a well-stocked gym, you can even perform your skull crushers using an EZ curl bar — a type of weightlifting bar that features a "W"-shaped bend. The form is generally the same, but you might be able to lift heavier than you can with dumbbells (they're a bit more challenging to control), says Pabon. Plus, the EZ bar can help you keep your elbows tucked in, he says.

Regardless of the weight you're using, doing three to four sets of 10 to 12 reps is a good place to start. Consider performing them on days when you're focusing on other "push" muscles, such as a chest or shoulder day, says Pabon. "It's a great way to really finish [the triceps] off after they've been used as secondary muscles for the first part of the workout," he says. For an added challenge, try doing skull crushers in a superset workout with a biceps exercise on arm days, suggests Wilking.

Or, you can use skull crushers as an active recovery movement. "For example, if you're doing a leg or full-body workout, use skull crushers while allowing your legs to recover between sets," says Wilking. No matter how you mix the exercise into your routine, your triceps will surely thank you for it.

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