This Classic Upper Body Move Can Help You Improve Shoulder Mobility and Effectively Targets the Triceps

Without any context, “skull crushers” might sound like a maneuver that only a daredevil would attempt. In reality, they’re a common exercise that’s suitable for athletes and casual gym goers alike. As long as you execute the movement with proper form and an appropriate amount of weight, the move is safe and effective for targeting the triceps.

To make the most of the exercise in your workout routine, it’s helpful to understand the benefits of the move and how to perform skull crushers correctly. We caught up with Peloton instructor Tunde Oyeneyin to get all the details.

What Are Skull Crushers?

“Skull crushers are an upper body exercise meant to work your tricep muscles,” says Tunde. If you need a refresher, your triceps muscles are located at the backs of your upper arms. They function to allow you to straighten your elbows, as opposed to your biceps muscles, which help you bend at your elbows.

During the skull crusher exercise, you extend your arms across from your chest, then repeatedly bend and extend at your elbows. Each bend of your elbows lowers the weight toward your head, hence the cautionary name “skull crusher.”

Equipment Needed for a Skull Crusher

Skull crushers only require a few basic pieces of equipment. You’ll need either a bench or a mat if you choose to perform the exercise on the ground. To load the exercise, you can use a pair of dumbbells, or you can reach for a barbell or EZ bar. With dumbbells, your palms will be facing each other, but with a bar, they’ll be facing forward.

What Muscles Do Skull Crushers Work?

Skull crushers mainly target your triceps muscles, but they don’t stop there. “By isolating and extending, the exercise strengthens the triceps, as an added bonus it also hits your shoulder and back muscles,” says Tunde. All three contribute to overall upper body strength.

Since the triceps alone do most of the work during skull crushers, they’re considered an isolation exercise rather than a compound movement, which works multiple muscle groups. While compound exercises tend to be the main focus of a well-rounded strength program, there are multiple benefits to adding some isolation exercises to the mix. Training with isolation exercises can help you strengthen the muscles needed to improve at compound movements to push past plateaus, correct muscle imbalances, and improve functional fitness, according to the International Sports Sciences Association. 

The Benefits of Doing Skull Crushers

As mentioned, skull crushers can help strengthen your triceps. More specifically, they engage multiple parts of the muscle. “The tricep is made up of three heads: the medial, lateral, and long heads,” says Tunde. “While the medial head is the main driver, the skull crusher works all three heads simultaneously.”

When it comes to functional fitness, triceps strength is important for everyday activities. Any time that you extend your elbows, whether you’re lifting something onto a high shelf, pushing yourself out of bed, or vacuuming your floor, your triceps muscles will get involved. Additionally, your triceps function to stabilize your elbow joints during fine movements like writing.  

In addition to building strength in your triceps, skull crushers can help increase shoulder mobility, allowing you to move freely, according to Tunde. Maintaining mobility in the many joints that make up your shoulders is essential for carrying out tasks during daily life, according to a 2018 study in the Journal of Physical Therapy Science. Pain and decreased shoulder mobility often result from repetitive poor posture, according to the study authors. All the more reason to add exercises that can encourage shoulder mobility, such as skull crushers, to your routine.

How to Properly Do Skull Crushers

By perfecting your form, you can ensure that you’ll strengthen your muscles as intended—and ensure you stay safe. Here’s how to do skull crushers, according to Tunde:

Skull Crushers: Variations and Modifications

You can adjust skull crushers based on your fitness level and the type of equipment you have at your disposal. Here are some additional options.

Skull Crusher Variations

The most common skull crusher variation is to simply swap in a barbell or EZ bar instead of dumbbells, says Tunde. 

“You can also perform the workout standing up straight with your core tight, feet shoulder width apart, in which case you can use any of the same weight options or even the cable machine, slowly pulling the cable to above your head and back to its resting position,” Tunde says.

When performing the exercise lying down, you can opt to maintain a tabletop position with your legs throughout the movement rather than keeping your feet planted on the ground. This will provide an additional core challenge, says Tunde.

Some people choose to perform partial reps of skull crushers, meaning that rather than using a full range of motion during each reap, they won’t fully bend or straighten their elbows. The hope is to maintain tension in their muscles the entire time, maximizing the muscle-building benefits of the exercise. 

A 2019 study published in the Journal of Strength and Conditioning Research investigated the effects of this approach, and those who performed partial reps of skull crushers had significantly higher levels of various indicators of hypertrophy (increased muscle mass) than those who performed full reps. That said, other studies exploring whether partial or full reps are optimal for hypertrophy have produced mixed results.

How to Modify Skull Crushers As a Beginner

Performing the exercise on a mat on the ground, rather than a bench is a good gateway to skull crushers, according to Tunde. “Instead of lying on a bench, take the workout to the floor,” she says. “Lie on your back with your knees bent and feet flat on the floor and do the skull crusher with a lighter weight. Perfect your form before increasing the weight.”

Common Mistakes to Avoid When Doing a Skull Crusher

With skull crushers, the most common mistakes that lifters make include improper weight selection, grip, and/or elbow alignment, says Tunde.

For one thing, it’s important that you don’t use too heavy of a weight when trying skull crushers, which can compromise your form. “Ten to 12 reps, three times through is ideal when performing this exercise,” says Tunde. “All but the last two reps should feel smooth. If the weights are beginning to shake or if your elbows are completely exhausted before hitting 10 to 12 reps, the load may be too heavy.” On the flip side, if all 12 reps are an absolute breeze, that’s a sign that you can safely go heavier.

An improper grip can cause you to drop the weight and injure yourself, so make sure to use a weight(s) that you’re able to grasp tightly throughout the movement.

Finally, make sure to avoid allowing your elbows to flare out away from your body which can make the exercise feel easier at the expense of effectively engaging the target muscles. Tucking your elbows into your body and pressing your shoulders down away from your ears will ensure you’re engaging your lats and isolating your triceps, says Tunde.

How to Incorporate Skull Crushers Into Your Workout Routine

How often you should add skull crushers to your workout routine will depend on your individual goals. That said, “A general recommendation would be once to twice a week with three sets of 10 to 12 reps,” using a weight that’s heavy enough so that the last two reps are difficult to perform, says Tunde. If you continuously perform the exercise over time, it will start to feel easier, and you can increase the weight that you use to continue making progress.

Whether you encounter the skull crusher exercise during a Peloton class or decide to add them in during a gym session, you’ll be prepped on how (and why) you should do them.