How to Do The Clamshell Exercise With Perfect Form
Don't be fooled; this simple exercise is one of the best for building stronger, healthier hips and protecting the knees.
By Kelan Ern Updated: `09/12/2025
A few years ago, during a personal training session, I was showing someone this highly underrated exercise when I told them:
'There are powerful corrective exercises floating around on the internet but people aren't doing them correctly.There are so many distinctions in even simple exercises that if you mess up - you no longer get the benefit.'
That's why I wrote this brief clamshell tutorial:
Help you master this powerful corrective exercise.
Work the right muscle groups.
And catch the little mistakes 90% of people make.
Before we get into how to do them - here's a quick metaphor so you know why this exercise is needed in the first place. So you know the purpose of the "tool" before using it.
Blaming Painful Knees and Gray Wolves
In the 1920's, the gray wolf population was nearly decimated in Yellowstone.
One reason was early park managers blamed them for causing 'wanton destruction' to the elk (and other animals). So they nearly exterminated the wolves from the area. And sure enough, over the next 70 years, the elk herds grew larger.
But there were unintended consequences.
These elk herds overgrazed the land which hurt the willow and aspen trees. Few trees meant beavers lost their primary building material for dams. Less dams meant the streams started eroding and becoming deeper (which hurt the trees even more).
Just like the gray wolves were wrongly blamed for problems in the ecosystem, the same goes for certain joints in the body.
The knee is one of them.
Over the years, people have asked:
How do you strengthen the knee? How do you stabilize the knee better?
The problem is they are going after the wrongsuspect.
They assume the knee is frequently injured because something is wrong with it. But when it comes to the human body:
The symptom of the pain is usually not the source of the pain.
There are exceptions to this.
For example, if you’re blind-sided by a NFL linebacker, chances are your knee can’t hold up to this force. But outside of these collision injuries, we can prevent many types of knee injuries.
And in many cases, the hips are the true culprit.
When the hips are functioning correctly, the knees tend to stay healthy. When the hips are functioning correctly, the low back tends to stay healthy as well. And one of the best ways to keep the hips functioning is with the clamshell exercise.
What Does The Clamshell Exercise Do?
Clamshells are a side-lying exercise for strengthening specific hip muscles. This little exercise doesn't look that impressive, but it helps you in three big ways:
#1: Strengthens Your Glutes When you do clamshells you’re activating your glutes (your butt muscles) which are crucial for walking, running, climbing stairs, picking up objects off the ground or even standing up. When these muscles aren't working properly, the lower back tends to compensate. The clamshell exercise strengthens not just the largest glute muscle (glute maximus) but also a smaller one called the glute medius.
#2: Knee Stability In my fitness workshops, I've had people try the following:
Stand up and put one hand on your glute (butt muscle)
Gently move your knee side-to-side.
Unless you have a little glute amnesia, you should feel the muscles in your hip contracting.
That's because muscles like the glute medius help stabilize your knee. Like I mentioned before, many people want to point their finger at their knee for not being strong enough. But in many cases they should point it at their weak butt (specifically glute medius) that isn't strong enough to control the knee. That's why you see this exercise frequently recommended by physical therapists for knee pain.
#3: Hip Health If you've ever had a massage (or foam rolled) the outside of your thigh (TFL-IT Band), you know how tender and tight this area can get.
When this area gets stiff it can literally rotate the hips and cause your femur to move improperly in the hip socket - causing hip pain and restricting your movement. However, when you build the glute medius, it helps the femur glide smoothly in the socket. And clamshells do exactly that.
Despite it's simplicity, many personal trainers, group exercise instructors and workout buffs tend to do this exercise incorrectly.
How to Do a Clamshell Correctly
Set-Up: Start in a side-lying position with your legs stacked on top of one another and your knees bent at about a 45 degree angle.
Movement: Keeping your feet together, open your knees without rotating at the spine. Then return to the starting position. You should feel this exercise working the glute muscles. Do 10-15 repetitions each side.
Common Mistakes
Rotating The Spine: This is the most common mistake. Make sure that only your leg is moving. Not your spine. Not your hips. Sometimes it helps to have your hand on your hip to monitor this. Once again you should feel this exercise working the glutes.
What If I Can’t Activate My Glutes?
Maybe you don't feel it in your glutes. Maybe you feel it along the side of your leg (IT band area).
Sometimes it’s because these muscles haven't been used in a while or are extremely weak. In other cases, it’s because your alignment is off which makes these muscles harder to activate.
Here are a few tips to help:
Tip #1: Hand on the glutes Place your hand on your glute when you do the exercise. Believe it or not, when you touch a muscle, it helps activate it.
Tip #2: Bring the legs back Another tip is instead of bending your legs forward at a 45 degree angle, you bring them backwards. Meaning: You would do the clamshell with your upper legs in-line with your torso.
Tip #3: Pillow underneath you Another way that’s worked for some of my clients is to have a pillow underneath them (right above the hips). This can help put the spine in better alignment – especially for those with larger pelvic bones.
How to Make Clamshells More Challenging
After you get the hang of the clamshell, here's a little variation exercise to try. In this video by physical therapist Mike Reinold - you'll see him do clamshells with a resistance band.
This is a sure-fire way to get your glutes firing way more! Even a small amount of resistance can make this exercise much more difficult.
Remember: Don't rotate from the spine (or the hip). All the motion should come from the leg.
When Should I Do Clamshells in My Work Out?
Clamshells are great at the beginning of a warm-up. Why?
Because when you activate the glutes (and other important hip muscles) you are more likely to use them during the rest of your workout.
Meaning: your activated glutes will help you when you do squats, deadlifts, lunges, step ups, tire flips or other leg exercises. And ultimately that's going to allow you to get much more out of your workout.
Do 10-12 repetitions on each side.
Final Thoughts
For the last 16+ years of coaching fitness, certain corrective exercises have come and gone. But the clamshell has been one of the most important exercises in my 'toolbox.' It's one that I come back to again and again.
It's easy to overlook it but it's a powerful injury-preventing exercise. It'll help you move better, positively impact the rest of your workout and help stabilize your knees during your daily activities.
Try them out and see for yourself.
Kelan Ern Elite Fitness Coaching
P.S. There are dozens of other tools and strategies to help you go further, faster on your fitness journey. Each week I share these fitness tips with my 'mind-body community' and do a monthly deep-dive on the fittest people on the planet inside the Mind-Body Breakthroughs newsletter.
