The clamshell is a side-lying hip exercise: with your knees bent and feet together, you lift the top knee like a shell opening while the pelvis stays perfectly still. It works the gluteus medius and the deep rotators at the back of the hip, which is why physical therapists often use it early in rehab for outer hip pain, runner’s knee and IT band pain. This page is general education rather than a prescription; a therapist who examines you decides whether it suits you and how much to do.
Last updated By the Monon Therapy Collective editorial team
- Main muscles
- Gluteus medius and the deep hip rotators
- Position
- On your side, hips bent about 45 degrees, knees about 90
- Equipment
- A mat; a loop band to progress
- Often used for
- Outer hip pain, runner’s knee, IT band pain
Which muscles does the clamshell exercise actually work?
The clamshell mainly works the gluteus medius, the fan-shaped muscle on the outside of the pelvis, together with the small rotator muscles that sit deep behind the hip joint. Those muscles keep the pelvis level and stop the knee from drifting inward each time you stand on one leg, which you do with every step of walking or running.
Because the hip is bent and the movement turns the thigh outward, the clamshell leans more on the rotators than a straight-leg side raise does. It is also low-load, so a sore hip can start working before standing exercises.
When do therapists use clamshells, and when do they move on?
Therapists tend to use the clamshell early, when standing on one leg still hurts or the hip muscles barely switch on. It often appears in plans for hip bursitis and outer hip pain, runner’s knee and IT band syndrome, where weak hip control is one of several things the therapist may find.
With outer hip pain from an irritated gluteal tendon, lying on the sore side or letting the top knee drop across the body can squeeze the tendon, so a therapist may put a pillow between your knees or pick a different starting exercise. The clamshell is a starting point rather than the destination: plans usually move on to standing work such as the monster walk and the step-down exercise, which carry over better to stairs, running on the Monon Trail or a pickleball lunge. To have a therapist check your hip, start with the hip questionnaire.
What are the most common clamshell mistakes?
The single most common mistake is rolling the pelvis backward to make the knee rise higher, which turns a hip exercise into a trunk twist. The others are smaller, but each one shifts the work away from the side of the hip.
- Rolling the top hip back as the knee lifts: keep a hand on the hip, or lie with your back against a wall.
- Letting the feet separate: the heels stay glued together throughout.
- Swinging the knee up fast, so momentum does the work.
- Feeling it mostly in the front of the hip: bring the knees forward a little and shorten the arc.
- Adding a heavy band before the movement is clean: the pelvis will rock to cope.
How do you make a clamshell easier or harder?
Make it easier by shortening the arc and lying with your back against a wall, and make it harder with a band, a pause or a side-plank position. Change one thing at a time.
- Easier: a small, slow opening with no band, back and pelvis touching a wall.
- Easier: a pillow between the knees if lying on your side presses on a sore hip.
- Harder: a light loop band just above the knees.
- Harder: a three-second pause at the top of every repetition.
- Harder: the reverse clamshell, lifting the top foot while the knees stay together.
Clamshell, step by step
Keep a hand on your hip to feel any pelvis movement.
Set up on your side
Lie on your side with hips and shoulders stacked, head resting on your lower arm or a folded towel. Bend the hips to about 45 degrees and the knees to about 90 degrees, so your heels line up with your seat.
Lock the pelvis
Put your top hand on the bony point at the front of the top hip. Tip the pelvis very slightly forward, so the top hip sits a touch in front of the bottom one; this makes it harder to cheat by rolling backward.
Open the top knee
Keep the heels touching and lift the top knee toward the ceiling only as far as the pelvis stays still under your hand.
Pause, then lower slowly
Hold the top for a moment, feeling the work in the side of the seat just behind the hip bone, then lower over two to three seconds. Breathe out as the knee lifts and in as it lowers.
Finish the set and switch
End the set when the pelvis starts to rock or the effort moves to the front of the thigh, then repeat on the other side. Your therapist sets the number of sets and repetitions.
Stop and get your hip checked if...
- You feel a sharp pain deep in the groin, or a catch or click with pain at the front of the hip.
- Pain lying on that side wakes you every night and is getting steadily worse.
- Numbness, tingling or weakness spreads down the leg.
- The hip pain started after a fall, especially if you are over 60 or have osteoporosis.
- The hip is hot, swollen or red, or you feel unwell with a fever.
If you think it is an emergency, call 911.
