Clamshell exercise: the setup, what the studies measured, and where it fits

The clamshell, or clam, is done lying on your side with your hips and knees bent: you keep your feet together and open the top knee, then close it again. One study describes it as combining "open-chain hip abduction and external rotation": the thigh moves out to the side and turns outward at the same time.¹ Another set it up as "a side-lying clam exercise" with the pelvis in a middle position "and the knee bent at 90°".² It needs no equipment; a loop band above the knees adds resistance.

This page covers the setup, what the muscle-activity studies found it works, how the hip angle and the pelvis change that, what a band changed, how it compares with the side-lying leg raise and the abduction machine, the faults, and the sets. The machine version of the same side-of-the-hip work is on the hip abduction machine, and the standing band version is the lateral band walk.

How to do a clamshell

1. Lie on your side with your head on your lower arm and your hips and shoulders in one line.
2. Bend your hips and knees. One study set the knees at 90 degrees and tested the hips at 30, 45 and 60 degrees of bend;² in another, the side glute was most active with the hips bent to 60.³ As a practical guide, 60 degrees is two-thirds of the way from thighs in line with your body to thighs at a right angle to it.
3. Stack your pelvis, top hip directly over the bottom one. In one study both glutes were more active when the pelvis stayed in neutral than when it was rolled back.³
4. Keep your feet together and open the top knee as far as you can without the pelvis rolling back.
5. Pause, then close it slowly. The way down is half the rep. Do all your reps on one side, then turn over.

What the clamshell works: what was measured

These studies measured muscle activity with sensors (EMG), usually as a percentage of each muscle's activity in a maximal test contraction. The muscles are the gluteus maximus, the gluteus medius at the side of the hip, and the tensor fascia latae (TFL), a small muscle at the front and side of the hip that also helps lift the leg out to the side.

• The glutes more than the TFL. In 20 healthy people with fine-wire sensors in the muscles, both glutes were significantly more active than the TFL in the clam. Of 11 exercises, the clam had the highest ratio of glute to TFL activity: 115, in a range of 18 to 115 across the 11.⁴
• The same order in a second group. In 11 healthy, active adults (5 men, 6 women), the clam with and without a band ranked first and second of 13 exercises on the same ratio. During the clam the gluteus maximus was the most active of the three, then the gluteus medius, then the TFL.⁵
• The side glute over the TFL in a third. In 11 men doing ten maximal reps, gluteus medius activity was greater than TFL activity in the clam, as it was in the other two exercises tested.⁶
• Not every study agrees. In 20 healthy runners (9 men, 11 women), with a weight just above the knee and the hips bent to 45 degrees, the muscles at the front of the hip that bend it were more active in the clam (54% of maximum) than the TFL (34%) and the gluteus medius (33%); the gluteus maximus (34%) did not differ significantly from the front-of-hip muscles.¹ The authors concluded that the side-lying leg raise is the one to use when the gluteus medius is the target.¹

Muscle activity on the day of a test is not muscle growth over months. No study cited here trained people on clamshells alone and measured muscle size, so this page makes no claim about what weeks of clamshells build.

Hip angle and pelvis position

In the studies, the setup changed how active the glutes were:

• Pelvis stacked, not rolled back. In 17 healthy volunteers doing six versions of the clam, gluteus maximus and gluteus medius activity were significantly greater with the pelvis in neutral than reclined. TFL activity was relatively low and generally unaffected by the changes.³
• More hip bend, more glute. In the same study the gluteus medius was most active with the hips bent to 60 degrees.³ In 30 healthy women, the upper part of the gluteus maximus was more active at 60 and 45 degrees than at 30, and the ratio of glute to TFL activity was highest at 60.² A later study by the same group, also in healthy women, found the back part of the gluteus medius more active at 60 or 45 degrees than at 30.⁷

The practical reading: bend the hips well forward and don't let the top hip roll behind you.

Clamshell with a band or without?

In the study of 11 active adults above, adding an elastic band to the clam significantly raised only the TFL's activity; the gluteus maximus and gluteus medius were unaffected.⁵ The band was a fixed level, one for the men and one for the women, and the authors suggest future studies try more resistance.⁵ So in that study the band raised the TFL's activity, not the glutes'; whether a heavier band changes that wasn't tested.

A practical way to use one, not a finding: start without a band, and add a loop band just above the knees once 20 slow reps on each side are easy, so you have a way to keep the set hard. Progressive overload covers the other ways.

Clamshell vs side-lying leg raise vs the abduction machine

In 11 men doing ten maximal reps of each, gluteus medius activity showed no difference between the side-lying leg raise, the clamshell and the hip abduction machine. The TFL was more active in the side-lying leg raise than in the other two, and the ratio of gluteus medius to TFL activity was higher on the machine than in the clamshell or the leg raise.⁶ In the runners above, the gluteus medius read 70% of maximum in the side-lying leg raise and 33% in the clam; that study compared the muscles within each exercise, not the exercises with each other.¹ The two studies used different loads and groups, so they don't settle which is better for the side glute. The machine and the floor versions are compared on the hip abduction machine.

The common faults, and a fix for each

• Rolling the pelvis back to open the knee wider. Stop where the pelvis would have to move; in the 17-person study the glutes were less active with it reclined.³
• Hips nearly straight. Bring the knees forward until the hips are well bent.
• Feet coming apart. Keep the heels touching so the move stays a turn of the thigh.
• Swinging fast. Open under control, pause, and close slowly.
• Pushing through pain. If you have pain, an injury or a medical condition, stop and ask a doctor or a physical therapist.

Sets, reps and where it goes

• What the studies used: five repetitions of each exercise in one, ten maximal repetitions in another.⁵⁶ Those were test sessions, not training plans.
• As a practical start: 2 or 3 sets of 12 to 20 slow reps on each side, stopping two or three reps short of failure. This is practical advice, not a finding. (How to judge reps left.)
• Where it goes: after the main lifts, beside the hip thrust or the glute bridge. The glute workout has a full session.

Common questions

Do clamshells work the glutes?

In two studies both glutes were more active than the TFL in the clam, and it ranked first of 11 and of 13 exercises on that ratio;⁴⁵ a third measured more gluteus medius than TFL activity.⁶ In the runners' study the muscles at the front of the hip were more active than the gluteus medius and the TFL.¹ Set it up with the hips bent and the pelvis stacked, as above.

Do clamshells build muscle?

No study cited here trained people on clamshells and measured muscle size, so this page can't say. What was measured is how hard the muscles worked during the exercise. For the side of the hip, the abduction machine page covers what training studies found.

Clamshell vs fire hydrant?

The fire hydrant is done on hands and knees: with the knee bent, you lift one leg out to the side. The clamshell is done lying on your side. No study that measured the fire hydrant on its own was found for this page, so it can't compare the two. A measured exercise on all fours is the kneeling glute kickback.

How many clamshells should I do?

No study set a number for training. As a practical start, 2 or 3 sets of 12 to 20 slow reps on each side, two or three reps short of failure, after your main lifts.

If you'd rather not plan it alone

The clamshell takes a minute to learn; deciding whether your week needs it, and how it sits next to the lifts that already work your glutes, is the part a page can't do. Coaching is the version where your workouts live in Trainerize, built around the days you actually have, and a weekly call looks at the week you just had and rebuilds the next one. Coaching to build muscle explains how that works, and here's how to apply. Otherwise: at your next lower-body session, finish with two sets of fifteen clamshells on each side, hips well bent, pelvis stacked.