At a glance

Lying on your side with the knees bent and lifting the top knee while the pelvis stays perfectly still. The usual first strengthening exercise for the gluteus medius, and one of the few an irritable gluteal tendon tolerates early.

What it works

Primary muscles

Gluteus medius, particularly the posterior fibres
Deep hip external rotators

Also involved

Gluteus maximus, upper fibres
Obliques and deep trunk muscles, stopping the pelvis rolling back

Equipment: None to start. A resistance band above the knees once the movement is clean.

How to do it

1
Lie on your side with hips bent to about 45 degrees and knees to about 90, heels roughly in line with your bottom, head resting on your lower arm or a pillow.
2
Stack the hips directly on top of each other and keep a small gap under your waist. Rest your top hand on the top hip so you can feel the moment it starts to roll backwards.
3
Keep the heels touching and lift the top knee, opening the legs like a clam. Small is correct: most people have 10 to 15 centimetres before the pelvis begins to help.
4
Hold for two seconds with the knee open, then lower with control over two seconds.
5
Stop each lift at the point the pelvis starts to roll back. That point, not the ceiling, is the end of your range.

How much to do

3 sets of 10 to 15 slow repetitions each side

Daily while the hip is irritable, then 3 to 4 times a week

Gluteal tendon problems are a three month job, not a three week one. Tendons remodel slowly, so consistency matters far more than intensity here.

Stop and get assessed if

Stop and get assessed if the clamshell produces sharp or pinching pain over the bony point on the outside of the hip rather than a working ache in the side of the buttock, or if it sends pain down the leg. Night pain that is getting worse week on week also needs assessing rather than more repetitions.

Common mistakes

Rolling the pelvis backwards to open the knee further: The extra range then comes from the spine and pelvis, so the gluteus medius switches off at exactly the point you were trying to load it. Keep the top hip stacked and accept the smaller lift.
Letting the top knee drift forward across the midline: Whenever the knee crosses the midline of the body, the gluteal tendons are squeezed against the bone underneath them. That compression is the mechanism behind most outside-of-hip pain, so keep the knee behind the line of your trunk throughout.
Working through pinching pain on the outside of the hip: A pinch over the bony point is compression rather than strengthening, and repeating it keeps the tendon irritated. Reduce the range, check the hips are stacked, and put a folded towel under your waist so the pelvis is not tipped across.
Adding a band before the movement is clean: A band multiplies whatever pattern you already have. If the pelvis rolls without one, it will roll harder with one, and you will simply get stronger at the wrong movement.
Rushing the repetitions: Fast clams bounce off the soft tissue around the hip instead of loading the muscle. Two seconds up, two seconds down, and the burn should arrive in the side of the buttock.

Making it harder

Hold the open position for five to ten seconds per repetition. Isometric holds are well tolerated by a sore tendon and often ease pain for a while afterwards.
Loop a resistance band just above the knees, but only once the pelvis stays quiet without it.
Move on to side lying hip abduction, which loads the same muscles with a much longer lever.

Making it easier

Reduce the lift to a few centimetres and simply hold there, building the range over days.
Start with no movement at all: press the knees gently apart against your own hand or a belt for ten seconds at a time.

Clamshell