“Anchor through your tailbone when you are on the mat”

This Pilates cue is a common one when it comes to ensuring your alignment and control of your ‘pelvis and spinal neutral’ postures in class. This is especially so when testing your deep abdominal strength in exercises such as table top. But what does this ‘tailbone’ actually look like? It is anatomically called the sacrum and may not look like what you imagine.

I often get asked about the coccyx with reference to this Pilates cue. When you study this image, you can see that your coccyx is just the tiny bone on the end of the sacrum. it is not through this that you find or control your ‘pelvis neutral’ of ‘heavy tailbone.’   

The sacrum articulates with the iliums of the pelvis on each side. These 2 joints are called the sacro-iliac joints ( you have a left one and a right one), and these are crucial role in providing stability, transmitting forces, and facilitating movement between the upper body and lower extremities.

Now you know what the ‘heavy tailbone’ cue really means — put it to work in our Pilates Fundamentals tutorials on spinal neutral and deep postural support.

What is the sacrum, exactly?

The sacrum is the triangular bone at the base of your spine. It is actually five vertebrae that fuse into one solid bone as you grow up, which is why it feels completely different from the rest of your spine when you press on it. It sits wedged between the two bones of your pelvis, with the tailbone (coccyx) hanging off its lowest tip.

Where is your sacrum?

You can find it yourself in seconds. Put the flat of your hand on your lower back and slide it down until you feel a broad, flat bone with a small nub at the bottom. The broad bone is your sacrum; the nub is your tailbone. It sits roughly level with the dimples you can see at the top of your buttocks — those dimples sit over the sacro-iliac joints.

Is the sacrum the same as the tailbone?

No, but they are neighbours, and the mix-up is completely understandable. The tailbone is the small bone at the very end of the sacrum. So when a Pilates teacher asks you to “anchor through your tailbone”, the bone that does the work is the sacrum — the tailbone is simply the point you are aiming through.

Why the sacrum is built for stability, not movement

The sacro-iliac joints each move only a few millimetres, and that is by design. They are stability joints: their job is to lock the spine onto the pelvis and pass force from your upper body down into your legs — and back again — every time you walk, stand on one leg or climb stairs. That is why Pilates teaching talks about finding the position rather than forcing movement into it. The control you build in class is what keeps those joints quietly doing their job.

“My sacrum is out of alignment” — what that usually means

Patients often arrive saying their sacrum has “gone out” or needs “adjusting”. In most cases it is not a bone that has shifted: the sacro-iliac joints barely move, so a sacrum that feels wrong is usually one joint that has become irritated, stiff or overloaded — commonly after a change in activity, a pregnancy, a fall onto the buttock, or a long spell of sitting. A physiotherapist assesses how your pelvis and lower back are actually moving, works out which structure is responsible, and treats that. That is a very different plan from trying to push a bone back in.

The sacrum in pregnancy

Pregnancy is when most people first notice their sacro-iliac joints. The hormone relaxin loosens the ligaments around the pelvis so that it can open for birth, which means the sacrum’s stabilisers have to work harder at exactly the time your posture is changing and your centre of gravity is moving forward. That combination is why pelvic girdle pain so often shows up around the sacro-iliac joints in the second half of pregnancy. Our pregnancy physiotherapy treats it, and our prenatal Pilates classes build the control that supports it.

When to see a physio about sacrum pain

If you have a deep ache on one side of your lower back or buttock, pain rolling over in bed, pain standing on one leg or climbing stairs, or pain that has been there more than a week or two, it is worth having looked at. At our Sydney CBD clinic we treat lower back and pelvic pain — our physiotherapy team can assess the sacro-iliac joints, the lower back and the hip together, then give you a home program. The lower back pain and pelvic pain pages explain how we work.

Related reading

Two short reads that pair with this one: actively sit and untuck your tailbone for the sitting version of the same cue, and visualising the pubic symphysis joint for the other end of the pelvis.