Let's have a look at a model of a pelvis so we can observe a part of the body that isn't normally visible. Then we'll discuss some common methods for measuring our 'sit bones'.

Images above are:

1. anterior (front) view of a female pelvis

2. SQlab sit bone measuring stool

3. SQlab measuring paper

4. SMP sit bone measuring pad

Your sit bones are more correctly known as the ischial tuberosities and are located at the base of your pelvis. This is the part of your skeleton that supports your weight when you sit on a chair or bench. See the yellow circles on the pelvis in the first image. We may also rest our weight on our sit bones when sitting very upright on a bicycle saddle such as on a beach cruiser or town bike.  However, when we adopt even a small amount of anterior or forward pelvic tilt, as most riders do on road or gravel bikes, we begin to roll forward onto our ischiopubic rami. Our weight shifting forward and resting on that narrower part of our pelvis.

From the sit bones to the front of the ischiopubic rami is only about 4cm in length. On a correctly fitting saddle, our weight should be supported primarily on our skeleton at all times, regardless of how upright or aggressive our position is. Only a small amount of weight, and therefore pressure, on soft tissue is acceptable. A correctly fitting saddle will be shaped such that the sides taper inward from rear to front and mimic the narrowing or your pelvis as you roll forward. Narrow enough that you don't rub your inner thigh on the saddle when pedalling, while still providing sufficient flat surface to support your skeleton.

So if we don't typically support our weight on our sit bones when riding, why do we measure our pelvis at that point when assessing saddle width. Put simply, this is the only part of the lower third of the pelvis that is measurable without expensive or invasive procedures. Fortunately, measuring sit bone width is ludicrously simple.

Some brands will have expensive electronic stools with pressure pads that sense the areas of greatest pressure under your bum when you sit on the pad. The resulting report tells you your sit bone width. When measuring sit bones, regardless of the method, you will need to wear normal shorts or pants so that your sit bones can protrude onto the measuring surface. Padded cycling shorts of bibs will prevent you from obtaining an accurate measurement or may result in no measurement at all.

The SQlab system (image 2) is very simple. You sit on their special stool, which has a plastic pad with many small spikes. Covering the spikes is a piece of paper. You sit down, lean slightly forward, grab the handles at the side and pull yourself down onto the paper. When you stand up, your sit bones should have pressed two sections of the paper onto the spikes. Find the centre of the two sections and measure the distance between them (image 3). That's your sit bone width!

The SMP gel pad (image 4) works in a similar way with the pressure from your sit bones creating two hollow sections in the gel. You can measure the distance between the centres with a ruler or use the measurements on the pad that can be seen faintly through the gel.

You can also measure your sit bones easily at home, although it may be less accurate. You need a thick piece of cardboard and a flat stool that is low enough that you can comfortably keep your feet on the ground with your knees bent at approximately 90 degrees. Sit on the cardboard and pull yourself down onto the stool as you would with the SQlab method. You should leave two flattened sections in the cardboard when you stand up. Find the centre of those two sections and measure the distance between them. 

In our next blog post we'll discuss the use of sit bone measurement when choosing a saddle and why it is only one of several important factors to consider.