Demystifying the sit bone width?
Over the last years, measuring the width of the sitting bones became the primary measurement tool to select saddles. By now, several saddle manufacturers use different versions of this tool to measure the distance between the ischial tuberosities.
However, can the distance of the sit bones really be the answer to every saddle purchase?
It’s time for us to have a closer look at this topic:
The basic idea behind this measure is reasonably simple: the athlete sits down on a stool that is either covered with a gel film, or soft foam or corrugated cardboard. He then moves his centre of gravity forwards or from left to right (depending on the manufacturer). In this way the athletes impresses his buttocks into the material and the retail assistant measures the distance between the two imprints, which are supposed to signify the sitting bones. This serves as a basis to select a saddle model with a matching saddle width.
In principle, there is nothing wrong with this approach. It can be a first step to categorize the width of the pelvis. However, this is only valid for an upright sitting position, requiring the saddle to be lower than the handlebar. In this case the cyclists indeed is mostly sitting on the sitting bones. The saddle should be having a sufficient width in the back to have enough bearing surface for both sitting bones.
With more athletic sitting positions (sportive MTB-position, road bike, triathlon), the saddle is higher than the handlebar. This normally requires a forward rotation of the pelvis – just to allow the hands to reach the handlebar. In this way, the straining of the sitting bones is significantly reduced and it moves forward to the pubic rami (i.e. the transition from the sitting bones to the pubic bone) and the pubic bone. The weight force of the rider is put stronger on the mid- and front-area of the saddle – this is accordingly the area on which a saddle analysis should focus!
In the extreme, the two sitting bones are no longer strained at all but ‘float’ unstrained above the saddle. A phenomenon that is especially common for aerodynamic road bike and triathlon positions. We could unequivocally prove the forward rotation of the pelvis in sportive sitting positions using the gebioMized pressure measurement film in several thousand saddle pressure measurements throughout the last decade.
For upright sitting positions the distance of the sitting bones can offer a first value to consider when buying a saddle. However, also in this case bony structures of the pelvis (besides the actual ischial tuberosities), nerves and blood vessels could be pinched off, causing tingling and numbness. When measuring, it is thus always advisable to capture the whole saddle area to maximize comfort. For sportive sitting positions the focus in bikefitting should be on reducing the pressure on the frontal pelvic area – for this the distance of the sitting bones only plays a subordinate role. In addition to selecting the correct saddle, its correct adjustment is of decisive importance as well!
Author: Daniel Schade