Why Triathlon/TT Fitters are obsessed with hips

Actually I’ve always considered myself more of a leg guy. With TT/Triathlon fits, however, it’s the hips that rule.

When I put on my bike fitter hat my consideration of the human form is no longer about the aesthetic - although my assessment of whether a client “looks good” does typically translate to a good fit (that would make a good blog article)- but about performance.  When I’m analyzing a TT position, I zero in on the hips. I’m obsessed with them. To put a finer point on things, I look very closely at the angle your torso and upper leg make at the hip.

Aero fits are distinct from standard road or mountain fits simply for the movement pattern restrictions the aero position places on the rider.  For a road fit or mountain fit, rarely do I look at the Retul metric “hip angle closed.” For aero fits, however, this is perhaps the single most important measure, one that I often fit around, and my eyes usually scan to this measure right after knee angle.  It is so important that I often recommend an ISM saddle, both because it allows for better hip rotation with less discomfort, but also because they make it easier to improve this measure.

Why?

To understand the why, you have to visualize what an aero position does to the rider and the potential restrictions that a more aero flat back can place on an optimal pedal stroke.  Think of the torso and the thigh as two segments that connect to each other. Where they connect forms an angle. This is essentially how Retul or any other motion capture system sees this relationship - as an angle.

The more aero or “aggressive” you get, the lower and flatter your back or that torso segment.  Yes aero nerds, I know, aerodynamics involves more than simply back angle, but I think you know where I’m going with this. As you get lower that angle between your torso and thigh becomes more closed.  

A question I’ll typically ask a rider when I’m assessing their aero position is:  “Do you feel any restriction or perhaps a loss of power when you’re coming up over the top of the pedal stroke?”   “Does it feel like your knees are coming up and almost bumping up against your stomach? A closed hip restricts the pedal stroke in that position, making the rider less powerful and efficient.

So, what do we do to enable a rider in the aero position to maintain a similar hip angle as a road bike position?  

To help with the visualization of this concept, what I like to do is explain to a client the difference in aero bike design to that of a road bike.  Triathlon and aero bikes look pretty funky compared to a road bike, not only because of the shaped tubes and other aero accoutrements designed into the frame, but equally importantly their seat tube angles are much much steeper than a standard road bike.  Where my road bike may have a 73 degree seat tube angle, most aero or tri bikes are 76, 78 even 80 degrees depending on the bike and seat clamp configuration.

Why?

Again, visualize that phenomenon of the rider “folding” at the hips when in an aero position with a low back.  With a saddle further back relative to the bottom bracket, the rider folds even more, closing up that hip at the top of the pedal stroke.  This is perhaps the biggest challenge a facing a rider when simply clipping on a set of aerobars on his road bike and assuming all will go according to plan. It usually doesn’t.

The very obvious geometric way around this is to move the saddle forward relative to the bottom bracket.  At the same time, when you move the saddle forward, you are effectively lowering the saddle. To maintain the same saddle height and knee flexion, a fitter would therefore also raise the saddle.   

So, if I determine the hip angle is a restricted at the top of the pedal stroke, one of my strategies is to increase the seat tube angle, either by physically moving the saddle forward, or trying the aforementioned ISM or other noseless saddle.  Because these are designed for the rider to sit closer to the tip of the saddle, they effectively steepen the seat tube angle, and I have had pretty good success with ISM saddles.

The other method is to be conservative.  With triathletes, and especially long course triathletes, I’m fitting to comfort as well as performance.  I want them to comfortably be able to stay in that position for hours on end if that is what the race demands.

If a rider is so uncomfortable over hours in the saddle that he is constantly fidgeting, getting out of the aero position to rest or stretch things or out, or is just so uncomfortable that it gets in the way of the business of pacing and applying power during that bike leg, then it defeats the whole point of optimizing that bike position.   Getting a rider more comfortable usually (not always) means a higher back angle.

So, in this case I raise the aero pads, which raises the torso angle relative to the horizon.   Because that torso is the one of the segments that forms that hip angle we were talking about, raising the front end effectively opens up that hip angle.  So, in this case, we accomplish two things: a more comfortable position and potentially more power due to a less restricted pedaling at the top of the pedal stroke. Win/win.

With crank length, is longer better?   

Maybe I’ll save a more thorough explanation of crank length for another blog article, but since you now have a little more experience visualizing the how certain equipment changes and positional changes influence that oh-so-important hip angle, I’ll leave you with a question:  How does crank length influence hip angle?