Why Inseam Measurement is Irrelevant (For Bike Fit & Saddle Height)

Summary

In this episode of the RCA Podcast, expert bike fitter and sports physiotherapist Neill Stanbury discusses the common misconceptions surrounding inseam measurement in bike fitting. He explains why traditional methods of measuring inseam are not reliable indicators of seat height and highlights the various anatomical factors that contribute to effective leg length. The conversation emphasizes the importance of understanding individual anatomy and the limitations of relying solely on inseam measurements for optimal bike fitting.

Takeaways

Inseam measurement does not correlate well with seat height.
Anatomical variations significantly affect leg length measurements.
The shape and size of the saddle can alter effective seat height.
Measuring inseam is often a waste of time in bike fitting.
Bike fitting should consider individual anatomy rather than generic measurements.
The LeMond method for seat height is flawed.
Effective leg length changes with pelvic rotation and saddle shape.
High inseam measurements can be misleading based on body composition.
Bike Fit Fundamentals course offers a comprehensive fitting guide.
Understanding your body is key to proper bike fitting.

Bike Fit Fundamentals: https://roadcyclingacademy.com/bike-fit-fundamentals/ 

 

Transcript: 

Cam Nicholls (00:00.174)
Welcome back to the RCA Podcast where today I am joined by expert bike fitter and sports physiotherapist, Neill Stanbury. Just be aware that this conversation is also designed for YouTube, so please excuse any visual references. Let's get into it. Okay, Neil, everyone wants to know my inseam height when we do a... Okay. It's not...

Fit video mine too. Yep relevant new I get this question a lot in the comments actually I do occasionally read the comments I usually check out of the comments because there's just 10 million of them now that we're a big big enterprise, mate There's just there's a lot of noise down there So I don't often delve into the comments because it's just too much to go through and I'm too busy But when I do read through them particularly with the bike fit videos and stuff like that a lot of questions about what my inseam is what's your

same.

Number one question. I think it's like 89 centimeters. Yeah, so mine's quite long for my like I've got a really short torso and long legs, right? You've never measured your inseam? No, not yet. Yeah, I never measure inseams. Back in the day I used to measure inseams and record it as part of my my fits and I've completely stopped doing that probably 10, 12 years ago, a long time ago I stopped doing it and the reason for that is I found no great correlation between the seat height that we were ending up at at the end of the fit and the inseam measurement. So I wasn't really using the number for anything.

measured my

Neill Stanbury (01:18.83)
Interesting. So I just ditched measuring inseams. Now if you're not sure what measuring an inseam entails, we basically get the person to stand up against a wall, flat feet on the ground, know, heels on the ground, knees locked out straight usually, no shoes on, no thick socks, and then we put like a book or a spirit level or something up in here. this rather, you know, centralized section of your anatomy here. We jam it up in there and then we get you to step away from the wall and measure how high it is. Right. So this is what they call inseam measurement.

Which is sort of supposed to be a proxy for leg length measurement, right? And I'm going to tell you today why that's a bad move. For those of you out there who've purchased our BikeFit fundamentals course, you'll notice that one of the things that isn't in there is inseam measurement because again, there's just no great correlation between the seat height and the inseam measurement and if you could say that there's a loose correlation I guess but there's so much variance. You might get the same person with an inseam of 80 centimeters and another guy with an inseam of 80 centimeters and their seat heights might vary by

50 millimeters, like huge variance that it just basically becomes so useless, the number, there's just no point even measuring it, Now, one of the methods of measuring seat height or of setting your seat height, I think they might have called this the LeMond method back in the day, is to measure your inseam and then multiply it by number, and that's just supposed to be your seat height, right? Now, let's delve into the many ways in which this is a terrible idea to set your seat height, people at home. First problem, let's say- How many problems are there?

I don't know, I'm gonna talk till I run out. There's quite a few. Let's say you've got two identical people with identical bodies and one of them has a slightly longer coccyx, this little bone in here which comes down off the bottom of your sacrum there. And one of the coccyx is it kind of comes down lower and the other one curls inwards more like that. You can just sort of see it kicking in or being very straight. The coccyx is almost, a lot of the time, the first thing when you jam something up you're gonna measure it.

The first thing that comes into contact with your book spine or your spirit level or whatever. So the length of your coccidial protuberance here determines your leg length. Obviously not. Makes no difference, right? So this portion of your anatomy, if it varies, it's gonna alter your inseam measurement. Second problem. The height of these ischial protuberances downwards here, inferior iliac spines here, what people commonly think of as your sit bones, right? The height of these relative to the center of the acetate

Neill Stanbury (03:44.174)
that height there, the difference between the two, varies hugely between people. So as a measurement of leg length, which is, you know, you can think of your leg length as being the center of the rotation of the acetabulum down to maybe the ground if you're standing still, the difference between that and that can be anywhere from 30 millimeters up to 90 millimeters, depends on the person, right? So the height of the acetabulum inside the pelvis relative to the sit bones is highly variable between people. So even if you do manage to measure,

somehow to your sit bones and you get a reasonable kind of dimension there for your inseam measurement, this number blows it out up, down, whatever. Doesn't make a difference. Another problem, if the person rotates their pelvis forward fairly well versus sitting fairly upright, the height of these relative to the rotational center of the hip changes the effective leg length more or less because this arc, this radius, if you think of it as a radius of movement here,

This arc changes the effective leg length a lot. So if the person was, say, extremely bent forward on the bike, this is kind of going to be level with that relative to the vertical plane that the crank is operating down there. Whereas if they're sitting bolt upright, you're going to get a completely different dimension effectively for the length of your leg. So inseam measurement for this reason is another terrible idea because as you roll forward, you just kind of...

change everything, all of the three-dimensional relationships between them alter so much that it's not even worth thinking about. Another one just to make it, we're going down lots of rabbit holes. As your femur comes out here and goes down, the angle at which it comes out and then kicks down, what we call the femoral neck here, highly variable between people, really, really different. The angle that it comes forward or backward is highly variable between people. Both of these two things alter the effective seat height.

when you're sitting on the bike, they alter the effective length of your leg, irrespective of where your actual in-scene measurement is, right? So in-scene measurements, if you've got a really lean person, you might get that spine and that book right up much higher. If you've got a person with slightly larger soft tissue around their backsides, for example, you might get a situation where that spirit level or that book spine that you're putting up in there is sitting down lower. So you're not getting a very good measurement there anyway.

Neill Stanbury (06:08.078)
So the common idea is you take this number and you multiply it by .883, I think was the old LeMond mechanism, and that's supposed to be your seat height. Terrible idea for all of those reasons. Another one is the saddle that you're sitting on, the shape and the size of the seat. Some of them, if it's particularly narrow seat, you're gonna sit deeper over the saddle. If it's a wider seat with a broader back, you're gonna sit up higher. So just by virtue of...

the nature of the shape of the saddle itself, you're get huge potential variations in the effective seat height that you're measuring anyway. So I've seen multiple situations like, I see this probably five times a week, where we change saddles for a given rider on a given seat height. We go from something like an SMP back to a flat style SLR boost or something like that. And you've gotta drop the seat 10 millimetres into the frame or lift it or move it forward or backward. So the effective seat height changes a lot.

based upon how your anatomy interacts with the saddle. And when you're trying to basically set the seat height based off a number, there is just for all of those reasons and more that I could, you know, we could go down endless rabbit holes with this, inseam measurement is a terrible idea as a method of measuring your seat height. So one of the reasons that my inseam might measure so high is because I'm so lean.

There's not much bum on me mate. You know, I don't know. You have noticed, yes. A lot of people are noticing that in the comments. getting a lot of ribald comments down there. So because I'm so lean down there, I think that inflates my inseam measurement. Yeah, okay. So, for a person who's 173, 174 centimetres last time I checked, an inseam dimension of 89 centimetres is crazy high, like really, really high. Does that mean I've got particularly long legs? Eh, probably.

but it could also mean that for all those reasons that we mentioned before, my inseam measures very high. So that is a more detailed explanation of why I don't even bother measuring inseam when I'm measuring people. And if you go and have a bike fit and they measure your inseam and they set your seat height based upon your inseam measurement. Ask for your money back. Red flags, Warning lights are flashing. And again, we've had a lot of questions in the Bike Fit Fundamentals course, people who've been going through that course.

Neill Stanbury (08:25.262)
They haven't asked me to measure my inseam and this is why there is just no point to it It is just such a highly variable method of measuring that it's just not worth doing. It's not even worth spending three minutes doing. Okay. Yeah

you sold me. So if you're watching this and you're like, I want to set my saddle height properly now, you mentioned bike fit fundamentals.

What's that? Yeah bike and fundamentals. That's our seven part course that we did last year our program that we did Detailed detailed 15 to 20 sometimes 25 minute modules seven of them and downloadable guides downloadable guides This will step you through the process one after the other Sequentially how to fit yourself to a road bike to a pretty high level We're very proud of it took a lot of work and it's out there for purchase if you guys want to go through the process yourself

you enjoyed that conversation with Neil Stanbury, we'll catch you in the next podcast.