odds and ends from a life under construction

, / 4575 3

Testing, 1, 2, 3: Hydrostatic Body Fat Analysis and the Saucony Stride Lab


One of the tasks I assigned myself for getting back on the workout wagon was having my body fat accurately measured. I needed data-backed starting point – I’m a wonk, and “how my jeans fit” wasn’t measurable enough for me. We have an Aria scale that uses bioimpedance to measure body fat, and it has consistently measured me between 37 and 39 percent (compared to the mid-40s just before surgery). And that number felt really, really high.


Last week I visited with Katy, my sometimes-trainer, at our neighborhood gym and she set me up with a strength training plan and measured my body fat using calipers. Her number was 27 percent; that sounded amazing, but much too low. And while she was taking her measurements, she commented that I had a lot of excess skin, especially around my abdomen, that would make it hard to get an accurate number. So, still not sure just what my body fat was doing, I hunted down a place to have the current industry standard measurement done – a hydrostatic body composition test.

Body Fat Test

The short version of hydrostatic measurement is that your body is composed of two things – fat, and everything that isn’t fat. Fat is less dense than water, so it wants to float, and everything else is more dense than water, so it wants to sink. Using a formula that takes into account your weight on land and the amount of water you displace in a tank (your “weight” underwater), a computer can figure out your rough body composition (minus errors introduced by the amount of air in your body).

For me, the test involved going to a mobile lab (essentially a large truck). Inside the lab was a changing area, a desk for the technician’s equipment, a scale, and a large tank filled with water (don’t worry, the water is circulated to keep fresh, and kept at the temperature of a hot tub for comfort). I had to put a weighted belt across my lap to hold me under the water, and I was instructed on how to quickly blow out as much air as I could to get as accurate a measurement as possible. Here’s a video of pretty much how it all went down (minus the video featuring a totally different person).

I went through the breathing-out process three times so I’d have a chance to “practice” blowing out all my air and get my number down as low as possible. And what was my magic number?


So just about the midpoint between my scale at home and the calipers at the gym. And I’m happy with that number. It’s not at the ideal place I’d like, but it’s also not exceptionally high and a goal below 30 is totally reachable. The technician also confirmed that, for the way I carry my weight, my BMI is completely inaccurate. I tend to build muscle easily, especially in my legs, so while a “normal” BMI would require a weight below 150, an ideal weight for my body type would be closer to 160 pounds (I’d like to get down to a racing weight of 155, so I have about 22 pounds to go).

The body composition report was full of interesting information, like how gaining strictly muscle or losing strictly fat (vs. a combination of both metrics) would affect my body fat percentage. I also got a definitive resting metabolic rate (1704 calories, which seems like a ton), and the report told me that on an average day just walking around, I burn 2434 calories. Pretty cool.

The last page of the report was the most useful though. You know how cardio machines or magazine articles tell you how many calories one hour of an activity will burn for the “average 125-pound person”? That person doesn’t really exist. Thanks to my body fat test, I know exactly how much a certain activity will burn. 30 minutes of ice dancing? 568 calories. 30 minutes of Pilates? 122 calories. 30 minutes of running 5-minute miles? A whopping 771 calories! Yeah…. that’s never going to happen.

Saucony StrideLab

Feeling good an assessed, I headed home, got cleaned up, and dragged Evan and Jason with me to the Saucony Stride Lab at the new Boulder Running Company location that opened just south of us. The store is now the largest specialty running store in the country, and the Saucony Stride Lab is the only gait analysis of its kind in a retail store in the world. I had made an appointment for noon, so we could get me analyzed before running our errands for the day, but another woman seemed to think she also had an appointment for noon (the computer at the front desk said that was not the case). Luckily, she was in and out in less than ten minutes and I had the fancy force plate treadmill all to myself (there was a nice little kids’ area nearby to entertain Evan, too).

Me and my trainer Katy

The technician had me run for a minute or so at a comfortable pace while she recorded my movements using the treadmills sensors and four cameras mounted around me to capture different points of my stride. Then I hopped off and the computer crunched the numbers and video and spit out some slow-mo captures with arrows positioned over the images to show the direction of force from my feet to my hips, the rotation of my shoulders, and some other fun metrics.


The short version: I’m not that bad a runner, as awkward as I think I look. I have a bit of a heel strike, but so do a lot of people and it’s not necessarily a bad thing. I also have a hip drop on one side, probably from an old ankle injury that I’m still overcompensating for. I don’t unnecessarily rotate my shoulders when I run; too much rotation can mean you’re trying to propel yourself forward with your upper body rather than your core and pelvis.


My biggest area of concern was that I am over-striding. My foot is landing in front of me instead of underneath me, so my knee is extended a little too much instead of landing with a bend for cushioning. The technician recommended I work on my lean, shortening my stride, and picking up my cadence – all things I knew I needed to be working on! I don’t know that I got a ton of new information from the Stride Lab analysis, but it was certainly a fun experience. And since my current running shoes have been discontinued, I got to try out some new kicks on the fancy treadmill. These colorful Asics came home with me and are currently in my training rotation. Yay!



  • showmeashley says:

    BMI has always sent me into an angry tailspin, you know especially after a doctors visit. When I get closer to goal I definitely want to try this water tank now.

  • Nicole Brady says:

    Oh my goodness. I want to do the stride lab. I am just getting into running (last year) and when I’m tired, I run like a lumberjack. Thump, thump, thump. No interest in seeing how horrible my BMI is though. 🙂

  • Candace says:

    That is way cool that they can analyze your stride like that..mine is horrible since I use my arms to hold my chest tight so I don’t hurt.

Join the Conversation