November 02, 2014

A super nerdy foot pod vs. GPS vs. accelerometer experiment

I was totally expecting to wake up at 4:00 this morning because of the time change, but I took half a Xanax last night before bed, and managed to sleep until the phone woke me up at nearly 8:00! That would have been 9:00 without the time change, and I can't even remember the last time I slept that late. It's always awesome when I can get a really good night's sleep.

I was very excited to track my friend, Renee, who was running the NYC Marathon this morning. Renee is the one who got me started running, and I've always looked up to her as a runner. She's always had a dream to run the NYC Marathon some day, and she finally got in a couple of years ago, in 2012. I even secretly bought a plane ticket to fly out there and surprise her for the race.

Then, the devastation of Hurricane Sandy happened. I made the decision not to go even before the canceled the race, but it worked out well for Renee to defer her entry until either 2013 or 2014, and she chose 2014. So, this morning, her long-time dream of running the marathon came true! I love that the live tracker had mile splits the entire course, so I could really follow along to see how she did. It was ridiculously windy today, and I haven't talked to Renee to see how she feels about her finishing time or how the race went for her, but she finished in 4:12:04. I know she was hoping for sub-4:00, and she's run a sub-4:00 previously, but with the wind today, I think everyones times were slower than anticipated.

Anyway, I got an email from Garmin about the foot pod. It's funny, I've been trying to get an answer from them for nearly a YEAR, and never heard a word back. Then I posted about it a couple of days ago on my blog, and suddenly, I get an response ;)

It's very long, but this is the general summary of what Garmin had to say: There is an accelerometer in the Garmin 620 and 220, which will give you the same info that a foot pod would; therefore, you don't need to use a foot pod. The accelerometer has to be "calibrated", which happens automatically once you do a couple of runs outdoors with GPS--apparently, the Garmin will "learn" about your running, and will give you an accurate reading. So, according to the email from Garmin, you don't need to use a foot pod because the built-in accelerometer will give you the same info--pace and distance--that you would get with GPS or foot pod.

So, if this is true, then my Garmin should "know" me VERY well, because I've been running with it for nearly a year now. If it just takes a couple of runs to calibrate, mine should be spot-on. I decided to test that today, out of sheer curiosity. Warning: This is about to get extremely nerdy and technical, so just skip to the page break if you don't care. Most people don't ;)

The only true way to test it out was on a measured track, so I went to the local high school 1600-meter track. I planned to do three runs: the first using GPS, the second using the foot pod, and the third using just the built-in accelerometer.

First up: GPS. I chose the same mark on the inner lane of the track to start and stop all my 1600 splits. Each lap is 400 meters, so I started the Garmin on the mark, and then stopped it on the fourth lap. I never had to change lanes or dodge people or anything, so I am 100% certain that I ran 1600 meters for all of the splits I did today.

Split one was done in 9:33. The GPS was very accurate, showing 0.99 miles at the 1600-meter mark. (For reference, 1600 meters = 0.994194 miles.)

Second: For this split, I turned off the GPS and turned on the foot pod (which I had just calibrated a couple of days ago, although I didn't have a chance to test the calibration yet). The calibration factor was 105.8%. Again, I ran four laps, starting and stopping on the same mark.

Split two (again, 1600 meters) was done in 9:16. The Garmin read 1.04 miles, which is 1673.72 meters. So, the foot pod showed me as going almost 74 meters farther than I had actually run. In pacing terms, my actual pace was 9:22/mi, but the foot pod showed a pace of 8:56. That's pretty significant for just a mile!

Third: For this split, I turned off the GPS as well as the foot pod, which activated the built-in accelerometer. This is what I was most curious about, especially after reading the email from Garmin. I ran 1600 meters, starting and stopping on the same mark as before.

Split three was done in 9:31, and the readout on the Garmin was 0.89 miles, or 1432.32 meters. In pacing terms, my actual pace was 9:37/mi, and the accelerometer pace showed 10:43. That's a minute and six seconds difference, per mile!

After I did those three miles, I still believe that the foot pod can be accurate when calibrated to the correct number. It's just hard to find that number! For a long time, I had it calibrated to my treadmill, and it was extremely consistent. So I think I just need to find the right calibration factor on the foot pod.

I went to the car, and decided to do the math to recalibrate it, and then go run another 1600 meters. It would have bothered me all day if I didn't ;) I got a calibration factor of 105.5, which wasn't that big of a change. After running the 1600 meters again (in a time of 9:30), the Garmin read 1.03 miles (or 1657.62 meters). Better than the second split by 16 meters, but still 57 meters too much. In pacing terms, my actual pace was 9:36/mi, foot pod pace showed 9:14.

I'm going to keep playing around with the calibration factor on the food pod until I can get it to give an accurate read. Then I'd like to experiment with it both on the treadmill and on the indoor track, just out of curiosity. But if there is one thing I learned with certainty, it's that the built-in accelerometer is not at all accurate (at least not for me).

So it has now been 48 hours since Halloween, and I've only had TWO pieces of my kids' candy! I had a fun size Butterfinger and a fun size Milky Way. Today was extremely difficult, because the kids keep offering me candy, and I truly want to just dive headfirst into the bag and eat it until I feel sick. I know that sounds terrible, but it's true! Instead, I've managed to continue counting my PointsPlus, keeping my goals in mind.

I'm really undecided about whether I'm going to run the Monroe Half on Sunday. On one hand, I really want to run it because it's my hometown race. I did it last year, and I LOVED it--I paced Stephanie to sub-2:10, and I felt fantastic. It's only about five miles away, so it's very convenient. Also, my friend Stacie is the race director, and I haven't seen her in forever.

Stephanie and me after last year's race

On the other hand, I'm totally unprepared to run a half marathon right now. I ran 10 miles on October 9th, but that's the farthest distance I've run since August 22nd! The weather doesn't look good as of right now (of course, that can change within the hour in Michigan). If I do the race, it would be solely for fun, and I would have to run very easy just to get through it. I'm sure I can finish, but it'll be harder than I'm used to. I'm leaning toward doing it--just nervous about it!


  1. oh man thank you - i am so annoyed with my garmin and the pace variables as well.

    today i ran 5 miles and the garmin was SO far off of runmeter on my iphone that i wondered who it was tracking!

    i bought the foot pod a few weeks ago to help with my treadmill runs cause the rreadmills at my gym are WAY off in distance ... but i tried it and it was further off.

    garmin website has woefully little about the calibration.

    i almost have given up - but you are inspiring me. the builtin accelerometer on the garmin has been acting up at the gym over the last week so i really want to get the foot pod working. will follow your examples

    thanks again! (btw i work in IT so i speak nerd too)

  2. I never went all out and bought the expensive Garmin. I used to run with an older one and a foot pod and now I wonder how accurate it was. These days I just use the GPS and RunTracker on my phone.

    P.S. I loved reading the nerdy part. I might despise math, but running math is another story.

  3. Wow--An Accelerometer? That's NOT EVEN A WORD! (As a fellow "Friends" fan, I hope you get that quote!) Funny how Garmin finally contacted you after you wrote about your frustration with them in your blog. Your blog gets results!!!

    I hear where you're coming from on the Halloween candy. I just bought a tiny bit, a few candy bars at Walgreen's for the grandkids, and about 8 extra, in case any kids showed up at our door Friday night. We had NO trick-or-treaters out here in the country, however, and I never got the grandkid's candy delivered to them, so it's still here. I did give Granddaughter, who is spending the night, some of it last night, and the rest of her's is in the fridge, (so the chocolate doesn't melt all over her hands, like it did last night), then I put some candy bars in the little cauldrons I bought for the other 3 grandkids, put them in a plastic grocery bag, put a twisty on it, and put them in my car, thinking I will get the candy to the kids when I take Granddaughter home later today. Plus I needed to get tjhat candy OUT of my house. I put the last three candy bars in hubby's lunch this morning. I think I bragged that I only ate some mini Tootsie Rolls this year, and no other Halloween candy. But then I remembered that sugarless Russell Stover marshmallow covered pumpkin I ate. I found it at Walgreens when I was buying the other candy, and it was only 80 calories. It was delicious! But I'm glad the rest of that darned candy is out of my house! Now all we have to do is get through Thanksgiving and Christmas. Lord---Give Us Strength!!!

    Diane at wrote about "Five Ways to Quit Thinking About Food," today. I use some of her suggestions: Finding something else to fill your time (crafts, computer games, etc.), and Getting Out of the Kitchen. But still I think about food all the time. Frustrating. I thought maintenance would get easier over time, but it really hasn't. I guess each day I get up and keep trying, I win.

  4. I had a suspicion that the built in accelerometer wouldn't be accurate. What a bummer. I have the same Garmin as you. I don't have a foot pod. I should see if I have access to a track so that I could try to calibrate one. Yikes! It seems daunting. I just trust the treadmills at the gym and I don't really track those runs. Which kind of sucks. I hope Garmin reads this post so that they can see how off it is!

  5. Ha ha, I opened up my daily "Bloglovin" email this morning and was stunned to see a picture of myself. ;) But just seeing that picture brings a HUGE smile to my face. I remember being SO HAPPY after that run (and exhausted and cold, ha!). We worked so hard in such tough conditions, but we met our goal and it was SUCH a great feeling!! You can just see it by the smiles on our faces. I can honestly say I haven't had that feeling since that day. Not even my marathon finish made me that happy.

    So I totally think you should run it and have a great time!!! If I could run it with you, I would!!

    PS: I loved the research on the foot pod and GPS. I swear, those things are never truly accurate, and you just proved it!! I love your math nerdiness. :)

  6. Yeah, the accelerometer definitely is not accurate, but it's not really Garmin's fault I guess... it can only measure the up and down (aka each time you take a step). It doesn't know how far forward you are launching yourself with each step... If I run slow I ran at like 172spm and if I'm running past I'll do 180spm. That's the difference between 9:45 and 7:00 a mile, but honestly anything in between there is a crapshoot. 176spm could be an 8:00 pace for me but it could also be a (;15 pace. It all depends on how much energy I'm expending with each step. I just have the 220, so I don't have anything super fancy... it is my understand the 620 is supposed to be able to measure things like stride length and ground contact time and whatnot... seems like it should be able to mesh all those together and be more accurate, but you're still getting the same results as I. It gives me about 90% of the distance I actually ran when I solely use the accelerometer.

  7. Go do the half-marathon just for the fun of it. Even if you go slowly or walk some. From your comments, it sounds like you will regret it if you don't.

  8. Question, I know my strides are much longer when I run faster so how would I calibrate? Fast? Slow? Maybe it won't matter? I would like to calibrate correctly so I can start using the pod while on the treadmill again. I just manually enter my runs but that doesn't show the speed changes. However even if I calibrate on teh treadmill it never matches. Thoughts?

    1. I just tested this out today! I calibrated mine using my regular, easy pace. Today, to test it out, I ran fast (basically a sprint) for 100 meters a few times during my run, and varied my pace a little, but thankfully, the foot pod was still super consistent. It'll be on my blog tonight :)

    2. Awesome!!! Thanks for being a number nerd :)

    3. I know I'm about a month late, but I just went through the foot pod calibration at the start of November, and what I finally decided to do was manually change the calibration factor until the distance on my Garmin matched the track distance.

      In other words, I turned off the GPS, set the calibration factor to 100 then did two loops. According to my Garmin, I'd run .52 miles, so I manually set the calibration to 95. Then I did another two loops. Garmin now said I ran .51 miles. I did this a few more times until my calibration factor was 93 and the distance was equal to .5 miles.

      I know the accelerometer isn't accurate because every time I move my watch arm differently, like to adjust my sunglasses or anything, it drops out foot strikes, so my cadence when running without the footpod always lags significantly behind what it is when I have the footpod on. As much as 15 strikes per minute, which is a lot.


I used to publish ALL comments (even the mean ones) but I recently chose not to publish those. I always welcome constructive comments/criticism, but there is no need for unnecessary rudeness/hate. But please--I love reading what you have to say! (This comment form is super finicky, so I apologize if you're unable to comment)

Featured Posts

Blog Archive