Discussion: Polar 625x footpod

I've been wearing my footpod recently while on the treadmill... which has led to a number of questions about calibration. (I wear it so that I can easily capture the speed/distance data to upload into the diary, and also in theory deal with any issues of treadmill miscalibration.

Firstly, my footpod driven speed is generally 1/2mph slower than that displayed on the treadmill... However there's been one case where its been the same (which I'd guessed was a treadmill miscalibration), and one the other day where there was a 1.5mph difference... This definitely wasn't all a treadmill calibration issue, as when I cranked up the treadmill to try to get to my target footpod-driven speed, I could tell from my heartrate/exertion that I was going a lot harder.

Is there any reason for the footpod to not be consistently calibrated from day to day? They do talk about you should recalibrate for each pair of shoes that you use, but would this account for all the difference? Do I take it that I'm better off using the treadmills as a guide to pace, rather than using the footpod?

With the S625 footpod it matters how you mount it on the shoe, this could be the reason for the different finding from day to day. I also observe differences of 5% in distance just by switching the footpod from one shoe to the other.

I've never heard that treadmills were well calibrated. I noticed that my 625x consistently gave me slower speeds than any treadmill, just by varying amounts. I wonder if it has to do with the difference in foot strike and movement on a treadmill versus a track?

To be perfectly honest, I am quite surprised that you get any sort of meaningful reading using the footpad at all. As I understand it, the foot pad uses accelaration (and length of) and decceleration (and length of) in order to determine your speed and distance, so it just doesn't make sense at all on a treadmill as during both accel and deccel your foot is going in two different directions (I hope that makes sense). The maths is all wrong!

While I was in SA (many moons ago in the late 80's) I helped on a study which was involved with the accuracy of treadmills (from bad ones - cheap - to good ones - REALLY expensive) and the comparision of running speed indoors (on a treadmill) and the same outdoors.

Generally the middle to better treadmills all tend to be exceptionally accurate (and this was 20 years ago - I can only assume they have got better), however the impression of speed was completely different, ie the same speed felt faster indoors (in terms of effort) than outdoors. Admitably these treadmills were set up and maintained correctly.

If I was using a treadmill and it was reasonable good one, then I would accept the treadmills reading.

It's really easy for a treadmill to measure distance accurately. I'd be appalled by one that was inaccurate. No excuse for that.

That's very helpful thanks all... Today's great news therefore seems to be that I'm doing my treadmill intervals 0.5mph faster than I thought..!!

Richi wrote:
"To be perfectly honest, I am quite surprised that you get any sort of meaningful reading using the footpad at all. As I understand it, the foot pad uses accelaration (and length of) and decceleration (and length of) in order to determine your speed and distance, so it just doesn't make sense at all on a treadmill as during both accel and deccel your foot is going in two different directions (I hope that makes sense). The maths is all wrong!"

Accelerations are the same in any inertial (i.e., non-accelerating) reference frame. This means, the acceleration of the foot pod is the same, whether running on a stationary surface (e.g., a track) or a moving surface (e.g., a treadmill). Discussed in any 1st year physics text. The maths is all right :-).

Perhaps a GPS mightn't be much use though.....

You could put the GPS on your foot...:-)

My 2 cents: distance on treadmill IS accurate, but speed is all relative. Let me explain: the speed of the belt IS accurate, but your speed is not. Try it: get on a treadmill, set it a a constant speed, and see how you can go way faster than the best, or slow down and match its speed, and still keep up with the belt. The reason for that is that you are running on the spot, so you can be moving faster than you have to, giving your pod a different reading than the treadmill does - if you are going 5% faster than you have to, then your pod may tell you that you ran 1050m while the treadmill will tell you 1km. My impression, they both are right, it just shows you how in tune you are with the belt that day!