This seems to be the hot topic among gadget obsessed runners lately. as a self confessed gadget junkie and techno geek i've noticed the following related trends:
(reasons for using an sdm?)
a fantastically simple device, dirt cheap and widely available. sadly however quite unreliable for running.
a sprung weight inside the pedometer closes a switch each time a foot strikes the ground. you have to tell it how far you go for each stride and it will multiply up the stride length by the number of strides it counts to give distance, and subsequently can divide distance by time to give average speed.
it's not difficult to see that this relies on accurate measuring of your stride length and a consistent stride length when you run.
i've used one of these and went through a learning process of adjusting the stride length to give the correct reading on a known course. eventually it was quite good at that course and at least gave me a rough idea of the distance of other courses, still drastically affected by walking, changes of pace and hills.
something we've all no doubt heard about, and increasingly widely available. some of the older chunky models can now be had for as little as £70 or so.
the geometry is tricky but basically using a network of satellites it can locate you anywhere on the globe to within a quite impressive accuracy (impressive relative to the size of the globe, not always so impressive in relation to the size of your average runner).
some gps related articles:
now you know how it knows where you are... but what you're really interested in is how fast you're going and how far you've been.
lets say for now it checks your position every 5 seconds, it's easy to work out how far it is between these two positions, divide this by time and we have speed (m/s km/h mph whatever) which is directly related to the pace us runners like to use (min/mile min/km). similarly it's easy to add the distance each time step and keep a total distance measurement.
so it does speed and distance, just what we wanted... right? well only kind of. if this position reading was 100% accurate it would be great, but of course it's not. it varies with the number of satellites in view at any point in time, and not having a clear view of the sky means it could lose it's position altogether.
When reading the following waffly section please bear in mind that the actual workings of these systems is much less public domain than the basics of gps.
Hence what i say is based on speculation, hopefully technically sound speculation, but speculation nonetheless.
Also i'm only just starting to see feedback from the newer polar system, so reserve the right to change my opinion at any stage!
The first one of these i came across was the fitsense system but i don't think that ever came to the uk.
at first glance as an engineer using acceleration as a main signal is fraut with problems; the signal is inherently noisy and liable to offset and drift problems, to get speed you need to integrate (acceleration x time = speed change done over very small time steps) and to get distance you need to integrate speed (speed x time = distance). this integration means that any error will accumulate with time, making for instance distance readings quite variable.
and that's before you worry about alignment of the accelerometer on the foot and the fact your foot waggles about throughout each stride!
Initial reports from fitsense and then nike systems seemed to reflect this but were very positive about the speed reading and quick response when doing intervals.
Looking at fitsense patents there is alot to suggest that they're not integrating acceleration but using it to detect when the foot contacts and loses contact with the ground. and relating what 'foot loft' time to running or walking speed. it also directly mentions (if i remember rightly) needing to calibrate such systems to the individual runner in both walking and running. this calibration requirement was born out (as far as i'm aware) in the instructions for the early systems.
Assuming such an algorithm is reliable and calibrated correctly it removes the first potential source of error, integrating the acceleration. this would give good fast response to speed changes, but could also br susceptible to drifting when integrating for distance (again reflected in initial feedback).
From a technical point of view the 3 accelerometer system has several advantages, tracking a much fuller motion of the shoe, allowing resolution of a 'true' longitudinal acceleration to integrate to speed and distance.
I'd love to be able to get my hands on the raw data from a footpod and see how much is possible with it!
see.. i told you! it's amazing what they'll put technology up to, theoretically fine & flexible if perhaps expensive (?). no idea how it copes with all the swaying and jiggling round of running though!
here's a comparison review i've been sent (nike vs timex), would be only to glad to put other's on here too (just email me below)
Sorry, work in progress email comments here