PDA

View Full Version : Help me understand footpod calibration



TRYING2GOFAST
01-11-2012, 06:45 PM
If my footpod calibration factor is 820 with one pair of shoes and 859 with another pair, does that mean I'm lighter on my feet (less impact per distance traveled) in the 820 pair, or the other one? I think it's the former?

BBECKER39
01-12-2012, 09:30 AM
I'm not an expert here, but I believe your foot pod calibration factors will have much more to do with your stride and gait than the weight of your foot impact. The ANT+ sensors use accelerometers to track your stride, and are not simply counting foot impacts.

TRYING2GOFAST
01-12-2012, 04:49 PM
The shoes aren't so different that I'd expect my gait to be so radically different. Heel/toe drop, support level, pretty similar. I didn't have any particular muscle soreness going to the newer pair (the 820s) that would suggest I'm running that much differently. I'm sure there's something I can learn from this if I only knew what it was. Hope someone with more knowledge of the footpods can weigh in.

KARLW2000
01-13-2012, 10:17 AM
I'm not an expert here, but I believe your foot pod calibration factors will have much more to do with your stride and gait than the weight of your foot impact. The ANT+ sensors use accelerometers to track your stride, and are not simply counting foot impacts.There is no doubt in my mind that shoes change the calibration. I have light racing shoes to heavy trail shoes and several training shoes in between. If I run the same distance (1 loop around my block) at the same pace, I will get significantly difference results. I just bought a new pair of Mizunos - same shoe, newer model. Incredibly, these new shoes felt a lot heavier and I got 3% longer distance on the exact same run/speed as I did the day before.

BBECKER39
01-16-2012, 07:50 AM
Sorry, I overlooked the most obvious point in the initial question. Changing shoes can cause a shift in the calibration because of a shift in the pod's orientation on the shoe in addition to any changes in your stride due to the shoe change. I still can't give any detailed insight into what is actually going on, but even just moving the pod to a different location on the same shoe can potentially change the calibration.

BBECKER39
01-16-2012, 04:17 PM
I got a more detailed explanation from a more qualified source. Here are his comments:



For most users, the Garmin footpod is very accurate without calibration in either mounting position, on either foot and at any speed. Calibration is provided as an option for some users who experience a below typical speed/distance accuracy.

Footpod speed/distance error is a consequence of each person's gait being as unique as their handwriting. As a matter of fact, even your two legs don't have exactly the same gait. In some ways, footpod speed/distance algorithms are similar to optical character recognition algorithms - for each stride, the footpod "recognizes" the appropriate stride length, in spite of the inherent individual differences in our gait.

Footpod mounting position changes the gait acceleration signature, as observed by the footpod. Consequently, the "recognized" stride distance may be slightly different. It is thus recommended that footpod calibration be performed in the mounting position where the footpod is going to be subsequently used. Furthermore, for best calibrated accuracy, the footpod should not be switched between left and right feet after calibration.

Gait also changes with speed, and for some users that change is quite appreciable. Unlike simple pedometers, the Garmin footpod analyzes each stride individually, and calculates stride length for each stride - when stride length changes, the foot pod notices. However, as speed changes, an appreciable change in other gait parameters may result in a bias which influences calibration. So, if a person calibrates a unit for a particular speed, it is possible that a change in gait concurrent with speed change reduces the effectiveness of calibration, and consequently, a speed/distance bias is experienced. Thus, to the extent that this is possible, it is recommended that calibration be performed at a "typical" speed.

TRYING2GOFAST
01-19-2012, 06:36 PM
Cool, thanks.

Now all I need is another watch to hold me to a "typical" pace while I'm calibrating my footpod and can't see my pace ... I always wind up at 5K pace even though I want it calibrated better for marathon pace. :o

KARLW2000
01-19-2012, 08:29 PM
Cool, thanks.

Now all I need is another watch to hold me to a "typical" pace while I'm calibrating my footpod and can't see my pace ... I always wind up at 5K pace even though I want it calibrated better for marathon pace. :oJust go to a track and run something like 12 laps or more. If like me, you will speed up. Pick one of the laps you like and use that for your calibration. It will probably be one of the middle laps.