November 10, 2008

Calibrating Polar foot pods

This past week there were few discussions on the Polar forums about the calibration of the foot pod with Polar watches. It does not really matter which watch you have S625X, S725X, RS400sd, RS200sd, RS800sd, RS800cx or the new FT60 or FT80. If you use foot pod you will get better results if you calibrate it. With all foot pods in the industry it is true that they are sensitive to your running style, running form, surface you run on, type of the course you run on, etc. Over the years I found that for the best results it is good to keep track of most common calibration factors for combination of surfaces you run on and shoes you run in. I keep a running tally of calibration factors for all pairs of shoes I run in and generally keep at least two calibration factors (CF) - one for running on my treadmill at home and one for running outside on the road. I do not do much of trail running, but if I did I would keep that as a separate CF.

All depends on how much accuracy you strive for. The S1 and S3 foot pods from Polar will give you result within the 5% range of accuracy out of the box. That may be sufficient for some folks, but if you want to be more accurate I suggest to calibrate. All Polar HRMs I used so far let you calibrate the unit by either running pre-determined distance and then correct the lap distance manually or by manually adjusting the CF. I prefer the manual calibration. In order to determine the new CF I run known distance of at least 1 mile. Sometimes I take the measurement multiple times to get better feel for the accuracy. I record each calibration run in a spreadsheet and look at the new CF from the run (or multiple runs). In case I have multiple runs I may take average of the CFs or eliminate the obvious error CFs and take average of the CFs that are quite close. Sometimes you may hit the lap button too early or too late and that may mess-up the CF calculation.

Actually I have a mile long distance measured from the corner of my street to another corner and I use that for all my outside calibration runs. That makes it easy to repeat the calibration when I need it - even like the day before the race if I want to be 100% sure the calibration is OK.

If you want to be super accurate - which may be important for some track workouts you will want to run mile repeats at constant pace and record the laps at different speeds - like 8 minute mile, 7:30, 7:00, 6:30 and 6:00 or faster. Your CF may be slightly different for different speeds, but this is too anal even for me. I usually run with just the CF for combination of shoes and surface. Most of my calibration is done at pace between 8 minute mile and 7 minute mile and they work quite well even for running intervals at 6 min mile. Especially the S3 foot pod is less susceptible to pace changes and accelerations / decelerations that the S1 does not handle as well. The S1 may not give you 100% great distance in interval run - it is best to calibrate it well for the interval part of the workout and just ignore the pace / distance recorded in the recovery sections of the workout.

If you are calibrating on treadmill it is best to use the exactly the same treadmill every time. If you go to the gym you may need to repeat this for few machines as they tend to be quite abused and each may produce different results. You also need to understand that all you do in this process is that you are calibrating your watch to show the same numbers as the treadmill - no guarantee that the treadmill itself is accurate. They quite often are not. If you run for a week in a hotel - don't even bother to calibrate. It is pretty much waste of your workout time. The calibration itself on the treadmill is easy - just hit the lap button at some nice round number, keep running until you ran 1 mile (or longer) and then plug-in the number to the spreadsheet and you know the new CF.

I use the spreadsheet I was referring to in the text above - you can download it from my website. It is very simple and all it does is that it takes your input of the actual distance, old CF and watch displayed distance. I then tells you the new CF and % accuracy of the previous CF. I use it to fine tune my CF over time. If you do this over time you may achieve quite good accuracy. I ran 5 mi race last December in which I had recorded 5.03 miles over quite varied terrain - hills and flats on the road. Now we can discuss whether it was the CF fine tuning or just dumb luck. I'll let you judge it.

Remember that the CF is quite important if you use the software like WKO+ to calculate your intensity factor and TSS as for running. The software uses normalized graded pace to determine how hard the workout was - if your calibration is off you may be working at lower or higher intensity than what the software tells you.

OK so you have determined new CF - what do you do with it? Look at your manual and follow the instructions to change the manual calibration factor. Most Polar HRMs support only one CF at the time so you will need to remember to modify it for each workout. If you have the new RS800CX it allows you to maintain up to 3 pairs of shoes each with different CF. I currently use two pairs of shoes for my running - one is calibration for running outside and the other one for running inside on the treadmill at my house. In other cases like the RS800sd and RS400sd you can define different sports and assign the CF to the sport. When you then define guided workout remember to set the 'Sport Profile' field to Sport-specific settings which will pick-up the pre-set calibration factor from the PPT5.

I hope this helps people deal with the calibration topics. Let me know if you have any questions.

13 comments:

Chris @ Polar said...

Another great post Jan!


We appreciate you taking the time and having such a passion for Polar products.

Feel free to give us a shout if you are ever in NY.

Jen's Journey said...

Thanks for posting this. I have a Polar RS 400, and I am getting ready to get the foot pod once we go back to the US - we live in Russia and I can not find it here.

I do have a question for you. Out of all the Polar HRMs which do you feel is best for using while training for a triathlon?

Thank you -
jen

kxux said...

Jen, out of all the Polar HRMs the best one for triathlon training is RS800CX and if you want to be really serious about the bike training you should get also a power meter. I use the CS600 with power and it is great on the road. It has some issues on the trainer especially in big gear. I did some testing on my coaches ComputTrainer and while the CompuTrainer was showing close to 300W in big gear the CS600 was at around 260W. This happens only on the trainer and only in big gear. In easy gear (small ring in the front) the readings were within 1-3% error range. I will need to ask other CS600 users about the error they are seeing.

Mark said...

V helpful blog, as I have found other sources (Polar websites, local retailers) very confusing. For a triathlete wanting bike speed/cadence and running speed/distance/stride cadence, would you recommend the RS800 or s725x. And if the RS800, would you recommend going with the G3 sensor plus bike cadence or the footpod/bike speed/bike cadence sensors? Doing onroad stuff, I don't think the G3 would add much for me. Many thanks.

kxux said...

Mark,

for triathlete training for on-road tri I would suggest RS800CX with foot pod, bike pod and cadence pod. G3 optional if you want to capture location - see my other posts for the Google Maps that this captures. I use it for running races, but would not waste time with G3 in T1/T2.

If you require HR under water and can go without guided workouts (well let's say can go with limited ability guided workouts with warm-up, interval 1/2 and cool down) you can consider the S725X with S1 foot pod, bike speed and bike cadence. Consider that the S1 has to be switched on in T2 which can add few seconds to your transition time (some people do not care I do so I prefer foot pod that works without any need to switch on like the S3).

Mark said...

Thanks, that advice makes sense. I don't feel that mapping my workouts will add anything to my training. Even though I could add a GPS later I feel that I should decide now as don't want to end up with a redundant bike sensor and footpod if a GPS would do all that anyway.
It's a shame that Polar didn't add the power functionality to the RS800CX (now THAT would be a great unit, being able to add the full range of sensors to the one watch). It seems total overload to get a CS600 w Power plus a RS800CX, and a pain to have to download from two separate HRM units for bike and run). Is there any better solution? I guess the key question here is whether measuring power is an essential - now that is a whole other topic.....

kxux said...

I use the RS800cx, CS600 w power setup and the PPT5 handles both quite well. They also work very well with WKO+ which is also very useful if you train with power. I have very similar view - Polar could have developed a smashing unit if they included power in the CX line.

Using two (actually in my case three) HRMs for three disciplines training may seem difficult, but it is pretty easy and straightforward. The planned workouts go from PPT5 to both units and I just take the CS600 unit when I train on the bike. When I run I carry RS800cx on my wrist anyways and the S625X is in my pool swim bag. I'm actually using the S625X for swimming for two main reasons - the watch gets pretty beat in the pool water and secondly the older Polar belt technology captures my HR during the swim training. It is not essential, but it helps me determine the level of stress of the workout.

One unit would be nice for all this, but there seems to be no one watch solution on the market yet with all the bells and whistles. S625X came close, but it was not ideal setup for triathlon if used with power - putting it on the bike in T1 and taking it off in T2 was just too much time lost. I have heard of people that had two S625X units - one sitting on their bike for power and one with them for splits. Which eliminates the need for putting it on and off in transitions.

With the RS800cx you should be able to fly through the T1 and T2 like the wind. I like to see my T1/T2 time under a minute if possible or keep it close to the leader of the race by less than 15 seconds.

As for training with power or not. That is for long discussion. I tend to be in the camp of moderating the effort on the bike by power during the races longer than Olympic distance and training with it on every ride if possible. It really helps give you idea of the type and level of stress from each workout. And if you use WKO+ to monitor your training there is no other way to get the IF and TSS right for your bike training sessions.

But then you can always look for other Power solutions. They are either very expensive (compared to Polar) or have some other disadvantage while still being much more expensive. I've read on discussion boards that PowerTap tends to flood in wet conditions, SRM is just way out of my price range and the only other option I considered was Computrainer, but that is limited to indoors. Although I may get it now that I used it for one session. It is a great piece of hardware for indoor training. One can actually use software to convert GPX file to Computrainer file and then train on the same profile as your target race. And I'm not even talking about the simulated courses they offer. But that took us little away from the pure HRM discussion...

RBand said...

Great Article...I have a few questions since it seems like you are very esperienced. I have been running with a rs400sd for almost a year.

1) Have you noticed a different calibration factor if you are wearing your heart rate monitor? It seems to me like the HR monitor takes some of the "CPU" and I need a lower CF for when I use the HR monitor vs. not using it.

2) How much more accurate does the 800sd seem to be compared to the 400?

I am a marathon runner, and have had the s1 footpod hurt my foot after very long runs/very high mileage weeks. To be clear, I don't notice the s1 when running normally, it is just the extra pressure of the device attached to my shoe when running very long mileage weeks that seems to affect me.

The s3 looks much smaller, and maybe less obtrusive.

kxux said...

RBand:

1) No I have not noticed any difference in calibration of the foot pod with or without HR belt. Neither with S625X nor with the RS800cx. There are other factors that play role, but HR or no HR does not seem to matter for CF.

2) The S3 is more accurate for workouts that involve a lot of rapid speed changes - like track workouts. Otherwise for long steady pace the S1 and S3 can be calibrated very well and provide accuracy of 1-3% at least based on my experience.


"I am a marathon runner, and have had the s1 footpod hurt my foot after very long runs/very high mileage weeks. To be clear, I don't notice the s1 when running normally, it is just the extra pressure of the device attached to my shoe when running very long mileage weeks that seems to affect me."

How tight are your laces? It sounds like you may need some additional padding between the foot pod fork and the shoe. I never had issues with this. Perhaps try moving the foot pod higher or lower on your shoe and see if that helps. The S3 is definitely much smaller and lighter so this may go away if you upgrade to RS800 line and S3. Alternative is always to go with G3, but then the immediate pacing info is not going to be as accurate as with foot pod. Especially in areas with weak GPS signal or when you go little slower.

Sorry about taking time to respond. Last week was little crazy with the work and prep for the race. Recovery days make it easier for me to maintain this blog :-).

Mark said...

Why is it that I always seem to be calibrating my S3 footpod down, ie it's always telling me I've run further than the actual distance? Unfortunately I never seem to get the opposite, that I have run shorter than actual distance. As shoes wear out or you get new shoes, does that change the calibration. For example, I just put the S3 on some new shoes - the previous calibration was 0.984 then today I ran 30km (watch told me 31.4km) so I had to crank the calibration down to 0.94 (per your very helpful spreadsheet). Does your calibration go up and down, or only down like mine? Thanks

kxux said...

Mark: I do have three calibration factors at this time - all three either spot on 1.000 or little below that like 0.984 which is for outdoor running. The 1.000 factor is for running outside with Zoot shoes that are very minimal - e.g. not much padding and they are great (at least for me) in anything up to 10k or 8 miles. For shoes with more padding like Newtons (neutral) I'm little below the 1.000. I do vary my calibration a little, but once I'm within the 3% error I do not bother re-calibrating as often. I must say that I did not pay much attention to how the calibration factor changes with the shoes wear. I'll check it on my next pair - although I may not use it for running as my first attempt to run in them lasted about 800 meters before my calf locked-up. I'm afraid these shoes as MC and I'm neutral - so much for shopping for shoes on-line...

Greg said...

One thing I've found, especially when changing the pod between pairs of shoes, is that you need to be absolutely sure that it is attached tightly under the laces. Uuse the the exactly same lace holes as reference. If the pod moves around at all you'll be way out of calibration. Anecdotally it seems that if it is loose then it will over predict your distance.

Anonymous said...

Since more than a year I use velcro to attach my S3. Also, I have several S3 supports ("fork") - one for each of my running shoes. So, I always have the same S3 position.
More than once, on 10 km I was off by 20-30 m, depending on the moment I stopped my watch after the finish line. Really a great accuracy!