Power output agreement between Polar and SRM

Power meters are very expensive and for many people it is a big decision to buy one. Polar offers one of the cheapest power measurement systems on the market, so it was interesting for me to see how it performed compared to the much more expensive SRM crank system. SRM crank system uses torque and angular velocity to measure power output, while Polar uses the vibration and speed of the chain to calculate power output. 

British scientists made a study with 12 trained men who made 12 all-out efforts separated with short recovery periods. This study setup is very demanding for the power meters, since the stress on the bike is very high. The purpose of the study was to compare the agreement between these two different systems. The results were published in Journal of Sports Science, August 2006.

Polar underestimates power output

The main finding was that Polar underestimated the power output and did not agree with the power output from SRM. One of the great problems with the Polar was that it was greatly influenced by chain vibration and sampling rates.

These findings agree with the experiences I have been told about Polar. If you ride on cobble stones it is not reliable at all and it is necessary to change chain often. It is obvious that you can not get a precise system like SRM and only pay 1/10. For me Polar offers a cheap entry level for riders who would like to explore power outputs without spending all their money. I believe that you can make analyzes of races and get a good feeling of how races are. But I am not sure, that Polar is good enough for pacing in intervals. Maybe I should go try one?!

Reference:
Hurst HT, Atkins S.  Agreement between polar and SRM mobile ergometer systems during laboratory-based high-intensity, intermittent cycling activity.
J Sports Sci. 2006 Aug;24(8):863-8.