One of the most common questions I have been getting recently from power meter users is what does this power balance metric mean? Or “I have been seeing a 47 – 53 left to right power balance on my rides, do I need to work on strengthening my left leg?”
Power balance is a relatively new metric to come from power meters and it is meant to show how much power is being produced by either legs. However, how that balance is determined by power meters does vary by the model. Most on the market are just dividing the pedal stroke by what side of the crank is on the drive stroke (i.e. 1 to 6 o’clock position). Note that these don’t differentiate from one leg pushing down versus the other pulling up. They purely report the net power being produced while either side of the crank is in the drive position.
The Garmin Vectors and Rotor 3D power meters are slightly different since they have strain gauges on either side and can give the net power measurement from each side for the entire crank rotation. While this measurement is significantly different than those that measure power only on one side, they are reported as the same metric on ANT+ computers and in post ride analysis software.
One other important factor to keep in mind when looking at power balance measurements is the margin of error. While there isn’t any reported margin of errors for power balance measurements, we can assume that they would be close to the 2.5% reported by most power meter manufacturers for power measurements. There is also potential for more error with the Vector and Rotor power meters due to having strain gauges on both sides. The vectors have also been shown to be very sensitive to installation torque and even a slight difference in installation may cause a large variance in power and balance measurements. It is also worth noting that a typical athlete’s left-right balance will not be static and it will vary with power outputs, cadence, and fatigue. I typically see athletes approach a 50-50 power balance as they approach threshold or higher intensities, and become more imbalanced at lower intensities.
The big question is should we strive for symmetry? Science is pretty clear on this, we are asymmetrical creatures. Pedalling asymmetrically is normal and not a limiter in performance. Instead of spending time working on using both legs equally, just focus on performance.
So does power balance have any value? I think it might have some application in bike fitting and diagnosing underlying neurological or circulatory problems, but most of those can be determined without seeing the numbers on your computer. There is potential to get a lot of useful information from systems like the Garmin Vectors in the future, (for example are you still pushing down on the pedal during the upstroke) but the current power balance number is not as useful as it is made out to be.