By Hunter Allen, Coach and Power Training Expert at Peaks Coaching Group
There are a lot of power meter choices out there on the market today. You can measure power in a variety of locations on the bike and in some of these locations, you can measure the power output of the left leg only or with both.
I have been involved in the power meter industry now since 1999, when I first bought a PowerTap hub, and over the years I have tested nearly every power meter that's come to the market, including some of which are long gone. In this piece, I will discuss a little bit about my experiences with and the differences between single-leg, dual-leg, and bi-lateral power meters. Before we dig in, let's consider an example from my own life.
Case Study for Consideration
When I tested the first single-sided power meter, it measured closely to my PowerTap, but not exactly the same. I found that the intensity of my riding had a large impact on the measurements.
When I was riding at endurance pace (56-75% of FTP), my right leg was doing more of the work than my left leg, but as I increased my intensity to tempo, sweet spot and FTP, my left leg started to do more and more work! My left leg appears to hold itself in "reserve" when riding easy, and then when I want to ride hard, the left leg responds with more power.
I found that at endurance pace, my left leg was roughly 10% lower in power output, than my right leg, but once I was at FTP intensity, it was 5% higher than my right leg! This trend continued and was especially pronounced when doing hard hill repeats at my anaerobic capacity and Vo2 max as my left leg could produce up to 30% more power than my right leg, which, as you can imagine is significant when you are trying to do focused intervals between 450-500 watts. Which number do you go with? The left leg that is doing 650 watts or the combined power that is saying 500 watts?
Let's consider the options.
Single-Sided Power Meters: Left Only
What is a left-sided power meter? It's a cycling power meter that doubles the power from the left leg to give an estimate of the total power. And it's really an estimate as the power from the right leg could be 10 watts higher, 30 watts lower or 300 watts higher depending on the timing of the measurement and strength discrepancy of the cyclist.
In most cyclists, there is a natural 5% difference between the right and left legs and this is a normal asymmetry. We humans, you see, are not even between our right and left sides. One side is more flexible, one side is stronger, one side is more coordinated, and this effects not only our power output, but the fit on our bike, our cornering ability (can you do right or left turns better?) and even our level of fatigue.
One issue with a single-sided power meter is that within power training structure, you're doing your best to ride within the prescribed power training zones, as these zones are critical to ensuring you are training the energy system that you want to train. If one power meter says you are in anaerobic capacity and then your total power says you are at Vo2 max, then it's difficult to know if you are training the correct system (see example above)!
One of the other problems with a single sided power meter is that it can give you a false sense of confidence or a lack of confidence. Imagine if you completed your "Power Profile" test and your results placed you as a Category 4 racer, but you were a Category 1. Or vice versa!? This wouldn't give you a lot of confidence in the test nor your power numbers.
Dual-Sided Power Meters: Measures Total Contribution of Both Legs
A dual sided power meter gives you the power of not only both legs, but the power you can generate from your entire body. The connection of all the muscles in your back, arms, shoulders, etc. all contribute to your power output and a power meter that measures the total power output is more accurate.
You want to know the total power output, so that you can make the correct decisions about your training and racing. This allows you to train with the confidence that you are in the correct power training zone and training the exact energy system you want to improve. If you want to train your FTP, instead of your Vo2max, then it's important to have accurate power measurement, as its impossible to try and hold your Vo2max power for 20 minutes, and mentally difficult as well! Additionally, in my case study above I wouldn't have been able to identify the ebbs and flows of my right leg's power at various efforts without a dual-sided power meter.
Both the PowerTap C1 chainring and hubs are examples of this kind of bike power meter. By capturing the data at the chainring, the contribution from both right and left sides are combined into a total number.The PowerTap hub captures the power that comes from both legs, is transferred into the drivetrain and then measured at the hub, or closer to what is actually transferred to the tire on the ground.Because of drivetrain loss, the hub is typically about 7 watts lower than measuring before the drivetrain. If you have a dirty or old drivetrain, you could be losing up to 15 watts!
Anytime you measure something, you want to know that you are measuring it correctly and that the numbers are consistently correct. Imagine trying to build a house with two different measuring tapes and one was off by half an inch and another read in centimeters! Your house probably wouldn't turn out so well!
It's critical that your power meter measures both legs and is accurate, so that you can make the best decisions about your training and racing.
Bi-Lateral Power Meters: The Whole Package
A bi-lateral cycling power meter measures each leg's contribution to power output independently and can display each side's power along with the total power or the summation of the two. The total power is always known and at the same time, the contribution that comes from either side. This is of great benefit to the user as it displays how different or similar one leg might be as compared to the other.
The PowerTap P1 pedals are an example of this type of power meter and an exciting advancement in the measurement of power. By isolating the power, we can analyze the Gross Power Released and the Gross Power Absorbed for each leg. The Gross Power Released or GPR is the power released primarily on the downstroke of the pedaling cycle and the "positive" power. The Gross Power Absorbed or GPA is the power that is being absorbed primarily on the upstroke of the pedaling cycle and the "negative" power. Each leg has some resistance on the upstroke, and actually creates enough resistance to be a negative contributor to the total power output.
One critical thing to remember is that the legs oppose each other by 180 degrees, which means that the when the left leg is producing force positively, the right leg is creating negative or absorbent force and therefore the difference between these two is the "Net" power created by the "Left Phase" of the pedal stroke. Obviously, the net from the Right Phase is the difference between the right positive force and left negative force.
Using PowerTap Mobile on your Apple device, you can also easily see these differences and the app nicely displays the correct left and right power while you ride. This is a major advantage for the PowerTap power pedals.
Most importantly, these additional metrics are meaningful because a bi-lateral power meter lets you correctly calculate the left and right phases of the pedal stroke, which will help you to determine if you have difference between the contributions of each leg.
Of course, this is just the tip of the iceberg in learning about your asymmetries in your legs, and I have a more in depth article on this at www.hunterallenpowerblog.com.
Hunter Allen is internationally known as one of the top experts in the field of power meter coaching.He co-authored, "Training and Racing with a Power Meter" with Dr. Andrew R. Coggan and it has been translated into eight languages.
He created and teaches the USA Cycling Power Certification Course for USA Cycling Coaches, along with teaching an online power certification course. He has traveled to over 20 countries teaching the principles of power training to more than 3000 coaches and cyclists.As a coach, he has coached athletes to World Championships, National Championships, Tour De France along with helping local beginners and juniors to excel.
He founded Peaks Coaching Group in 1996 to focus on developing the artful science of efficient power training for which Peaks Coaching Group is still known for today. With over 50 coaches, the Peaks Coaching Group continues to lead in coaching cyclists with power meters. You can follow Hunter on Twitter @hunterpeaks or over at his blog.