Finding the Balance, An Introduction to Left/Right Power Data
By Hunter Allen, originally printed in Road Magazine, October 2015
Capturing each leg's contribution to the total power is a relatively new concept in the world of power analysis. Devices that capture left and right leg data have been available for three years, but no one has really been able to discern whether or not this new information is useful or meaningful. Like the arrival of power data in the early 2000s, we had no real way to analyze or understand how this knowledge benefited performance. For the past two years, I have studied left/right metrics to create meaningful analysis and coaching advice from the data. During this time, I worked on the core development team for Training Peaks WKO4 software with Dr. Andrew R. Coggan, Tim Cusick and Kevin Williams. In the process, we created some new analytics, along with unique charts and graphs to better understand each leg's contribution to total power. What exactly can a left/right power meter tell riders? Is a left/right power meter right for all riders? The metrics offer a wealth of new information for the data driven cyclist, and this article will be the first of many describing how the data can turn into results.
Left/right power analysis follows the same basic principles of traditional power training. Baseline testing is key with the new meters, and it is just as critical as determining functional threshold power (FTP) and power profile values. The first test requires a commitment of four consecutive days, so plan that week in advance as to not throw off your overall training plan. Ideally, perform this first test on a five-minute hill. However, you can also test on a flat road, preferably into a head wind that provides more resistance to push against. The hill test alternates between standing and sitting while climbing. The flat test focuses on the left and right legs. For the remainder of the article, I will describe the hill-climbing test, with alternative adaptations for a flat test. Both methods will reveal how to create power from each leg, and how that contributes to the total power output. Before you can embark on your new journey, you'll need to understand some of the new analytics in the Training Peaks WKO4 software, so you'll be able to interpret this data.
Understanding Gross Power Released and Absorbed
Gross power released and gross power absorbed are critical to understanding left/right power metrics. During pedaling, each leg releases power and absorbs power throughout each pedal revolution. On the down stroke, the left leg releases power while the opposing right leg absorbs some of this energy. The opposing leg can absorb significantly more power than expected, and this can negatively impact the total power. The opposing leg's job is really just to get out of the way on the upstroke while preparing for the down stroke. When the opposing leg gets out of the way, you produce more releasing power. This ultimately moves the bike forward faster. When the left leg is on the down stroke and the right leg opposes it on the upstroke, we call this the left power phase. When the leg roles reverse, we call this right power phase.
Each leg of each rider has a unique pedal stroke. Some of us release more power on the left leg than the right. Other riders punch at the pedal stroke with the right leg, while the left smooths the power over the arc. The same side dependent discrepancies exist during power absorption. Many of us absorb more power with one leg than the other. There are other intricacies occurring during each pedal stroke, but I'll explore those in later articles. For now, let's stick with gross power released (GPR) and gross power absorbed (GPA). They are the most important concepts to understand during initial tests.
Understanding Gross Power Review Charts
Figure 1 (above) displays the Gross Power Review Chart. This is the first chart you will look at in the WKO4 Pedaling Metrics Pack that comes in the WKO4 software. In Figure 1, it's the lower part of the screenshot with the red/blue/green/purple lines in it. The red line is right leg GPR over a given time period. The blue line represents the left GPR. Remember, GPR is the power that you are releasing on each pedal stroke to propel the bicycle forward. The green line is GPA on the right leg and purple line is GPA on the left leg. This represents the power produced that does not move the bicycle forward and GPA is generally resistance or negative power.
Minimizing GPA produces more overall power. This chart shows the big picture of how leg asymmetries can sap energy. In Figure 1, the blue line (left GPR) is generally higher than the red line (right GPR). This indicates that the left leg releases more power than the right leg. Concurrently, the green line at the bottom of the chart generally sits above the purple line. This shows that the right leg absorbs more power than the left leg. This chart shows that while the left leg releases more power, it also absorbs more power. This suggests that the left phase does not contribute more power than the right phase to moving the bike forward. We need a closer look to see if this is indeed the case.
Figure 2 (above) shows a five-minute standing interval test—five minutes at VO2 max while standing the entire time—to demonstrate the relationship between left/right and standing/seated power outputs. This chart is the Mean Max Gross Power Curve. It contains the same data as the Gross Power Review Chart, except plotted on a logarithmic curve. The log curve makes the power absorption and release of each leg more apparent. This chart is especially valuable for reviewing intervals or very hard efforts. Again, reducing the GPA while maximizing the GPR increases total power. Notice, that in Figure 2, the left leg releases more power while both legs absorb the same amount. The GPA's are nearly the same but the GPR on the left leg is almost always 10-15 watts higher than the right leg. This indicates that during standing efforts the left leg contributes more to total power than the right leg.
Both the Mean Max Gross Power Curve and the Gross Power Review Chart are critical along with the pedaling reports to better analyze your left/right power.
Leg Asymmetry Pedaling Tests
With this better understanding of left/right concepts and charts, let's dig into the pedaling tests! Each day will be the same essential test (3 x 5 minutes) at VO2 max, but each day you will have a different goal in your pedaling emphasis.
With your cycling power meter, you will complete three, five-minute intervals at your VO2 max power, roughly 113-115 percent of FTP. The first interval involves standing the entire time, and the second requires seated pedaling the entire time. The third interval incorporates both standing and seated pedaling—you will stand when you want to and sit when you want to.
Pedaling Asymmetry Hill Climbing Test
Day 1: Complete the test with no emphasis on either leg. Just climb naturally.
Day 2: Now emphasize the leg that releases less power to see if you can balance out the GPR and GPA.
Day 3: Emphasize the left leg only for all efforts.
Day 4: Emphasize the right leg only for all efforts.
Pedaling Asymmetry Flat Test
Since the flat test does not require standing, perform the first interval with your hands on the hoods and the second with hands in the drops. For the third interval, place the hands wherever you like. Complete the four days exactly as the hill-climbing test, just stay seated the entire time and modifying the hand position. Those aero bars can substitute an aero tuck for one of the other hand positions to determine pedaling asymmetries.
Pedaling Asymmetry Testing Protocol
Pick a route that can be repeated each day. It must be the same route, and you must start all efforts at the same locations. Try to make each of the four testing days as identical as possible.
Warm-up: Ride for 20 minutes toward the testing area. If you have to ride a little longer, that's fine, but maintain a nice endurance pace (56-75 percent of FTP).
Main Set: After warm-up, do 5 x 1 minute fast pedals, getting your cadence to 110 to 120 rpm. Hold it there for one minute. Recover for one minute at 80 rpm. Proper warm up is the goal here and be sure to preserve muscle glycogen for the intervals. Once at the testing area, get psyched and do 3 x 5 minute VO2 Max efforts. During the climbing test, the first interval is standing, for the entire five minutes. The second interval is seated. The third is sitting or standing depending on preference. For the flat test, the first interval is on the hoods, and the second is in the drops. The third is whatever position you prefer throughout the interval. Rest for 5 to 8 minutes between each interval. After completing the third interval, ride home at tempo pace (76-90 percent of FTP).
Once home, upload your power meter computer head unit to the WKO4 software to look for key data and to make notes in post-ride descriptions. Record keeping is key since you want to compare notes between each day of testing. First, determine if one leg releases more power than the other when standing compared to sitting. If there is a difference in the GPR between the left and right, also check for a difference between GPA when seated and standing. For example, Figure 3 (below) reveals that the rider releases 10 watts more power with the right leg while seated compared to the left leg. This is a significant discrepancy! Concurrently, the GPA of the left leg is 5 watts higher.
From this data, we can calculate the net power released. For the left phase, we take GPR Left (165 watts) – GPA Right (12 watts) = Net Power Released Left = 153 watts. For the right phase, we take GPR Right (175 watts) – GPA left (17) = Net Power Released Right = 158 watts. In this case, the right leg releases more power, but it also must overcome a higher GPA from the left leg. This indicates that if the GPAs were equal, the right phase would be 163 watts, a full 10 watts higher than the left phase.
Addressing Bilateral Leg Discrepancy
What can we do about right/left power discrepancies? In this case, the rider can try two things. First, just using more effort/intention to push harder on the downstroke with the left leg closes the net power release. Additionally, lifting the left leg can seemingly increase the net power released on the left phase. Since the left GPA reduces the net power release of the right phase, subtracting that resistance would make the right phase even higher than the left. Thus, I suggest starting with pushing harder with the left leg while seated and then re-assessing. Fortunately, this is exactly the protocol for the next test!
Figure 4 (below) displays the results from the second day of testing for a seated five-minute effort. In this test, right leg GPR is much higher than the left for natural pedaling, so emphasizing higher force with the left leg will release more power on that side. The left leg indeed got much closer to balancing the left/right GPR, but the GPA of the left leg stayed the same, thereby effectively reducing the net power release (NPR) on the right side. This second test indicates that better symmetry on both the right and left legs while seated results from pushing and pulling to even out the power production. The following two days of testing prove this theory correct.
The third day of testing emphasizes the left leg for all efforts. This replicates the observations from day two, and it also shows the impact on the GPR/GPA of the left leg for both standing and seated pedaling. With this athlete, the left leg becomes more balanced in GPR, but the GPA of the left leg remains about 4 watts higher than the right. However, this is very close. In Figure 5 (below), the GPA's are much closer for the entire five-minute interval. This most likely indicates more pulling up on the left leg.
The fourth day of testing emphasizes the right leg, and it shows a very high right leg GPR when seated. Therefore, the data does not give the rider any actionable intelligence. However, day four was highly revealing regarding the standing interval. I'll have to save that for another article!
While it may take some time to understand the theory behind left/right power analysis, the four-day testing protocol will help you discover the wealth of new power meter data. The WKO4 software has a built in pedaling chart pack that makes it really easy to understand these charts, so the hard work can be reserved for the intervals! Determining leg-strength imbalance can help riders fine-tune power output. It can also possibly identify a problem with the way you ride your bike (your actual position and form). These issues can be tough to solve without digging into the data with some testing thrown in for good measure. Remember, testing is training and training is testing.
Hunter Allen is the co-developer of the new TrainingPeaks WKO4 software. You can give it a free 14-day trial on www.TrainingPeaks.com/wko4 . Hunter is the co-author of Training and Racing with a Power Meter, and he is the CEO and Founder of the Peaks Coaching Group. He specializes in coaching cyclists with power meters, and he uses the latest in data acquisition devices for this pursuit. He has online training programs available at www.TrainingPeaks.com/hunter and you can contact Hunter directly at www.PeaksCoachingGroup.com.