Which Leg is Stronger? Using a Dual-Sided Power Meter to Investigate

This article about analyzing asymmetry between left and right legs with a dual-sided power meter is written by registered Physiotherapist, cycling coach and Cat 2 bike racer Jem Arnold

The author, on the right, chasing. Photo by Tammy Brimner

Using 4iiii Precision Pro to Investigate Left/Right Power Asymmetry

I’ve been using a 4iiii Precision Pro dual-sided power meter for the past two seasons. Having a dual-sided power meter has been critical for me, as I have a chronic injury which produces a significant Left/Right power asymmetry.

A single-sided power meter that just doubles left leg power (my weak side) would give me unreliable data, not to mention I might never have realized I had such a severe imbalance at all.

Having a true dual-sided power meter like the 4iiii Precision Pro has allowed me to investigate the asymmetry and take steps to correct the issue through treatment and rehabilitation exercises off the bike.

This article is a summary of a more detailed investigation I published on SpareCycles.blog. The original article was not sponsored or influenced in any way by 4iiii, but I thank them for allowing me to publish this summary for their site.

L/R Power Balance

L/R balance should be 50/50 on average, but a mild asymmetry of +/- 2% either direction is nothing to worry about. If you observe a consistent imbalance greater than 2% that might warrant further investigation.

For me, my left leg begins to drop power as intensity increases past threshold, leaving my right leg to cover the difference. Let’s take a look at a stress test I did the other day that was designed to demonstrate this power imbalance at its worst.

Note, the charts below come from WKO4 Training and Analysis Software, produced by TrainingPeaks.

The first chart is a simple display of power, heart rate, and cadence for the workout.

  • Power is shown in yellow. The dashed yellow line is my Functional Threshold Power (FTP) which gives context for medium vs high-intensity effort.
  • Heart Rate is shown in red. HR rises toward the dashed red line, which is my Lactate Threshold HR (LTHR).
  • Cadence is given in green.
  • The legend along the top shows power, HR, & cadence over the cursor at time 23:42.

This stress test included a ramped warm-up, some work at threshold with various cadences, and some high-intensity intervals. It appears that I hit my power targets and my HR reached threshold. All seems normal so far.

However I could feel my left leg begin to fatigue at some point during the stress test, so I know there must be something going on under the surface.

L/R Power Balance

Let’s look closer. By using the L/R balance reported by 4iiii Precision Pro power meter, we can split power into left and right legs independently.

  • Total power for the same point (23:42) is shown in the legend for reference.
  • Single-leg FTP (FTP / 2) is the yellow dashed line on the chart.
  • Left leg power is the red line.
  • Right leg power in the blue line. (These numbers are what each leg is doing independently)
  • L/R Balance is shown in white, smoothed to easily visualize and compared to 50/50.
  • Average, min, and max L/R Balance for the workout are also given.

It’s very easy to see where my L/R power balance begins to drift away from 50/50 as the intensity rises. This corresponds very closely to the increased left leg fatigue I felt during the workout.

For example, at the cursor (23:42) my power is 449 W. This is what I would see on my power meter at this moment. However my left leg was contributing only 207 W, while my right leg was overcompensating at 242 W. By the end of the stress test, my left leg power (red line) and right leg power (blue line) were mismatched by as much as 50 W!

So my right leg had to drag along my left leg as I worked above threshold.

The author, on the left. Photo by David Gillam

Further Investigating L/R Asymmetry

4iiii Precision Pro collects even more advanced pedaling metrics, allowing even greater insight into pedaling technique and L/R symmetry. Next time I’ll discuss how Torque Effectiveness and Pedal Smoothness can be used to investigate how each leg generates power through the pedal stroke. For a more in-depth version of the analysis in this article, head over to SpareCycles.blog.

Bonus video: Go onboard with Jem in the final lap of the Tour de Delta UCI road race in 2017, complete with power data for the final sprint: 

Why a Powermeter? 5 Reasons Why Power Measurement is Useful to Cyclists

Powermeters have become a lot more popular in recent years. What was once the domain of pros only, power measurement is now prevalent in all forms of racing and has trickled down into recreational cyclists as well.

Ask your fellow club cyclists where their sudden boost in strength came from, and you’re likely to head them tell you about their powermeter. But we’ll be first to admit—the powermeter itself doesn’t make you any faster. It’s all in how you use it. So why buy a powermeter?

1. Track and Measure your Power So You Don’t Tire Too Quickly

Whether it’s a race, a fondo or even just your weekly club ride, it can be easy to ride too hard, and end of having to limp home after the dreaded “bonk.” With a powermeter, you can be a lot more careful about the energy you expend, and have a better idea of what you’re capable of. Heart rate measurements work for this too, but with heart rate, there is lag. If you put in several sprints, for example, your heart rate might not even ride until after the effort is over. Watts are watts though, and sprints take a lot out of your legs. Only a powermeter can give you an accurate measure of just how much energy your ride is costing you.

On any given ride, factors like headwinds, whether you’re drafting other cyclists, or the grade will affect your time and your heart rate. Only a powermeter will give you an accurate measure of the actual effort.

2. Follow Prescribed Workouts from Coaches or Training Programs

Ask a coach for training tips, or look at any training program for cyclist online, they will recommend some form of interval training. Intervals are efforts that are harder than your average pace, for a shorter durations of time, and are designed to build strength. How they often work is by taking a baseline measurement (FTP is a popular place to start) and then following a prescribed workout with intervals and recovery based on a percentage of your FTP. For example, you might do several 4 minute efforts at 120% of your FTP, with several minutes of recovery in between.

3. Make the Most of Your Training Time

With interval training, you can maximize your training time on the bike. Before the advent of powermeters, the prevailing wisdom was to log log miles in the saddle; a process that would take most of a weekend. If the ride was outdoors, this could mean many long hours in miserable, cold weather in preparation for a season of riding or racing.

With powermeters and interval training, racers and recreational riders alike can maximize their training time and build more strength with less time spent.

4. Recovery Properly, With True Recovery Rides

You can only train as hard as you can recover. If you’re not taking time for recovery, you can easily fall victim to overtraining, and do more hard than good to your level of fitness and strength. Training programs as prescribed by coaches often have a healthy dose of recovery rides scheduled into them. These are rides when you have to have the discipline to not put out too much power, so your legs can remain active without tiring you out further.

That kind of discipline can hard to maintain, especially when you’re being passed by other riders on your usual loop. With a powermeter though, you can watch your watts, and feel secure in the knowledge that you are staying within the prescribed power zone and getting a good recovery.

5. Sharing and Analyzing Your Data After Rides and Workouts

Cyclists come in all shapes, sizes, and disciplines, but that doesn’t stop us from comparing all of our power numbers. With your powermeter connected to a program like to 4iiii app, Strava, TrainingPeaks or the like, you can brag to your friends about your FTP, your maximum sprint power, or measure your performance against your numbers historically. Measuring your efforts will give you a better idea of any gains you experience.

You can do periodic FTP tests, or even measure your power output on your favourite segment out on the road. Having a historical record will give you a better idea of your progress, and before long you’ll be the one answering questions from your club mates about where all your newfound strength came from.

How cycling continues to change my life

A 4iiii employee profile with Jerold Hoshowatiuk, Customer Experience Specialist


This is the story of how Jerold from our customer service team became a tester in our labs and rediscovered himself as an athlete. This year, Jerold has embraced training with power, has participated in a number of events, lost over 120 lbs, and has set goals for where he’d like to take his training. We’ll follow his progress in a series of updates this year.

My love of riding

I love how riding makes me feel. I know it sounds cliche but when I ride, I forget about everything. I don’t care about anything except feeling free. When I ride it’s just me, pushing me, wanting to be a better version of me.

My history of training with power

I started training with power when I was asked to be a test rider at 4iiii Innovations. Right away it turned into a competition: we kept a running tally of the maximum power that we would hit during our sprints. I peaked at over 1600W and was proud to see my name at the top of the list for a long time.

Testing was fun and hard. We spent a lot of hours in a room with some of the smartest people I have ever met. They would push us and the equipment to the edge of what we were capable of. One time, I was on the fourth or fifth interval, and I cracked—I was face deep in a garbage pail. One of the techs helped me get back on my feet. Another, who was in charge of data collection, gave me a snack bar and a bottle of water and asked me to go again… I wasn’t finished. Believe it or not, this is a fond memory and I’m sure a lot of other athletes can relate.

The author, before and after

The Tour of Sufferlandria, a true test of strength and resolve

This winter I took part on the Tour of Sufferlandria, a virtual tour consisting of trainer workouts, hosted by The Sufferfest. It was fun and not fun at the same time. Riding the stages were really tough but I worked through it. Each day, I had it planned to ride and I did. Some were harder than others, but that doesn’t mean that any of the stages were easy. There is a Facebook group for ToS and people from all around the world encouraging each other. We came up with the idea of making a video of myself and Martin, another 4iiii staffer, riding one of the stages outside, in -30C weather here in Cochrane. That was a lot of fun for us, but recovering was harder than I bargained for. I don’t recommend it.

A return to cycling, and training with focus

2018 is kind of a return to cycling of sorts for me. I have always been a guy that struggled with weight. I love to eat! There was a time in my life, before working at 4iiii, that things just went off the edge and I managed to pack on enough that I tipped the scales at over 410 lbs! I was 38 and I was on the couch watching TV with all my snacks when I saw a show called “too fat for 40.” That was my ‘aha’ moment.

One major catalyst for getting back on track was meeting my buddy Dan. He owns a boutique bike shop in Cochrane and when I met him I was huge. We worked together to build a bike that would hold up under my weight. I pushed myself, I cried, and I tried, but I kept going on. My wife was amazing through this ordeal.

Fast forward to December 2017. My friends from work introduced me to The Sufferfest. I rode a few times and I started watching my food. My first 4DP Fitness test was brutal. The thing that was amazing was that my legs just seemed to remember how to do this. It fired a spark. I’m down over 120 lbs from my peak weight, and my power on the bike is returning in a big way. I feel better, I look better, and most of all I acknowledge that there’s still work to do.

New life goals, events, and family fun

Some of my goals this year involve racing BMX again and being able to be on the track with my boys. Being on the track with them is amazing.

I am going to ride in my third MS Bike Tour this year as well. It’s a great cause and I look forward to riding with my team, The MS Spokes People. As the year progresses, I’ll add more events to my calendar and train to achieve goals.

The importance of training with power

Training with power is key. I have always been a strong sprinter, but I don’t always have the gas to hold the power that long. By having a power meter, I am able to measure my efforts, building them up to last longer. To train against that weakness. Using programs like Sufferfest and my PRECISION PRO powermeter enables me to train smarter—to get myself to where I am able to produce more power and hold it longer.

An enduring love for the bike

These days I feel really good being part of the bike industry. Working with so many great riders and teams, teaching my kids about how fun and awesome bike racing is—that’s what makes me want to be better.

At this stage of my life, I like being able to throw my leg over the top tube, clip in and smash the watts with the younger kids on lunch rides. I may not be as fast as I once was, but I can feel myself returning to that former glory and I’m not done yet. Not by a long shot.

We’ll continue to follow Jerold’s progress and provide updates on his training and events. Have a training story you want to share with us? Leave us a comment or email Jerold directly at jerold@4iiii.com to share your story.

A Beginner’s Guide to Zwift

For some cyclists, racing and training outdoors all year round isn’t always feasible. With the kind of weather we get here in Canada for example, that means a lot of indoor training, and for many, it means riding in the virtual world of Zwift.

Zwift — The Basics

What is Zwift? It’s a 3D virtual world, connected to the act of training on a bicycle (or a treadmill). It’s like a video game where you actually have to do the pedaling to advance.

Taking the components of “gamification” and adding them to the act of sweating on a bike trainer, Zwift has grown in popularity all over the world.

Athletes have several options. They can explore the various online “worlds” within the game: there’s a London course, the 2016 World Championship course from Richmond Virginia, and their own world called Watopia, with volcanoes, underwater tunnels and all manners of “scenery” to keep you in the spirit of riding.

As a rider, you can also choose whether to just ride, to follow one of Zwift’s training programs (like the 4-Week FTP builder), or you can make or upload your own custom workouts. Whatever option you choose, your mileage earns you points toward in-game upgrades like kit designs and fancy wheels and frames for your virtual bike. And whatever you choose, Zwift guides you through a ride, encouraging you to work harder as you ride among other virtual riders from all over the globe.

They also have group rides and racing, which is where things get interesting, and where an entire sub-culture of road cycling has emerged. The key metric in Zwift is “watts per kilogram,” so riders are expected to enter their weight honestly.

How to Get Started

To ride in Zwift, you need some means of connecting your bike to a device like a laptop, mobile phone, smart tv, or Apple TV. The app is available for download to a computer, or on a mobile device.

For the truly dedicated, this means buying an expensive smart trainer, one that can respond to the game mechanics. For example, to ratchet up the resistance on a climb. But the minimum requirement is a means of reading the data from heart rate monitors, powermeters and/or speed and cadence sensors. A small Ant+ USB dongle does the trick, and they cost about $40-50. If you’re using a 4iiii powermeter and Viiiiva heart rate monitor, your device may read the Bluetooth signal as well; no dongle necessary.

What Riders Think

Janna Gillick on the road

“Over the course of a week, I make use of all the different types of Zwift rides in order to meet my training goals for the week,” says Zwift Canadian Champion (yes, Zwift has National Championships) and Women’s BC Premier Series Champion Janna Glick. “If I have a long ride, I’ll either pick a Fondo event or choose a long route to ride in its entirety. For workouts with specific power targets, I’ll add my own workout into their system.”

Her advice to new Zwifters? Janna, who races with Glotman Simpson Cycling, says: “Run what you brung! I’m usually on rollers with my iPad mini on a fold-up music stand that I found by the side of the road at the end of a ride. I’ve got good, consistent data coming from my powermeter and heart rate monitor and that’s all you really need.”

Stuart Lynne is a race Commissaire, racer, and bike race organizer with Escape Velocity Cycling Club in Vancouver. When he’s not doing the timing at a Spring Series, crit or cyclocross race, he’s training or racing on Zwift.

“I find doing structured workouts on the trainer and IRL boring and hard to finish,” says Lynne. “Doing the equivalent with Zwift Racing works for me. Effectively every race is an unstructured workout with a mix of high intensity and VO2Max Intervals, and recovery at tempo/threshold. Zwift trainer workouts are also very time effective—I don’t have to drive or ride somewhere to do a workout or race. More quality time on the bike, less wasted with prep and overhead.”

If you haven’t already, try Zwift for yourself.

Need a powermeter and a heart rate monitor with a Bluetooth connection to use as a bridge to Zwift? Check out our shop to learn more about our PRECISION and Podiiiium models.

What is FTP? (Functional Threshold Power) and how to take your first test

What is FTP? Your FTP is the highest power that you can maintain in a quasi-steady state, without fatiguing, for approximately one hour. Both FTP and the commonly used 20-minute testing protocol were developed by Dr. Andrew Coggan, co-author of Training and Racing with a Power Meter.

FTP – Why it Matters

When we explain the value of powermeters in cycling training, we sometimes use a simple analogy: You wouldn’t load up a barbell at the gym and just start doing squats. You’d take a careful approach to how much weight you could handle, for how many repetitions, to achieve a desired kind of growth. In cycling, powermeters give us the ability to measure our effort and follow prescribed workouts to maximize the effects of our time spent training.


4iiii’s own Alex Stieda, the first North American ever to wear the yellow jersey at the Tour de France, explains the benefits of training with power.

Your FTP serves as a baseline measurement of your strength and informs your workouts. For example, a coach or training program might suggest a series of intervals of three minutes at 120% of your FTP, followed by three-minute rest sets at 50% of your FTP. Without FTP as a measurement of your current strength, you’re blindly “loading the bar” with no idea of the implications it will have to your training.

But what about your local hill climb or time trial? Isn’t that a good enough indicator of your strength? Simply put: no. Any number of factors can affect your time: headwinds, tire pressure, clothing, humidity, equipment…the beauty of power measurement is that it doesn’t lie. If you were 20 seconds slower on your local time trial because you were battling a headwind, your power numbers will tell the story accurately.

Setting the Standard: The 20-minute FTP Test

To determine your FTP, you need a powermeter and some form of training program or measuring device. Most training programs and head units will automatically note any increases in your 20-minute power numbers, and some will even guide you through the testing protocol. You can take the test outdoors or indoors, as long as you have enough road to ride an uninterrupted 20-minute all-out effort. Below, we’ve highlighted a protocol for an FTP test.

To take a self-directed test, follow Dr. Coggan’s steps:

  • 20 minutes easy warm-up
  • 3 x 1-minute hard effort wind- ups with a minute rest between (100 RPM pedal cadence)
  • 5 minutes rest, pedaling easy
  • 5 minutes all-out (hard at first, but make sure you can complete the 5 min)
  • 10 minutes pedaling easy
  • 20-minute time trial effort- This is the actual test. Pace yourself so you can last the full 20 min, maintaining the highest level of average power that you can
  • 10 to 15-minute cooldown

At the end of the 20-minute test, take your average power number and 95% of that figure is your FTP.

20min power X .95 = FTP

Cyclists love to compare FTP figures at the cafe, but how your numbers compare to that of another rider isn’t the whole picture. Your FTP is just for you. The highest FTP won’t always win a race, for example. The rider with highest power-to-weight ratio will have the easiest time on a hill climb — but even in a flat time trial, factors like drag and pacing will help determine the winning time and not a rider’s FTP. Still, it is a measuring stick and thus comparisons will be always be made.

Training to Increase FTP

Once you’ve established your FTP, it’s time to choose a training program, if your desire is to improve FTP over time. Training programs like Zwift and The Sufferfest have prescribed training programs, or you can enlist the help of a coach to guide you through a program that is purpose-built for your goals. In future articles, we will go into greater detail about these training programs, including advice from coaches and athletes who use these workouts successfully.

With regular training, it’s a good idea to retest your FTP every six weeks or so. That way, if you’ve seen some gains in your numbers, your prescribed workouts will change to your new number as well and you can continue to get stronger.