So Stryd is in its second generation of running power meters and I thought it would be a good opportunity to explain everything you need to know generally about running with power and also some key elements of Stryd's Power Centre, pacing, training and racing.
We will start with the basics and build from there, if you have a Stryd power meter already then scroll down for the detailed breakdown on elements such as CP, RSS V TSS, RE, IF etc.
If you are new to running with power then we start with what is it.
What is running with power?
Running with power means you can objectively quantify how much work you are doing while you are running by getting an exact power number in watts.
For example, if you run uphill, your pace will slow down, but most likely your power will not. If you have a running power meter, you will be able to see exactly how hard you're working on that uphill by looking at your watts.
How is this different than with a bike power meter?
On a bike, the higher the power number (watts) the better, because if you put out more power, you’re going to go faster. If you have a higher FTP (Functional Threshold Power) and if you’re power to weight ratio is high, that’s fantastic. You are always working on trying to improve these numbers on the bike.
This is not always the case with running. With running, if you increase your numbers, that doesn’t necessarily mean that you’re running faster.
You could be doing an all-out sprint and you’re really pushing your power numbers up, but if those extra watts that you’re pushing aren’t pushing you forward, but instead they are pushing laterally and horizontally, then that’s wasted watts.
When doing an RFTP test or Critical Power (CP) test for running, you could be in a situation where your FTP goes down compared to a previous test, but you still run faster. Your Functional Threshold Pace increases.
It uses accelerometer readings on how you’re quickly turning over, moving through space, elevation changes, etc. Then it computes all of that data into a real-time power number.
You need your weight as input as well for it to accurately calculate your power.
Stryd has those accelerometers in 3 dimensions, as compared to some other power meters that don’t have that 3D power measurement. Note that 2D power meters can be just as good functionally, but it's important not to compare apples to oranges - your power numbers will be different with 2D versus 3D power meters.
What is the difference between running power zones and bike power zones?
Running power zones are higher. The reason for this is because part of that wattage is going to push you in different directions other than forward. So you’re wasting watts which is adding to the total number. Depending on the coach that you’re working with, there’s a lot of different zones for different specific types of pacing.
For running zones, I like to break it up where you have your Zone 1 which is your recovery/walking when you’re doing hard intervals. It is about 74-78%. A big difference in cycling is that you can do 50-60% of your RFTP as your recovery.
If you try to do 50-60% of your RFTP running, you’ll be walking. It’s very difficult to hold your form effectively when you dip under 72-73% RFTP.
Then you have your 78-80% which will be your Zone 2. This is where you will be doing most of your foundation runs in that low intensity zone.
Then 80-82% is your long distance or marathon pacing zone. After you get over 82%, those are more of your specific tempo. This is what most consider as your grey zone.
For tempo, it’s more like 90-95%. Above that when you have 95-105% RFTP while running, this is going to be your 10k pace. Above this you get into your 5k, sprints, and your repetitions.
What is the difference between the TrainingPeaks running with power zones and Stryd’s own Power Center zones?
Stryd’s Power Center zones - the 5-zone based system - are very general and not very specific. When they say Zone 1, it goes all the way up to 80% FTP. Zone 2 is 81-90%.
When people try to run Zone 2 according to power center, it’s at a higher intensity than they should be running. With Jim Vance’s zones in TrainingPeaks, it’s a lot more specific with recovery zones, then your endurance zone, tempo zone, and threshold zone.
Running stress score – Training Peaks
The running stress score that you get from TrainingPeaks based on running with power are too low for high intensity work and too high for low intensity work. Whereas Stryd’s power center has a more polarised running stress score that more accurately corresponds to what you actually have been doing in training.
However the training stress score (TSS) in TrainingPeaks and the running stress score (RSS) in Stryd’s power center have two different formulas. But they are similar because they’re based on intensity and normalised power but the major difference is that RSS is to the power of K, which is a constant, which incorporates the load on the body when you’re running. This is because your running is more aggressive than cycling, you have to accommodate for that within your TSS.
What I do when Stryd’s power center gives me an RSS number after my runs, I take that RSS number, copy and paste it into the TSS place in TrainingPeaks to get a more accurate measure. Its worth noting that when you’re going at lower intensities, that TSS and RSS will have a higher discrepancy between the two numbers. As you get closer to your critical power, those start to become more aligned.
What are the implications of RSS and TSS in training plans, workouts, and pacing?
Because you have this new metric and you’re more able to accurately measure your stress and intensity, you can map your annual training plan (ATP) to be more accurate than if you just used your running paces.
Instead of trying to do paces and distances at specific paces to simulate a stress, you can just switch over and do durations at a specific power and derive a TSS score from that.
In essence, this is what we’ve been doing on bike trainers and turbos. Instead of measuring by distance and wattage, now we’re measuring it by time and wattage. Distance doesn’t really matter as long as you’re holding that specific wattage.
This means that we can take pacing work off the controlled environment of the track and put it on the roads as long as you’re holding the specific power for a specific time.
Pacing with power
It is an issue in triathlon, knowing how hard you should be running off the bike. For a long while coaches have been using rate of perceived exertion (RPE), heart rate, and pace.
This offers a wide variety of problems because when you hop off the bike and your heart rate is to the roof, it could be because you are dehydrated or because of how much caffeine you had on the bike. It could be a whole range of things that lead to an inaccurate heart rate number.
So it’s really hard to pace, especially in Ironman and half Ironman, using heart rate.
It’s the same thing with pace. It depends on how hard you push on the bike and how hilly the course is, you can’t really get an accurate pace running off the bike.
This is where power really comes into play because instead of trying to go for those subjective, fluctuating measures like heart rate and pace, you will have a constant number that you can hold on to. This is where you can get a more accurate pacing measure.
How do you use power in your workouts?
When you’re running fresh, some good workouts using power would be something like hill sprints or hill repetitions where you’re running for 90 seconds all the way up to 2 minutes 30 seconds trying to hold a specific power. The reason why this is good with power is because you don’t have to wait for your heart rate to come up and you don’t have to worry about pacing on hills when you’re using different hills and different grades.
It’s really effective trying to repeat and hold a constant power going uphill. It’s doing between 6-10 repetitions of 1 minute all the way up to 2 minutes 30 seconds running uphill at a good grade.
Also doing threshold or tempo workouts of 3 x 15 minutes at a specific power anywhere between 90-95% is another good workout.
Fast finishes for long runs where you build every 20 minutes starting at 82% FTP building all the way up to 95-98% FTP. Then for the last 5 minutes going all out to see what power you can hold for that last 5 minutes.
Another good one in your build phase of training is over-under runs. Applying the bike workouts where you go over your threshold and under your threshold for a set amount of time is now possible to do in run workouts with a power meter.
Then when you get into your brick workouts, it helps doing race simulations. For example, if you’re focusing on a half Ironman you can maybe do a 3 hour 30 minute ride. This is where you can maybe do a 20-minute build warm-up and then go straight into 4 x 30 minutes at 85% where the 1st and 3rd one is at a lower cadence which really helps you in hilly courses and it’s a good way of fatiguing the legs before the run.
Then, hopping off the bike and doing something like 3 x 10-20 minutes at 85-95% FTP where you do the 1st one at 85%, 2nd at 90%, 3rd at 95%. Then when you get back and look at the data to see if you were able to hold that 95%. If you were able to hold that 95%, then you can most likely gauge from that that you can most likely hold 90% of your FTP for that 13.1 miles. So this is a good assessment of fitness.
Another extra dimension that you can add on to this when analysing your workouts is how effective that 95% was. You can hold 95% of your wattage but is your pace faster? Is your running form lower?
How do you calculate RFTP?
Using a 3-6 test where you do 3 laps all out on the track. Take a 20-minute break. Then do 6 laps all out on the track. Plug that into power center and it generates a critical power pace and an FTP power number. Your FTP or critical power is approximately the power that you can hold for approximately 50-60 minutes.
Another is way is using a 5k or 10k result. I find these really accurate as far as aligning with the 3-6 test using previous results. A high-tech way to do it is to use the power curve in WKO to get an approximation.
Important metrics in running with power
1. Critical power or RFTP
Approximately the power that you can hold for approximately 50-60 minutes. In well-trained triathletes, it’s usually a bit higher than their FTP on the bike.
2. Critical power per weight: W/kg
This is essential in running because if you’re carrying excess weight then you’re going to go slower because you to generate more power compared to somebody who is carrying less weight. Power to weight is a more accurate representation rather than focusing on a pure power number. Because if you’re 200 pounds, it takes a lot more watts to push you forward.
For the elite group, it’s the higher 4’s, low 5’s as far power to weight ratio. Mid 3’s would be your typical competitive, recreational runner. Low 3’s would be the more recreational runner.
3. Running effectiveness
Speed (in m/s) per watt per kilogram. This is how much speed you’re producing for your watts per kilogram.
Elite runners, when running at FTP or CP are typically around 1. This goes down to mid-0.90s for easier runs.
This is how well you can overcome the horizontal retarding force that you encounter as a runner.
If you have a higher running effectiveness, you need to apply less power to overcome the retarding forces.
Running effectiveness is also good to compare to your RPE. When you feel that you really struggled on that last interval, if you look at the RE, you can figure out whether you really did struggle or it was mental fatigue.
4. Efficiency index
This was developed by Jim Vance. In my opinion, it’s not as good as running efficiency because it doesn’t normalize your weight. It’s just looking at how fast you are going for how much watts. This is speed (in m / min) over power. If you stay at a constant weight, EI might be useful, for comparing your own results to each other. You can have a high efficiency index but a low running effectiveness. This might mean that your limitation is that you need to lose some weight.
5. Form power ratio
This is the percentage of your overall power that is not pushing you forward. The founder of Stryd said that it’s your "running in place power".
What you would want is to have a very low form power percentage, because that means that more of your watts are pushing you forward compared to other directions.
This allows you to see what happens to your form over time. Is it getting worse, meaning that more of your watts are pushing you in other directions? Or is it remaining the same or even going down, meaning more of your watts are pushing you forward.
Benchmarks for different kinds of runners:
The form-power ratio not only varies for different types of runners but also for what pace you’re doing. At slower paces, your efficiency is going to be lower, so your form power ratio percentage is going to be higher.
For an average, easy run, the form power ratio is going to be between 29-31%. When you get into a marathon pace, it should drop down to about 27-28%. For FTP, you really want to be between 24-26% or lower.
I hope thats been informative and also explained some of the details behind how running with power works, what metrics are used to measure your training and how this can be applied to your race, Have fun and stay safe.
Sisu Racing Triathlon Coach