Cyclists have had power meters around for decades but they are only just starting to catch on for runners. The premise is the same as it is for cyclists in that power is a measure of the rate at which work is being done, whether that’s uphill downhill or on the flat, into a head wind or a tailwind, the work you are doing is what matters. Pace will vary in all those situations and heart rate is slow to respond and influenced by a lot of other factors hence the idea of having one metric that responds quickly to change in effort and can take into account gradients and wind is very appealing.
Power meters come in two main types- foot pods and watch-based. Currently Stryd is the market leader with their footpod and there have been a number of studies on its merits showing it be a reliable metric to delineate exercise intensities.
Its not without its detractors though – some argue that its not true power given it is not measuring the force at the foot ( power = force X velocity ), instead it uses accelerometers in the pod combined with a barometer and then uses an algorithm to calculate the power output. But Styrd have validated their power numbers by measuring the metabolic power of athletes on a treadmill and comparing it to the power numbers it calculates, as have several other independent studies.
See the bottom of this section for some links to relevant studies.
Wrist-based power meters have less accuracy as they are either based on GPS ( hence subject to any GPS inaccuracies ) or use an accelerometer in the watch ( so assumptions are made to derive the leg and foot movements from accelerometers in the watch ). Brands include COROS, Suunto , Polar , Apple and Garmin with Garmin a distant last in terms of usability.
Determining Anchor points for Power
Like any metric of intensity, it needs to be anchored to something. Power is best anchored to one’s threshold – commonly referred to as Critical Power or Functional Threshold Power ( which is very similar to lactate threshold )
The best way to determine this is using a specific Critical Power test which involves a 3 minute and 10-15 minutes all out effort. We will talk about how to test threshold power in more detail in the power section.
Advantages of power
Unlike heart rate, power responds quickly to changes in effort and unlike pace you can compare power on different inclines even though ones speed is different.
For example you can give your athlete a workout of 5 x 4 minutes uphill efforts at 240-250W and it doesnt matter if the hill has a 4% gradient or a 10% gradient – 240-250W will feel the same. This means we can compare the data from one hill session to another even if the gradient is different ( something we cant do with pace ) and we can set targets for hill reps that are valid for any incline. Power also overcomes the problem of heart rate lag as it responds within seconds to changes of intensity .
Stryd Power also takes into account head winds – so if you are running one direction with the wind and then turn and run into the wind, power will take the extra effort running into the wind into account For example you might be running 5 minutes ks with the wind and 6 minutes ks against the wind but power will show 220W both directions if the metabolic load is the same.
As good as that all sounds like any metric it does come with some disadvantages.
Disadvantages of power
Environmental Conditions
Power needs to be adjusted for elevation and environmental conditions. If running at higher altitudes or in hot humid conditions then we need to lower our power targets – fortunately there are some good calculators that make this easy.
Technical terrain
On technical trails power can be quite jumpy and practically impossible to use as a means of guiding intensity. However in most of these cases, skill is the limiting factor to one’s speed anyway. If you cant run and look at your watch at the same time then no metric is going to be of help!
Trail Surface
Different surfaces can affect power – eg running in soft sand or mud will mean power is under-reported in relation to effort. That doesnt mean we completely disregard power, we just need to know how to use it effectively.
Some will argue that because power can’t be used on technical trail, muddy trails , sandy trails its of no use but as we have seen no metric is perfect. The goal is to use the best available metric ( even if it’s not perfect ) to give our athletes a means of pacing a run at the appropriate intensity.
In trail races – rarely if ever is a course made up of 100% technical , muddy trails or sandy trails . So if we use power on the runnable sections to calibrate perceived effort then when power is not reliable we can fall back on perceived effort.
For example if there are some soft sandy trails in a race we can use the runnable trails to establish what race power feels like and direct our athlete that when they hit the soft sand maintain the same feeling of effort. Power might be 200-210W on the runnable trails but 150-160W on soft sandy trails but if the athlete just keeps the same perceived effort on those soft sandy trails then the intensity will be optimal.
Stryd power is quite consistent on different surfaces so if your race does have lots of mud or soft sand then with experience you can calibrate the power in your head – eg 200-210W on road might be equal to 150-160W on soft sand.
Uphill
The other thing we see is some athletes can generate more or less power for the same effort up steep hills. For the typical hills we encounter in road running there is little to no difference – ie if you can hold 200W for 30 minutes on the flat you can do the same uphill , but once it gets to some of the steeper hills that we see in trail and ultra races there can be a difference.
This doesnt mean we can’t use power – it just means we need to test to see if the athlete has a higher or lower critical power uphill ( will discuss this more in the power section )
Downhill
On all but the most gentlest gradients, we often see a difference in what an athlete can maintain on the flat vs downhill. Better skilled and conditioned runners can handle higher power levels downhill vs less experienced and conditioned runners
In this chart its almost impossible to tell just by looking at power when the athlete was running uphill vs downhill
But in this chart its very obvious
This doesnt mean we cant use power downhill – it does mean we have to take into account the athletes downhill skills and conditioning ( how we do that will discuss in another video )
Hiking
Power when hiking is lower than power when running so we need to determine different targets when hiking hills vs running hills
Practical use of power in trail races
This means in practice to use power on trails you need 4 targets – flat, uphill , downhill and hiking.
Now whilst that might sound like a bit of effort let’s discuss what it can give you and what the alternatives are.
For a trail ultra – if we can determine 4 power targets for our athlete it means they have an objective target to base their pacing off . Yes it falls down on tech terrain and muddy sandy trails but on the majority of trails we see in ultras it’s very useable. Power numbers do fluctuate up and down a little ( Styrd power a lot less than the others ) but we dont need 100% accuracy – I usually give power targets in a 10W range which with a little bit of practice is not hard at all to stick within that range on trails.
Without power we are stuck with pace (which can only use on the flat) , heart rate which is slow to respond and affected by many variables and perceived effort ( which at the start of a race is usually very much underestimated ).
My own data of literally thousands of races by athletes using power shows that if they stick to their race power targets from the start they maximise the chances of having their best race. Whenever there has been a blow up , cramp or stomach issues – the vast majority of the time the data shows they have gone out harder than planned even though it felt very easy at the time.
In a race when legs are tapered and feeling fresh, perceived effort is usually lower than in training for the same pace. For example, in long runs one might be running 6 min ks on the flat but on race days 6 min ks feels VERY easy so you take off at 5:40 min ks as it feels about the same effort wise as 6 min ks did in training. This usually results in a large slow down later in the race and significantly increases the chances of cramp and stomach problems.
For a road marathon we can usually determine with good accuracy what a target finish time should be and from that a target pace. Marathoners know that no matter how easy it might feel at the start if they go too fast they will likely blow up at 30-35km. But in ultras the terrain means we cant use pace so I see people going out too fast all the time because it feels easy. With power we get around that problem – 200W is 200W whether your legs are fresh or not. It gives us a reliable metric to set our intensity for both racing and training.
For these reasons Power is my metric of choice for the athletes I coach. However to use power effectively you will either need to use software such as WKO or Golden Cheetah or have a Stryd Subscription. This is because to effectively use power you need to separate uphill, downhill, hiking and flat in the data analysis. If you dont like data analysis or your athlete prefers to run by feel then power may not be the best option.
However even if they dont like running by numbers you can still use power ( or any other intensity metric ) for post workout / race feedback to help develop their sense of perceived effort. ( see the workout analysis video for more details )
Studies
Is Running Power a Useful Metric? Quantifying Training Intensity and Aerobic Fitness Using Stryd Running Power Near the Maximal Lactate Steady State
https://www.mdpi.com/1424-8220/23/21/8729
“…. it provided a stable, sensitive, and reliable metric that can estimate aerobic fitness, delineate exercise intensities, and approximate the metabolic requirements of running near the MLSS.”
Is Stryd critical power a meaningful parameter for runners?
“Stryd offers meaningful data for runners including a realistic estimate of CP.”
Are we ready to measure running power? Repeatability and concurrent validity of five commercial technologies
https://www.tandfonline.com/doi/abs/10.1080/17461391.2020.1748117
“…although the PolarV, GarminRP and RunScribe technologies maintain a certain relationship with VO2, their low repeatability questions their suitability. The Stryd can be considered as the most recommended tool, among the analyzed, for PW measurement.”