Using Running Power on the Treadmill

by Steve Palladino

coach and consultant, Palladino Power Project

January 4, 2020

Treadmill running is a common practice, whether to avoid temperature/humidity extremes, or dark/dangerous outdoor running conditions, or simply for convenience.   Particularly during the colder/darker Winter months (in the Northern Hemisphere), like cyclists that turn to the indoor stationary trainer, runners turn to the treadmill.

However, to optimize the running with power experience on a treadmill, the user must be savvy of the specificity of the treadmill.   For example, treadmill running has been documented to be associated with different mechanics (1), and lower Running Economy (2) than overground running outside.  And, for power users, there are considerations to understand and apply when taking their runs to the treadmill.  (Figure 1)

Figure 1.  Considerations for Running with Power on the Treadmill

FTP/CP Portability

Running with power is typically executed to a relative intensity.   In the majority of applications, the intensity that is executed is relative to FTP/CP (Functional Threshold Power and Critical Power).  However, one cannot assume that a FTP/CP value is universally portable.  A FTP/CP established at sea level should not be assumed to be portable to 1800m elevation.   A FTP/CP established a 15C should not be assumed to be portable to runs at 25C.  Depending on the circumstances, the FTP/CP value on which one has been basing outdoor training may not be portable to indoor treadmill running.  And, if a FTP/CP value is invalid for indoor treadmill training, then all training executed relative to that FTP/CP will be relatively wrong.

For example, let’s examine a case in which a runner’s outdoor FTP/CP is 300W, and treadmill FTP/CP is actually 288W (4% less).  If the runner fails to recognize that the outdoor FTP/CP is not portable to the treadmill and runs relative to the outdoor FTP/CP, what would happen?  A 45 minute easy aerobic run at 80% of FTP/CP, instead of being properly executed relative to the treadmill FTP/CP value, (288*0.80) or 230W, would erroneously be executed relative to the outdoor FTP/CP, (300*0.80) or 240W - effectively overpowered.

Do not assume that your outdoor FTP/CP is portable to the treadmill.

Heat / Humidity

An important consideration when moving your running to the treadmill would be awareness of heat and humidity differences between outdoor overground running and indoor treadmill running.  If you have been training in cooler temperatures, and FTP/CP was established outdoors in those same cooler temperatures, then you go to treadmill running indoors, where it is warmer and may not have sufficient air flow, the outdoor FTP/CP is not likely directly portable to treadmill training.  It is likely that your outdoor, cooler temperature FTP/CP is relatively too high for use in a warmer treadmill environment.

This consideration applies equally to Stryd “non-wind’ version, Stryd ‘wind’ version, and any other run power meters that can be used on a treadmill.  (purely GPS-based power is not usable on a treadmill)  

To make the conversion, so that FTP/CP is correct relative to the conditions found indoors at the treadmill, one would use the SuperPower Calculator, converting a FTP value ‘from’ the cooler outdoor running set of environmental conditions ‘to’ the conditions associated with the treadmill environment.

Here’s how to make the correction:

1. Determine your current outdoor FTP/CP environmental condition basis.  If your current outdoor FTP/CP is from:        

2. Determine the conditions for the treadmill environment, and use them as the ‘to’ conditions.
3. Use the SuperPower Calculator to adjust for the different environmental conditions (video)

For example, let’s say that a runner’s FTP/CP was 300W based on a recent CP test conducted in 10C / 50% humidity conditions.  And let’s say that the runner will be running on a treadmill indoors at 20C with an estimated humidity of 70%.  The FTP/CP value would be corrected from 300W to 295.5W.  (Figure 2)

Figure 2. Conversion of 300W FTP/CP @ 10C / 50% humidity to treadmill environment of 20C / 70% humidity.

To mitigate the impact on the indoor treadmill environment on FTP/CP and associated training power targets, one should optimize the treadmill environment:  

• Lower the room temperature
• Use large electric fan(s)

Once the treadmill environment has been optimized as much as can be, then run through the necessity to apply temperature/humidity correction to your outdoor FTP/CP, rather than simply assuming portability from outdoor running to the treadmill.

Stryd Air Power

The Stryd ‘wind’ version adds another twist to the issue of portability of FTP/CP from outdoors to the treadmill - air power.  Outdoors, running overground, a runner must overcome air resistance generated by both, a) the speed of the runner and b) any headwind that is present.  The ‘wind’ Stryd pod reports the power to overcome air resistance as ‘air power’, and adds ‘air power’ into the total power that the Stryd unit reports.

The ‘non-wind’ Stryd version does not detect air resistance.  Consequently, the power reported by the ‘non-wind’ version is devoid of the extra ‘air power’ component.  To display these relationships in abbreviated form, I follow the format presented by VanDijk and VanMegen in their book, The Secret of Running:

When running outdoors overground, if you generate a FTP/CP value with a Stryd ‘wind’ version, that FTP/CP is based on Pt, which includes both Pa and Pr.  In other words, the outdoor, overground FTP/CP includes an air power component.

When you run indoors on a treadmill, you are not overcoming any air resistance, and the Stryd ‘wind’ version rightfully will report 0 ‘air power’.  The problem begins when you use an outdoor FTP/CP inflated by ‘air power’ to generate power targets for a treadmill workout that will not produce any ‘air power’ component to the reported power.  

Let me illustrate with an outdoor CP test using a Stryd ‘wind’ version device and a 3 minute / 10 minute test protocol.  With WKO software, I can report the Stryd power components separately.  Figure 3 provides the results of the outdoor CP test.

Figure 3. Results of outdoor overground CP test with Stryd ‘wind’ version device.

AP = average power

Pt = total power reported by Stryd

W = Watts

Pa = air power reported by Stryd

Pr = Pt-Pa, or the equivalent to the power that would be reported by a non-wind Stryd

The 3 minute test component resulted in an average total Stryd power (Pt) of 422W.  Five percent of that power was ‘air power’, and the Pr (Pt-Pa, or the approximate equivalent to the power reported by a ‘non-wind’ Stryd device.) was 400W.  The 10 minute test component resulted in an average total Stryd power (Pt) of 363W.  Four percent of that power was ‘air power’, and the Pr (Pt-Pa, or the approximate equivalent to the power reported by a ‘non-wind’ Stryd device.) was 348W.  

As an aside, note that air power was a higher percent of total Stryd power in the shorter, higher intensity / faster 3 minute segment than the 10 minute segment - because air power includes the power to overcome the increased air resistance from running at a higher speed.

The Pr values, because they are devoid of ‘air power’, represent not only the approximate equivalent to the power reported by a ‘non-wind’ Stryd device, but also the approximate equivalent to the power that would have been reported on a treadmill for the same speed/intensity.

If you calculate the outdoor, overground CP estimate from this test, you would use the Pt values and arrive at 337.7W.   However, if you use the Pr values to calculate the CP estimate, you would get a value of 325.7W.  This 325.7W CP estimate based on Pr would be devoid of ‘air power’, and would be a suitable CP estimate for the treadmill for this athlete.

If this athlete were to go to the treadmill to do a workout of 4x 3:00 @ 104-106% of FTP/CP,  using a FTP/CP value of 337.7W on which to base the workout would be cruel and unusual punishment.  The athlete would be trying to execute 4x 3:00 @ 351-358W.   However, with no ‘airpower’, those would be essentially Pr Watts - no air power add on.  In contrast, the athlete should use the 327.7W FTP/CP value based on Pr.  If so, then the athlete would execute the workout as 4x 3:00 @ 341-347W - a big difference, and more appropriate.

With the Stryd ‘wind’ version, the outdoor overground FTP/CP is not valid for use on the treadmill.  

In contrast, the outdoor overground FTP/CP from a Stryd ‘non-wind’ version is more likely portable to the treadmill (with the previously discussed heat/humidity corrections applied as needed).

So, how do you go about making sure that you have a FTP/CP value for using your Stryd ‘wind’ version device on a treadmill?  Here are some options:

1. Use the data from a recent outdoor CP test, race, or time trial.  You would need to determine Pa from the test components or race.  Then subtract the Pa value from the Pt (total average power) - this will give you Pr for the test components or race.  Then use the Pr value in the SuperPower Calculator, using either the ‘Calculate FTP/CP and AWC from 2 or more maximal effort Activities’ or ‘Calculate FTP/CP (and AWC if possible) from a Prior Race Power/Time and Riegel Exponent’ function depending on the scenario (CP test vs race/time trial).  After getting the FTP/CP estimate based on Pr, remember to then make any necessary temperature/humidity corrections from the test/race/time trial conditions to the treadmill conditions.
2. If you wish to rely WKO’s modeled FTP for your FTP estimate, you will need to set up a power-duration curve for Pt, Pr, and Pa, along with a model FTP for Pt and Pr.  (Figure 4)  The expression for mFTP based on Pr is: FTP(pdcurve(meanmax(runpower-@air_power)))  You would use the mFTP Pr for treadmill running.  Refer to the Heat/Humidity section above one making a temperature correction for this scenario.

Figure 4.  WKO Chart with model power-duration curves for Pt, Pr, Pa, and model FTP for Pt and Pr.

3. If you wish to rely on Stryd’s auto-CP, you’ll be unable to do the prior data manipulations possible in WKO.  Instead, you could do an outdoor run that includes a segment of about 10 minutes at your outdoor auto-CP value.  Then, find the ‘air power’ for that segment.  Find your CP based on Pr by subtracting the average ‘air power’ for the segment from your Stryd auto-CP value.  The resultant value will be a reasonably close CP for the treadmill.  Don’t forget to heat correct.
4. Do a CP test on the treadmill.  The advantage here is that your treadmill CP will already be heat corrected.
5. If you don’t want to be troubled by any of the options 1-4, try subtracting about 4% from your outdoor FTP/CP value to get your treadmill FTP/CP.  Don’t forget to heat correct.
6. (also see addendum)

Motorized vs Non-Motorized Treadmills

Non-motorized treadmills may have the advantage of greater mechanical similarities to overground running than motorized treadmills.  However, non-motorized treadmills have been noted to be associated with a significantly higher oxygen utilization, heart rate (HR) and rating of perceived exertion (RPE) compared to both, motorized treadmill running and overground running.(3, 4, 5)  

“Running on a non-motorized curved treadmill at three different velocities results in a higher oxygen uptake (37%) and heart rate (22%) and is subjectively much harder than running on a track or a motorized treadmill at the same velocities.”(3)

It is clear that the power demands for running a given relative intensity are much different than both, overground outdoor running, and motorized treadmill running.  

An outdoor FTP/CP is not portable to a non-motorized treadmill.  For that matter, a FTP/CP that is specific to a motorized treadmill is also not portable to a non-motorized treadmill.

Quite simply, if you are going to train on a non-motorized treadmill, you need to do a CP test on the non-motorized treadmill to be absolutely sure of power targets.

Another anecdotal method suggested by Guy Aroshas is to increase the Stryd weight setting by a scaling factor.  He increases outdoor Stryd weight setting by 1.2 times when running on a non-motorized treadmill to equate power to outdoor overground running.  Based on HR and oxygen utilization differences cited in the literature, a Stryd weight setting multiplying scaling factor of 1.20 to 1.35 might be applicable (increasing outdoor running Stryd weight setting by 1.20x to 1.35x to allow the Stryd unit to report Watts equivalent to outdoor running when running on a non-motorized treadmill).  In one athlete that I have data on, the difference in outdoor FTP (in Watts) and non-motorized treadmill FTP (in Watts) - both verified by specific CP tests - is non-motorized treadmill FTP 1.25 times less than outdoor overground FTP (at the same weight setting).  Another consideration in adopting this approach with a Stryd weight scaling factor of 1.20 to 1.35 is that non-motorized treadmill running impacts lighter runners moreso than heavier runners, as exemplified by the relative impact of body mass on Running Economy (Figure 5).  In other words, a lighter runner may find a Stryd weight scaling factor closer to the upper range than the lower range (1.20 to 1.35) to be more appropriate.  

Figure 5.  From: Edwards et, Frontiers in Physiology, November 2017 (4)

In the end, though, if one wants to be certain of the correct training intensities on a motorized treadmill, it is best to do a CP test on the non-motorized treadmill.

Incline

It has been demonstrated that, in terms of oxygen utilization, running on a 1% treadmill incline best matches outdoor overground running at the same velocity.(6)  Keep in mind though, that the 1% treadmill match is a generalization.  The actual best match varies a bit, depending on the running speed selected (Figure 6).  However, an incline setting of about 0.5% to 1.5% would allow for a best match across a range of speeds from 2.92 m/s (9:11 min/mile / 5:42 min/km) to 5.00 m/s (5:21 min/mile / 3:20 min/km).  At speeds slower than 3.75 m/s (7:09 min/mile / 4:26 min/km) it probably is of no importance whether you run on a 0% incline or a 1% incline.

Figure 6.  From: Jones and Doust, Journal of Sports Sciences 14(4):321-7, 1996. (6)

Note: 

2.92 m/s =  9:11 min/mile / 5:42 min/km

3.33 m/s =  8:03 min/mile / 5:00 min/km

3.75 m/s = 7:09 min/mile / 4:26 min/km

4.17 m/s = 6:25 min/mile / 4:00 min/km

4.58 m/s = 5:51 min/mile / 3:38 min/km

5.00 m/s = 5:21 min/mile / 3:20 min/km

If one runs at a treadmill incline of 0 to about 1.5%, the power reporting from the pod to the watch will be reasonably accurate and useful.  (Keep in mind, though, the issue of training relativity, and that the training needs to be relative to an appropriately specific FTP/CP setting.)  However, if one wishes to run at an incline of 2% or higher, then for more accurate power reporting, it will be necessary to use the Stryd phone app, in which you select the incline setting, and the app does the power calculation to report the appropriate power for that incline.  

Whether running on higher treadmill inclines adequately match the mechanics of overground hill running is another matter, and is likely not as specific to overground hill running as one might hope.  For some, being able to target a higher relative intensity on a higher incline while running at a slower speed might feel safer.

In the end, my recommendation is to set the treadmill incline to 0.5% to 1.5%, depending on speed (or just run at 1% and not worry about the fine variances), and run by power, using the power reported from the pod to the watch.

Calibration - Stryd, Treadmill, Zwift

Why bother trying to writing this section, when it has been covered so expertly by Steve Bateman (visit the Facebook group, from1runner2another).  Read the article, HOW: Calibration - Stryd, Treadmill, Zwift (7) - it is a must read for treadmill running.

A few final points

• Remember that for treadmill running, one must set the watch to distance and speed from Stryd, and to turn off distance and speed from GPS.
• Training on a treadmill will likely be executed in many cases to a lower power than outdoor running.  As such the training load metrics, RSS or TSS, will result in a slightly lower value at same effort than for overground running.  If running occasionally on the treadmill, my advice is not to worry about these small differences - they are really not that important in the grand scheme, or when looking at training load from the 25,000 foot level in a performance manager chart.  On the other hand, if you are switching to predominantly indoor training for a period of more than a week or two (like moving training indoors for the Winter), I suggest resetting FTP/CP to the treadmill-specific FTP/CP, so that the training load metrics are accounted for more properly over the span of indoor training.
• Running power is stochastic.  People will note that when they run overground, the power is “bouncy”.  Such is the nature of human power - it is stochastic.  However, when running on a motorized treadmill, with near-constant belt speed, you will notice that the power tracing for a run is much smoother - less stochastic.  Interestingly, on a non-motorized treadmill, power will appear to be more stochastic than seen on the motorized treadmill, and closer to, but maybe not quite as stochastic as overground running.

Summary

An overview of the various factors to consider in running with power on a treadmill has been presented.  Perhaps the most important consideration is to understand, that for various reasons, outdoor FTP/CP is not necessarily portable to indoor treadmill use.  Figures 7, 8, and 9 outline the process to assure that training targets are appropriate.

Figure 7.  Using the ‘non-wind’ Stryd on a motorized treadmill.

Figure 8.  Using the ‘wind’ Stryd on a motorized treadmill.

Figure 9.  Using the ‘non-wind’ and ‘wind’ Stryd on a non-motorized treadmill.

References

1. Is Motorized Treadmill Running Biomechanically Comparable to Overground Running? A Systematic Review and Meta-Analysis of Cross-Over Studies

2. Better economy in field running than on the treadmill: Evidence from high-level distance runners
3. Comparison of perceptual and physiological variables of running on a track, motorized treadmill, and non-motorized curved treadmill at increasing velocity
4. Non-motorized Treadmill Running Is Associated with Higher Cardiometabolic Demands Compared with Overground and Motorized Treadmill Running
5. The physiological and perceptual demands of running on a curved non-motorised treadmill: Implications for self-paced training.
6. A 1% treadmill grade most accurately reflects the energetic cost of outdoor running
7. Stryd Wind White Paper

Addendum

1. Another method of addressing the % air power lost between running outdoors and running on a treadmill would be to adjust the Stryd weight by by the same percentage as the typical % air power at CP. (Figure 10)  Information and calculator by Steve Batemant here.  

Figure 10. Using the ‘wind’ Stryd on a motorized treadmill - changing weight setting option.

2. Non-motorized treadmill users poll.

3. 12/4/2020  Update on thoughts regarding weight settings specific for treadmill use:

When running on a treadmill, typically there are multiple factors that can change the relativity of reported power on the treadmill to outdoor FTP/CP (see figure).  

Factors that change the relativity of reported power on a treadmill to outdoor FTP/CP.

 *NMCTM = non-motorized curved treadmill

For power to be meaningful it must be relative.  Power must relate to something….5K power today relative to 5K power 6 months ago....10K power at 6000 feet / 1829 meters of altitude relative to sea level...and so on.   For most Stryd users, the key relativity of their power is to FTP / CP….. this training run is to be at <= 80% of FTP/CP….that 10K was at 101% of FTP/CP...we relate power to FTP/CP.

When one establishes their FTP / CP outside, running overground, we relate training to that value, and indirectly, we relate racing targets to that value.  However, there are those factors (see table) that prevent relating power to one’s outdoor FTP/CP.  

Something has to change.  In short, to account for those factors on the treadmill, one has to change either, a) their FTP/CP, or b) the weight setting on the Stryd device.  Either FTP/CP is lowered or the Stryd weight setting is increased.  There are either indoor and outdoor FTP/CP values that are used, or indoor and outdoor Stryd weight settings that are used.

Fortunately, whichever way decides to go, the percent change is the same.  If one calculates that the loss of airpower and heat/humidity difference between outdoor running and running on the treadmill account for a 5% loss relative to outdoor FTP/CP, then either, a) the outdoor FTP/CP is lowered by 5% to arrive at a treadmill FTP/CP, or b) the Stryd weight setting is increased by 5% for treadmill running.

If a runner elects to lower FTP/CP to account for the treadmill factors in the table above, then:

• Weight setting is kept fixed
• There are two FTP/CP values to be used - one for outdoor, one for indoor
• The same % of FTP/CP can be used as long as the correct FTP/CP is used as the basis

If a runner elects to increased the Stryd weight setting to account for the treadmill factors in the table above, then:

• There are two weight settings to be used - one for outdoor, one for indoor
• There is only FTP/CP value to be used
• The same % of FTP/CP can be used as long as the correct Stryd weight setting is used

The advantage of the former is that one does not have to fiddle with the weight setting depending on whether the run is outdoors or on the treadmill.  The disadvantage is that unless one is also changing their Powercenter CP value, treadmill running RSS will be under-reported, and training metrics impacted to a small degree.   Also, one would have to change the FTP/CP on which power targets are based for training - either some maths are involved, or one has to change FTP/CP when building structured workouts.

The advantage of the latter is that there is only one FTP/CP that is used - power targets are relative to one FTP/CP, structured workouts are built on one FTP/CP, and RSS and downline training load metrics are more accurate.  The disadvantage is that one has to remember to change the weight setting, depending on whether the run is outdoors or on the treadmill.

Either way can work - choose what seems best for you.  For the athletes that I coach, I have shifted my preference from the former (two FTP/CP values) to the latter (two weight settings).  I prefer the one FTP/CP, two weight settings approach, and the more accurate training load metrics.

Resources

1. Video demo: Stryd 'wind' version users: How to obtain a treadmill-specific CP from an outdoor CP test.
2. Video demo: Using the SPC to adjust weight setting for TM running.
3. Video demo: case example of calculating weight setting for treadmill running
4. Discussion and calculator: Adjust Stryd Weight for Treadmill Usage
5. Article: Understanding the Stryd Weight Setting

4) 12/12/2022  Stryd power and the Treadmill - Incline

Per the Stryd folks, one must use select devices/apps to manage the incline setting and to record a TM run for any incline other than 0-1%:

Currently, it is necessary to tell Stryd the incline you are running on by recording your activity with either the Stryd mobile app (Android or iOS), our Apple Watch app, or the Stryd Workout App on Garmin (available for Stryd devices using 2.1.16 firmware or higher). This will adjust the reported power appropriately and factor in the incline into the power value. You can make multiple incline changes during a workout. Once an incline change is applied your power will adjust accordingly.

If you sync your Stryd to the Stryd app (Offline Sync) you will not see any of the incline adjustments that you made during the run. This is because the Stryd pod never knows what the incline is. The power values are adjusted in the app in real time.

Note: If you are running at a 0% or 1% incline indoors, you can record your activity with the device you typically use such as your Garmin watch or other sportswatch. There is no need to record using our app or tell Stryd the incline.

link to Stryd blog post

For what it’s worth: I prefer that the athletes that I coach only use 1% incline. In my opinion, increasing grade does not provide a meaningful hill training effect, and should only be considered if belt speed at 1% is insufficient for power prescription or too dangerous for power prescription.

5) 1/9/2024  Treadmill distance reporting / Distance calibration factor.

While I personally do not care much about distance accuracy/validity in a run power-duration paradigm, I do realize that for some it is important.

Thus, here is a thought regarding distance reporting on a treadmill:

Using Running Effectiveness can be a way to more closely the speed:power relationship on the treadmill vs overground running.

With a Stryd “wind” pod type, it is generally observed that Running Effectiveness is higher on a motorized treadmill (MTM) than outside overground at same relative intensity due to the loss of airpower. And, on the non-motorized curved treadmill (NMCTM), Running Effectiveness is much lower on a motorized treadmill (MTM) than outside overground at same relative intensity due to mechanical factors associated with the NMCTM.

If one where to perform a CP test (3’/12’ single session protocol, for example) both, outside/overground, and on the treadmill (either MTM or NMCTM), one could:

• Generate FTP/CP estimates for both outside/overground, and on the treadmill (specific to environments and also likely specific to the individual’s TM). From these two FTP/CP estimates, one could determine the appropriate TM weight setting to be used to equilibrate power reporting. Example: (outside FTP/CP estimate / TM FTP/CP estimate) * outside weight setting = TM weight setting
• Calculate the Running Effectiveness from the 12’ test segment from each, outside/overground and on the treadmill. From the two Running Effectiveness values, one could then calculate the appropriate distance calibration factor to be used on the watch when running on the same treadmill. Example: Outside watch distance calibration factor setting + ((outside 12’ test Running Effectiveness - TM 12’ test Running Effectiveness) * 100) Note: a MTM will typically yield a lower distance calibration factor setting for the TM than for overground running, and a NMCTM will typically yield a higher distance calibration factor setting for the TM than for overground running.

After making the correction to one’s Stryd pod weight setting and one’s distance calibration factor, then power targets (relative intensity) become more equilibrated, and the speed:power relationship becomes more equilibrated, resulting in a similar distance:duration relationship between overground running and TM running.

Note: it is incumbent on the runner to change weight setting and distance calibration factor when changing between overground/outside running and TM running.

6)   2/6/2024  Difference between outside/overground power and treadmill power and associated applications

Outdoor/overground power *at same effort* (eg, @ CP or @ 20' max effort) will typically be higher than on the TM. The primary reason* is that airpower (Pa) is added to the base running power (called Prc) to result in the total power reported (Pt) when running outside/overground. Airpower is not reported on the TM, so total power reported is only base running power (Prc).

outside/overground

treadmill

* "The primary reason* is that airpower (Pa) is added to the base running power (called Prc) to result in the total power reported (Pt) when running outside/overground."....the secondary reason can be difference in temperature and humidity, particularly at this time of the year for northern hemisphere folks.

Consequently, an auto-CP estimate based on outdoor/overground testing and racing is typically too high for TM running. And, if testing on a TM, that test data is going to a) not be useful (too low) for outside/overground running, and b) only really useful if all running and racing is limited to the TM, c) confounding to the Stryd race planner estimates. Do not test on a TM unless all of one's running and racing is on a TM.