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Juerg Feldmann

Fortiori Design LLC
Posts: 1,530
Here an example, where we  fundamentally go apart from training ideas, based on power ( wattage ) in cycling.
1.  I am  absolutely  fan of a power meter, but only if it  is used  in combination with physiological feed backs.
 Here for new readers  why in a short picture.

FTP and heat and sugar.jpg   We discussed this before.
 As well any coach or athlete knows  from experience, that when pushing a fixed wattage  the effort  on the physiological systems may differ  by a lot.
 Here just some examples.
 Above you see changes in temperature changes the FTP and with it the calculated zoning .
 Nutritional intervention will change performance  and therefor FTP and therefor the "zoning "
 Altitude  can do the same.
 A  simple cold with some respiratory reactions can change it easy.
 Change in RPM cab change performance.
 Time of day, jet lag and many more.
 So  using any current  " zoning "  have all the clear discussion gong on, whether we really train in the calculated intensity or, whether we may be off  and how much.
 So suing a power meter has this problem as well.
 Now the fun part actually is, that we all believe in power meters and many argue on what is the most accurate one.
 Many many years back, when Power meter got  more  mainstream  the SRM  was one of the top  and still is a great model.
 Interesting  with this technology is, that even  technology  ( not just human physiology ) will react on some outside changes like temperature.
 So the below article shows, that they take this in consideration which is really great.

Ride: teaching ultimate accuracy

A good power meter should enable you to focus on your efforts (or the ride) and save you from worrying whether the data in front of you is correct. The Pioneer does just that, but only after it’s ‘taught’.

Pioneer has a unique feature, in that for ultimate accuracy it must be taught in ranging temperatures via zero calibrations – a process similar to resetting a digital scale down to zero (the zero calibration isn’t actually a reset, however, but rather a data recording that’s stored by the power meter itself). The reason for this is to teach the system how the crank’s material responds in various temperature conditions. The system holds up to six data points; riders living in mountainous regions or riding long distances in which temperatures can vary greatly over a single ride will benefit most from this feature.

 So if we change from a cold area into a sunny hill side ( which we often had in Spain in our training camps,) the power meter  can adjust to it  so we are sure 300 watt is still 300 watt.
 Problem. Thanks to the ability to stay in the 300 watt  zone  we may in fact go out of the physiological 300 watt zone, as we may need some blood to control body temperature  and as such  we change the physiological demand  for ATP  production by changing most likely SV as well as HR and as such as well respiration and more.
 So a 300 watt maintaining power out put may shift us into another zoning physiologically. The training  book will not understand this as I write down a 2 h ride on 300 watt. (which is a bad example as it is pretty  high ) but in fact we may have passed  through different zones physiologically  but not  based on power.
  Suggestion. Much more efficient would be to have a great power meter like that combined with a real physiological feedback  so i can see how much I have to adjust  power done or up to maintain a  stable physiological stimuli  . Now  we can talk about a great team work telling  me  for a race how much I have to adjust the power to have an optimal race by looking at the delivery and utilization of  energy ( O2 ) simultaneous to the power readings..
 Here 2 workouts with a stable energy delivery and utilization picture from a  top class 24 hour  runner.
 This are  2   2h 30 min workouts on a 400 m track  with   first workout  1 . 52  minutes lab times  and than  second time to  calibrate  a  1.45 min per lap workout.
 The `power`speed  was  incredible regulated for each lap +- 1 sec. you can see   that the SmO2  as a sign of  energy delivery and utilization where pretty much stable but a  slightly lower level as  the O2 needed to run 1.45  compared  with 1. 52  is high.
 What appears to be a perfect balanced situation when we looked  at SmO2  and  time  as well as RF  and HR   is not a completely  perfect  balance.
smo2 both runs.jpg    
 . a) HR drifted  slightly up in the second  part  which opens the question why. ?
 Without going into much details here  for the advanced  MOXY users the answer.  from the " delivery " systems  respectively vascular reactions.

  thb both.jpg 
So the  clear question  comes  up:
  Why do we  not embrace  a new  technology, where we can see live  in what " zoning " we are  and compare it with classical feed backs  like HR or wattage in cycling  an start to understand, what causes the   reaction    so we can't maintain a  fixed  wattage zone  due to physiological adjustments ?



Fortiori Design LLC
Posts: 65

Some thoughts on this point. The only way performance parameters such as speed or power can be used for some kind of physiological zoning is if all effecting parameters stay unchanged, something which is extremely unlikely. In the end we need to consider what factors create the performance parameters we are measuring? The answer is undoubtedly physiological factors (with others such as biomechanical). This means in order to understand change in performance parameter we need to understand physiological factors. As Juerg points out performance parameters will remain the same regardless of temperature or recovery and so on, but the physiological factors will reflect all these changes. For this reason is it much more logical to utilize physiological factors to guide training. This is not to say that performance parameters are not important. Long term performance parameter trends are vital for training prescription. The goal of most training is performance improvement, and therefore this needs to be measured in an objective manner. Physiological information may be a little vague when it comes to determining performance gains. Emphasizing long term performance parameter gains (day to day highs and lows are common in all athletes) will help identify if your training program is getting you the gains you want, and if the physiologically based intervention is helping or if adjustments need to be made. 

And to finish up this post, I want to address the idea of physiological factors, and more importantly measurable physiological factors. For training purposes a physiological factor must be simple to apply and not interfere with training. This is the reason why heart rate monitors have had such success. However, a few basic questions need to be asked about the usefulness of heart rate monitors. Firstly, how can we guide training with a heart rate monitor? Zonings. Basically, HR will climb at a linear rate with increasing performance and therefore there is not recognizable points that could be used for zonings (HR deflection point was an attempt at this). This means zoning are basically taking max HR and calculate a % (VO2 is the same). In other words a mathematical/statistical analysis. Secondly, the question is why do we increase HR with performance? Numerous reasons, but one central reason during exercise is undoubtedly to increase oxygen delivery to the muscle. If the availability and consumption of oxygen in the muscle is what we are trying to understand with HR, why not do this directly? Taking both of these points Moxy and NIRS make much more sense. Firstly, SmO2 is not a linear reaction, but reflects constant change in oxygen supply and demand, and secondly the exact same point SmO2 reflects the actual performance related physiological parameter. 

Any thoughts?

Juerg Feldmann

Fortiori Design LLC
Posts: 1,530
We  are getting more and more   mails   asking for accuracy  and validation  and  as well stressing the point, that power meters are much more accurate that NIRS  .
 So  I decided  to add some  more information to this discussion. The beauty of this discussion is, that  more and more power users  start to   do some thinking on  what  may  go on with NIRS, It is always a  time  process   to  learn to integrate new ideas  and accept , that   we  can  make progress  by  looking  at optimal changes.
 Here  for all this  questions to independent    answers  on the discussion on accuracy.

Bike Power Meter Accuracy

The mobile bike power measurement systems from products like SRM and Power Tap use strain gages to measure torque or twisting force on the crank arm or on the rear hub. Torque is multiplied by axle RPM to determine rider power. Strain gage technology and accuracy are discussed on an internet site2. Strain gages are subject to both zero drift and span drift. To understand these two types of drift, think of a bathroom scale. Zero drift is the failure of the scale to read zero when you get off. Span drift is an incorrect reading of weight when you step on the scale. Both forms of drift, zero and span, result from change in strain gage properties with temperature and with aging of the glue used to attach the gage element to the measurement point. Strain gage systems can be calibrated using weights to produce a known force on the bike pedals. There are potential errors in weight calibration because the force can both twist the measurement element and also bend it. Strain gage signals due to twist or torque is data and any due to bending is an error. Re­ported accuracy in terms of mean error scores for SRM and Power Tap factory calibration over a range of 50 - 1000 W were 2.3 +/- 4.9% and -2.5 +/- 0.5%, respectively3. Accuracy for SRM and PT was not largely influenced by time and cadence; however, power output readings were noticeably influenced by tempera­ture (5.2% for SRM and 8.4% for PT). During field trials, SRM average and max power were 4.8% and 7.3% lower, respectively, compared with PT. Calibration and strain gage errors are also discussed in reference 4, which reports also a comparison of SRM, Power Tap and Polar mobile bike power measurement systems. This article4 suggests checking zero on each ride and checking span calibration at frequent intervals.

CompuTrainer Accuracy

The CompuTrainer system uses the bike rear wheel to drive a copper flywheel, spinning in the field of an electromagnet. The accuracy depends on knowing the rolling drag of the bike wheel driving the flywheel and the accuracy of calibration of the drag versus rpm versus magnet electrical current. The rolling drag is determined by a calibration procedure from the rate of slowing of the known mass flywheel at a given force on the friction roller and determined by user test. The drag generated by the electromagnet on the spinning copper disk depends only on the electrical properties of copper, RPM and the intensity of the magnetic field. The electrical and magnetic properties of copper are predictable functions of temperature and compensated for in software. The drag versus RPM versus current are constant because the geometry of the electromagnet and the location of the copper flywheel are unchanging. The drag versus RPM versus current were initially calibrated during development of this product with literally thousands of measure­ments for an accuracy of better than +/-2.5%.


Near Infrared Spectroscopy (NIRS)


C. Dean Kurth MD




Cincinnati Children’s Hospital


Professor of Anesthesia and Pediatrics


University of Cincinnati College of Medicine






Determination of the accuracy of the NIRS devices has been problematic. In order to determine accuracy, the device must be compared with a gold standard. Because there is no gold standard for NIRS (ie, no other device measures O2 saturation in the tissue circulation), determination of accuracy remains an estimate. There is currently one FDA approved device, made by Somanetics. Its accuracy on the FDA application was compared relative to a weighted average (SwO2) of arterial and jugular bulb O2 saturation. In adults and children, the device is not that accurate (+10-15%) on an absolute level of oxygenation (rSO2 vs SwO2), but is fairly accurate (+5%) on a change in oxygenation ( rSO2 vs SwO2). In other words, the device indicates a change in oxygenation accurately but does not indicate accurately what the oxygenation actually is.


Other devices purport greater accuracy than the Somanetics device. However, these devices are not FDA approved, not commercially available, and have not been subject to accuracy testing on large scale. Nevertheless, several of these devices have been tested in animal models and have been found to be accurate +3% on an absolute.

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