November 19, 2010 cburnham

Power File Analysis: Part I

I get a lot of questions about how I go about analyzing a power file and what information can be gained from power data.  Without going too far down the rabbit hole, I thought it would be helpful to go through the process here.  To start off though I want to touch on some basics and define the terms of power. 

So what do all these lines mean?

What is a watt, a kilojoule, or functional threshold and why do we care?

One of the best tools in sport if you know how to use it!

First off, what is power?  Power is simply the rate of work being done where work is equal to force times distance.  It is measured in a unit called a watt, which in cycling terms is average effective pedal force (AEPF) x circumferential pedal velocity (CPV) or cadence.  A watt is a completely objective measure of performance so it will accurately reflect changes in ability on a day-to-day, year-to-year basis.  When comparing performances on a long term scale or against other riders it is best to use a power-to-weight ratio to account for changes in body mass, usually reported in watts per kilogram.

A kilojoule is basically the amount of energy or work done.  One watt for one second is equal to one joule.  A thousand joules is a kilojoule.  Great, why do we care?  This gets a little math heavy so if you want to skip the nitty gritty just jump down to the bold section below.  1 kilocalorie is equal to 4.184 kilojoules.  Most trained athletes are only 23 – 25% efficient.  So if an athlete does a 1000kj ride we can figure the amount of calories burned by dividing the kilojoules by 4.184 and then multiplying by 23% (its better to underestimate kilocalories) to let us know we burned 1039 kilocalories.  The equation would look like this:  KJ/4.184 *(100/23) = KCal.  This is a lot of math to basically tell us that 1KJ is roughly equal to 1KCal. That is important to assess our energy and recovery needs (tip: don’t try to make up all burned calories at once or you will be bloated like a Cartman on weight gainer 4000.)

Functional Threshold (FT) is a concept developed by exercise physiologist Andy Coggan, Ph.D. that uses your best 1-hour effort to estimate a physiological threshold.  There are a million and half concepts of threshold based on lactate, ventilation, aerobic, VO2, blood type, height, taste in music, etc.  Functional threshold is a more pragmatic concept that is actually using your true performance as determinant of a physiological threshold and is one of the best predictors of race performance.   Why an hour?  A solid hour effort will give us a maximal steady state effort with minimal fatigue from fueling or hydration that is primarily aerobic.  It is possible to use a shorter effort to estimate FT but it is important that we remove the anaerobic contribution by only taking a percentage of that effort.  For example, you could do a 20-minute effort and take 95% of that effort to determine FT. Since we are able to contribute roughly 5% of our effort anaerobically for 20-minutes we will want to “remove” that portion of the effort to get a true aerobic effort.  Note that the 5% estimate of a 20-minute effort is just that, an estimate and your exact percentage may vary depending on your personal strengths and weaknesses.   There are actually several ways to estimate your threshold.  Here is a good article by Andy Coggan on the different ways you can estimate your threshold.

Any estimates on their functional threshold?

The concept of functional threshold is important because there are a lot of other training metrics we use that are determined by threshold including estimating the stress of a workout (Training Stress Score), the overall intensity of a workout (Intesity Factor), training zones, chronic and acute training levels, normalized power, and training stress balance.  These concepts allow us to develop a performance manager chart for a quick overview of our training.  These are all terms used and copyrighted by Training Peaks that their software will figure automatically for your training.  Here are some quick definitions taken from the Training Peaks site:

Training Stress ScoreTraining Stress Score (TSS) is a composite number that takes into account the duration and intensity of a workout to arrive at a single estimate of the overall training load and physiological stress created by that training session.  By definition, one hour spent at Functional Threshold Power (FTP) is equal to 100 points. In my opinion, this is the best way to compare your training to races and make sure you are meeting the overall demands of races.

Intensity Factor: For any workout or part of a workout, the ratio of the Normalized Power to the rider’s functional threshold power, which gives the user a relative intensity in relation to their threshold power. IF is used to calculate TSS.

Normalized Power: An estimate of the power that you could have maintained for the same physiological “cost” if your power had been perfectly constant, such as on an ergometer, instead of variable power output.  NP is used to calculate TSS.

Chronic Training Level: Cumulative effect of workouts done 15 days ago and older with the norm being 42 days as determined by research.

Acute Training Level: Cumulative effects of workouts done in the last 14 days with the norm being 7 days as determined by research.

In the second part of this I will address how we can go through a ride file and analyze our performance.

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