Before we discuss how to effectively use a power meter in your training and racing, it is important to define common terms associated with power. Many of the below terms were created by Andy Coggan and Hunter Allen as ways to objectively model the stress of training. If you are interested in learning more about these therms I highly recommend you read their book, Training and Racing with a Power Meter.

Power - A measure of the work over time done by the rider to move the bike forward. Typically measured in watts. Power = Torque x RPM

Average Power (AP) - The mean power output over a given period of time.

Functional Threshold Power (FTP) - The highest mean average power an athlete can sustain for one hour. This is not to be confused with Anaerobic threshold, Lactate threshold or Ventilatory threshold, which are all terms used to describe points in the intensity spectrum when an athlete is accumulating lactate and hydrogen ions in the body’s fluids. However, when a person is in good shape all of these these various measures of intensity can be maintained for about one hour.

Normalized Power (NP) - Is calculated by taking a 30 second rolling average of the power values. This can be thought of as the average power (AP) without the spikes and zeros (when you are coasting). The normalized power provides a better measure of the true physiological demands of a given training session. It is an estimate of the power that you could have maintained for the same physiological "cost" if your power output had been perfectly constant.

Peak Power (“Critical Power” or “Mean Max Power”) - The average power (AP) one can sustain for a given number of minutes.

Power/Weight Ratio (W/kg) - A measure of power over a given period of time relative to the body weight of the athlete. This is usually expressed as the power to weight ratio at functional threshold.
W/kg = FTP / Body weight in kg

Variability Index (VI) - A measure of the continuum from steady to non-steady pacing. It compares normalized power (NP) and average power (AP).
Variability Index = NP / AP

Intensity Factor (IF) - Indicates how hard a ride is in terms of intensity only. Calculated buy taking the ratio of the riders normalized power (NP) to the riders functional threshold power (FTP).
Intensity Factor = NP / FTP

Training Stress Score (TSS) - An indicator of a workouts training load or physiological stress resulting from the interplay of intensity and duration.
Training Stress Score = (duration in seconds X NP X IF) / (FTP X 3600) X 100 or more simply as
TSS = IF^2 X duration in hours X 100

Written by Kyle Visin
Santa Barbara Triathlon Club Co-President
USAT Level 1 Coach
Certified CycleOps Power Coach
Co-Founder of KillerTri 

A duplicate post is also available at