Power Meters: A Beginners Guide – Part 1 – Understanding Metrics

Written by: Editor in Chief

So you have been riding for a while and finally decided to take your training to the next level.  You have probably purchased one of the many available units on the market (eg. Quarq, Powertap, Stages, SRM, etc) based on your needs and budget.  But now what?  Power meters are a fantastic tool in the training arsenal, but without the proper knowledge they are just an expensive toy.  This will be the first of a series of articles which will aim to provide you with enough information to use and interpret the data from your new power meter. We won’t go too deep into the details, but we will guide you in the right direction.

Terminology

PowerHeadUnitShot

A measurement of energy used over time, in this case expressed in watts.  Unlike heart rate which can drift with dehydration, altitude, or heat – a watt is a watt and is constant.  Usually expressed and displayed in real time (usually to jumpy), 3sec average, 10sec average, or 30sec average.

Average Power

The average output of the whole of your ride. This include the uphills when you are pushing 300w as well as downhills when you are coating at zero watts. Average power will be important later when we start to explain field testing.

Kilojoules (kJs)

A unit of work. If you are trying to lose weight, kJs are what you want to rack up.  Many newer head units (eg Garmin) will give you a kJs readout.  This is a good guide for eating on the ride.

Threshold Power

The single most important metric we will use.  Essentially this is the power you can sustain for one hour.  Usually expressed in Watts/Kilogram or w/kg – this provides a way in which two riders can see how they compare.  Better yet, it allows you to see which way your fitness is progressing. For instance if you produce 250w at threshold and weigh 70kg you are producing 3.57w/kg at threshold.  Now lets assume you lose 2kg and now produce 265w at threshold.  You are now producing 3.89w/kg.  However, if you gain 2kg yet put out 275w at theshold you are only producing 3.76w/kg.  In an ideal world you will gain fitness, increase threshold power, and lose a few pounds.  Your mileage may vary.

Normalized Power (NP)

A proprietary and standardized algorithm that accounts for time spent coasting, downhills, and intervals or efforts where sharp increases in power are produced. It aims to create an estimated power number for the ride or interval if you had averaged a constant power output.  Normalized power will almost always be higher than average power, especially on hilly rides or in group rides/races.

Watts/Kg

Exactly how it sounds, the relationship of watts produced to kilograms for the rider who produced the watts.

Training Stress Score (commonly referred to as TSS)

Developed by Andrew Coggan and licensed by TrainingPeaks.  This is a way in which to quantify the effort level of a ride.  A TSS of 100 is equivalent to a 1 hour all out effort. This is one of the most tracked metrics in a training plan by most coaches.

  • TSS less than 150 – low (recovery generally complete by following day)
  • 150-300 – medium (some residual fatigue may be present the next day, but gone by 2nd day)
  • 300-450 – high (some residual fatigue may be present even after 2 days)
  • Greater than 450 – very high (residual fatigue lasting several days likely)

Intensity Factor (IF)

Ratio of the normalized power to your threshold power. Provides a valid comparison between rides and riders.

Typical IF values for various training sessions or races are as follows:

  • Less than 0.75 recovery rides
  • 0.75-0.85 endurance-paced training rides
  • 0.85-0.95 tempo rides, aerobic and anaerobic interval workouts (work and rest periods combined), longer (>2.5 h) road races
  • 0.95-1.05 lactate threshold intervals (work period only), shorter (<2.5 h) road races, criteriums, circuit races, longer (e.g., 40 km) TTs
  • 1.05-1.15 shorter (e.g., 15 km) TTs, track points race
  • Greater than 1.15 prologue TT, track pursuit, track miss-and-out

Variability Index

Defined as normalized power (NP) divided by average power.  This is an indication of how smooth or even effort was distributed throughout a ride.

Part 2 will go into the various testing protocols to obtain your threshold power, a critical step in effectively utilizing your new power meter.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s