(*Normalized PowerTM (NP), Intesity FactorTM (IF), Training Stress ScoreTM (TSS) are trademark metrics of TrainingPeaks.).  This is a summary of an article by Andrew R. Coggan, Ph.D.

Why is a power meter so variable, or "jumpy" when training outside?

• Resistance changes because of elevation and wind
• It is very hard to keep power constant or in a zone during a training session outdoors

What data do you need to look?

• Normalized Power (NP)
• Intensity Factor (IF)
• Training Stress Score (TSS)

What does each of these mean and what do they tell you?

Normalized Power (NP) – it is an algorithm.  It has two pieces of information incorporated.  One has to do with how the body is impacted by intensity during duration of a work effort.  The other has to do with how physiological responses tend to follow a curved line rather than being linear.  Keeping track of normalized power quantifies the actual intensity of a workout session.  It is an estimate of the power the rider could have maintained for the same physiological "cost" if the output power was constant. 

“Normalized power during a hard ~1 hour long criterium or road race will often be similar to what a rider can average when pedaling continuously during flat 40k time trial - the normalized power from mass start races can therefore often be used to provide an initial estimate of a rider's threshold power” (Andrew R. Coggan, Ph.D.)

Intensity Factor (IF) - ratio of the normalized power to your threshold power.  This allows for comparison of workouts, and between riders.

Coggan uses these IF values for workouts:

• 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 (under 2.5 h) road races, circuit, criteriums, longer (eg 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                                

Coggan goes on to say that an IF of more than 1.05 for an hour duration race is usually an indication that the threshold power is higher than what is being used in the calculation; it eliminates the need for frequent formal testing.

Training Stress Score (TSS) – combines both the intensity and the duration of each training session.  It is a good predictor of glycogen utilized and whether a rest day(s) or easy day is required.  It is calculated as follows:

TSS=(sec x NP x IF)/(FTP x 3600) x100

Coggan suggests this as an approximate guide:

• 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)

If you are training with a power meter and have a mid to higher end bike computer, all these data points are available in the summary of your workout.

A knowledgeable coach will know how to use these data points to train their athletes appropriately.

I was asked a question on indoor training versus outdoor training and why it is common to experience higher perceived efforts working on an indoor trainer.  Here is a link to a great explanation:

http://highlandtraining.net/indoor-cycling-training-vs-riding-outside.html