Oly87 said:
... I am interested in knowing how people define the work done in a training session when viewing Power outputs...
Follow Alex's links for more information, but in terms of quantifying a training session folk's look at:
- Sustained Average Power for various durations of interest where the durations relate to specific metabolic systems/processes targeted during that training session. So it might be looking at AP for 5 minute intervals if the workout was targeting VO2 Max work or AP for 15 - 30 minute sustained efforts if the workout was targeting Threshold work. Approaches like that are generally more useful and informative than looking at overall AP for the workout.
- Total energy in kilojoules for entire workout is a literal measure of 'work' in the physics sense and useful in terms of planning refueling or guiding weight loss programs. It's the simple integral of AP over time and doesn't account for varying stresses very well but it maps pretty closely to Calories burned which is a useful metric.
- Normalized Power (NP), described in detail in Alex's link but it estimates effective steady state metabolic load from a variable workout on the premise that short duration hard efforts take more out of us than lower intensity efforts. So a workout with a bunch of short sprints followed by rests is tougher than a steady ride with similar AP. NP only starts to make sense for rides/efforts longer than 20 minutes or so as it attempts to estimate effective 'metabolic' stress so it's most applicable for durations that are long enough to be primarily metabolic. Not a lot of value in calculating NP for 3 to 5 minute efforts as a training metric but NP is pretty useful in terms of understanding why a one hour race with many speed changes (like a criterium) is much harder than riding an hour steady to yield the same AP.
- Intensity Factor or IF derived from NP and the rider's sustainable one hour power (FTP) is a good indication of how hard the workout was relative to the rider's individual and current fitness level. So it helps to describe why sustaining 250 watts for an hour or more is tough or impossible for many newer riders or amateur racers but hardly a workout for top professionals relative to their capabilities.
- Training Stress Score (TSS) is a good single ride metric for overall training stress taking into account both the relative rider specific IF described above and the way that sustained relatively hard workouts are more stressful than sustained relatively easy workouts. So the score is weighted both to the individual rider's current fitness level and to show that intense workouts are more stressful than similar workouts even if the total kj or Calories burned are similar. TSS is an aggregate metric similar to total work in kj so it doesn't in and of itself give insight into the composition of the training that led to that score. It's still a very useful metric but has to be handled with caution when viewed in isolation or it can lead us down the high mileage/low intensity path as that's an easy way to rack up a lot of TSS but not necessarily the best way to develop bike racing fitness.
As Alex points out there are additional metrics that accumulate over time and are very useful for tracking training over days, weeks, months and years(CTL, ATL, TSB) and there are other ways to process power data to gain insight into things like torque/leg speed requirements for different activities(Quadrant Analysis),. individual rider physiological characteristics (Monod-Scherrer CP Analysis), rider/bike aerodynamics (CdA regression or Chung method testing) and so on but in terms of single ride metrics it usually comes down the bullets above in addition to the traditional metrics of ride duration, mileage, possibly elevation gained, etc.
Good luck,
-Dave
