Why changing FTP changes your zones
Why your zone totals change
Choose a different FTP in the experiment. The recorded watts do not move. The boundaries do, so the bars for time in each zone change. A different zone report can be a classification change rather than a different workout. Check this before deciding that your training intensity has changed.
This distinction matters when comparing exported files, old rides and platforms with different threshold histories. Check the reference value before explaining a dramatic change in time spent at intensity.
Change FTP to see which zone 200 W falls into. The ride itself stays the same.
Power during the ride
Time in each zone min:sec
Both example rides have the same duration and average power. Coggan-style bands; boundary values go into the lower band.
Why your zone totals change
Choose a different FTP in the experiment. The recorded watts do not move. The boundaries do, so the bars for time in each zone change. A different zone report can be a classification change rather than a different workout. Check this before deciding that your training intensity has changed.
This distinction matters when comparing exported files, old rides and platforms with different threshold histories. Check the reference value before explaining a dramatic change in time spent at intensity.
How these zones are calculated
The example uses Coggan-style power bands based on FTP. These bands are a useful language for describing intensity, but are not sharp biological switches. Other systems use different counts and boundaries.
For exact reproducibility we place boundary values in the lower band: up to 55%, 75%, 90%, 105%, 120% and 150%. The remaining watts occupy the top bin. That top bin is a plotting convention here, not a claim that all neuromuscular work can be identified by one percentage.
Average power and time in zones
Compare a steady ride with a variable one. In this example they average the same watts while spending very different amounts of time in the bands. Even identical time-in-zone totals can be arranged as long continuous efforts or many short bursts.
The order of the seconds has disappeared from a histogram. Return to the trace when you need to understand interval length and recovery.
Checking the settings behind your zones
Record which zone system and threshold the report used. Check whether a platform applies today’s FTP to old rides or retains the value from the ride date.
The same classification principle applies to running pace or heart-rate zones: changing the threshold or boundaries can change time in each zone without changing the session. The FTP percentages in this cycling example must not be copied into a pace or heart-rate system.
Treat zones as a way to describe and organise training. A colour change at a boundary does not mean your body abruptly changed energy systems, and a single histogram cannot establish workout quality.
Why one threshold creates several zones
A zone system takes a reference value and applies boundaries to it. In this example, Zone 2 ends at 75% of FTP. At an FTP of 200 W that boundary is 150 W; at 300 W it is 225 W. A recorded 200 W is unchanged, but it moves from Zone 4 to Zone 2 under these particular boundaries.
This can make an old ride look different after you edit your FTP. Some tools preserve a historical threshold and others may recalculate past reports. Before interpreting a sudden shift in time-in-zone, check which threshold and zone system were applied to the ride date.
A zone is a category, not a body switch
The body does not abruptly change how it makes energy at the precise watt where a colour changes. Aerobic and anaerobic pathways contribute together, and people can have different physiological responses at the same percentage of an estimated FTP.
FTP itself depends on a testing or estimation method. A useful zone system gives you a consistent way to plan and describe sessions; it does not prove that every minute in a coloured band produced the same adaptation. Pair the category with duration, the original trace and how the session felt.
Use the graph when you review a workout
First look at the power trace: where were the long efforts, surges and recoveries? Then look at the zone totals to see how those seconds were classified. The histogram loses their order, so six minutes in a zone might be one interval or many short bursts.
Try changing the FTP while leaving the ride selection alone. The white trace remains the same while the coloured boundaries and totals move. Switch between the steady and variable examples to see a different point: equal average power can hide very different distributions.