Metric guide
Every zone boundary inherits the error in its anchor
The useful version
Choose the best anchor you have, keep the model consistent and treat every boundary as a range rather than a cliff.
Why zone models disagree
Some systems divide percentages of maximum heart rate. Others use heart-rate reserve or lactate threshold. The same workout can therefore receive different labels in different apps.
Before comparing distributions, compare the model, the anchor value and the number of zones.
Choosing a better anchor
Age-predicted maximum heart rate is a population estimate. A reliable observed maximum or a well-executed threshold test is more individual, though neither is perfectly fixed.
Sport matters too. Running and cycling can produce different heart-rate responses, so one shared set of zones may be a poor fit.
Reading a zone distribution
The weekly pattern can show whether easy work is drifting harder or whether intended intensity is actually present. It cannot judge the plan without knowing the purpose of each session.
Look at time near a boundary with common sense. A one-beat difference does not create a different physiological state.
Go one step further
Questions
What people usually ask
Which heart-rate zone model should I use?
Use the model that matches your available anchor and training plan. A measured threshold or a reliable observed maximum is generally more personal than an age-based estimate.
How much time should be in Zone 2?
There is no universal percentage for every athlete. Your sport, training phase, session definition and zone model all change the answer.
Why do Garmin zones differ from another app?
Platforms may use different anchors, percentages and zone counts. Check the configured maximum heart rate or threshold before comparing the labels.