TRAINING TERMS
Zone 4
A higher band that the App arranges in work bouts with easier recovery between them.
80% to below 90% of reserve
Zone 4 starts at resting heart rate + 80% of reserve and ends just below resting heart rate + 90% of reserve. This is a band based on an estimated maximum; it is not a measured physiological threshold.[1]
The recovery is part of the work
When the App selects Zone 4, it includes at least two work bouts with planned Zone 1 recovery between them. The selected durations depend on actual training evidence and the complete session’s limits. They are not a standard interval recipe for everyone.[1]
Read the phase, not just the pulse
After harder work, heart rate can remain elevated as you ease off. Follow the recovery phase rather than treating an elevated reading as a reason to extend the work. Reaching this band does not prove that a particular pace or duration is appropriate.
Sources & further reading
Reviewed September 14, 2026