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.
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.
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