The number can peak during recovery
Suppose you speed up for a short hill and then slow down. The display may continue rising briefly after the hard part ends. That does not necessarily mean the easier movement is physiologically harder; the cardiovascular response and measurement have their own timing.
Using heart rate as the only control signal can therefore be awkward for very short efforts.
The chasing problem
If you accelerate repeatedly because the target heart rate has not appeared yet, you may overshoot the intended effort. By the time the number catches up, the work period can be much harder than planned.
For a short interval, exercise-specific pace or power and perceived effort may describe the task more promptly, although each also has limitations. The appropriate measure depends on the session and the person.
Compare the right parts of a session
A low average heart rate across a workout with long recoveries does not show that its hard intervals were easy. Conversely, a high peak does not summarize the entire session. Record the work and recovery arrangement alongside the summary number.
Heart-rate targets are tools for describing exercise, not requirements that every repetition must satisfy before it counts.
