Cardio Zone Trainer

Effort intensity 55%

Interval cycle length 30 s

Age (sets max HR) 30 yr

Heart rate

–

Zone

–

Calories burned

–

Session time

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How it works

Heart rate does not jump to a new value instantly — it chases a target set by effort, and closes the gap a little every frame, faster on the way up (sympathetic drive) than on the way down (parasympathetic recovery). Interval mode swings the target between a hard effort and an easy one on a repeating cycle, which is exactly what makes HIIT push heart rate through more zones than steady pacing at the same average intensity.

maxHR = 220 − age
targetHR = restHR + effort · (maxHR − restHR)
HR += (targetHR − HR) · k · dt      (k faster rising, slower recovering)
zone  = round((HR / maxHR) · 100)  →  Z1..Z5 bands

Effort intensity — fraction of heart-rate reserve demanded right now; in interval mode this is the "work" effort, alternating with a fixed lighter "rest" effort.
Interval cycle length — total seconds per work+rest pair once HIIT mode is on; shorter cycles never let heart rate fully settle before the next surge.
Age — sets max HR via the 220−age estimate, which rescales every zone boundary and therefore the color the runner marker turns at a given effort.
Zones — Z1 warm-up (50–60% max), Z2 fat-burn (60–70%), Z3 aerobic (70–80%), Z4 threshold (80–90%), Z5 max effort (90–100%); the runner speeds up and the heart pulses faster as heart rate climbs through them.

Real-world relevance: this is the same target-zone logic behind chest-strap and wrist heart-rate training guidance — steady-state cardio parks in one zone, while interval training deliberately oscillates across several to build both aerobic base and anaerobic capacity.

Z1–Z2 · easy Z3 · aerobic Z4 · threshold Z5 · max

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