Cardio Zone Trainer (2D)

This simulator models how heart rate actually behaves during cardiovascular exercise: it chases a target set by effort rather than snapping to it, rising faster than it recovers, exactly like a real cardiovascular response. A runner circles a five-zone training ring — colored from easy green through max-effort red — speeding up as simulated heart rate climbs, while a pulsing heart shape beats in sync with the live BPM. Switch between steady-state pacing and interval (HIIT) mode, which swings effort between hard work and easy recovery on a repeating cycle, and watch how each strategy moves the runner through different zones and accumulates calories differently over the same session.