🧪 Cardio Training Simulation

This simulator models the actual physiology of a cardio workout: a runner on a treadmill whose heart rate does not snap to a target the instant you change effort, but chases it through a first-order lag exactly like a real cardiovascular system does. Set the exercise intensity and the target heart rate is computed from the Karvonen heart-rate-reserve formula; pick a fitness level and watch how a trained heart both ramps up and recovers faster than an untrained one; switch to interval training and see the heart rate oscillate between work and rest targets without ever fully settling, the same overshoot-and-lag pattern a smartwatch shows on a real interval session. Live readouts track heart rate against its target, the current training zone, and calories burned as the session runs.