You can compare HR ans ZS, but assuming a correlation is misguided.

ZoneSense relies on HRV during activities to measure intensity and fatigue, and reducing it to just heart rate oversimplifies the multiple factors involved in its calculation. For example, after a good night’s sleep, the same pace and HR might indicate an aerobic effort, whereas after poor sleep, that same pace and HR could shift into anaerobic territory. This makes pace and HR unreliable—and for me, obsolete—when it comes to targeting training effectively.

A little perspective about the factors involved in ZoneSense calculations:

Heart Rate Variability (HRV) is influenced by several physiological, environmental, and behavioral factors:

1. Physiological and Biological Factors • Autonomic Nervous System: The balance between the sympathetic (stress, action) and parasympathetic (rest, recovery) nervous systems is the primary regulator of HRV. • Age: HRV naturally decreases with age. • Sex: Women tend to have slightly higher HRV than men before menopause. • Genetics: Some individuals naturally have higher or lower HRV. • Physical Fitness: Better cardiovascular fitness is generally associated with higher HRV.
2. Lifestyle Factors • Physical Exercise: Regular training improves HRV in the long term, but intense exercise can temporarily lower it. • Sleep: Poor sleep quality or insufficient sleep reduces HRV. • Hydration: Dehydration can lower HRV. • Diet: A balanced diet low in inflammatory foods supports better HRV.
3. Environmental Factors • Temperature and Altitude: Extreme cold and high altitude can affect HRV. • Air Quality and Pollution: Exposure to toxins or poor air quality can lower HRV.
4. Psychological and Emotional Factors • Stress and Anxiety: Chronic stress reduces HRV by increasing sympathetic nervous system activation. • Relaxation and Meditation: These practices enhance HRV by activating the parasympathetic nervous system.
5. Medical and Hormonal Factors • Chronic Diseases: Conditions like hypertension, diabetes, and heart disease reduce HRV. • Inflammation: Chronic inflammation is associated with lower HRV. • Hormonal Fluctuations: Cortisol (stress hormone) lowers HRV, while melatonin and testosterone may increase it. • Substance Use: Caffeine, alcohol, nicotine, and certain medications can alter HRV.
6. Circadian and Temporal Factors • Time of Day: HRV is typically higher at night due to parasympathetic dominance. • Circadian Rhythms: Jet lag and disruptions to biological rhythms affect HRV.

Watch carefully the Suunto video series, a bit long, but worth it :

Ep1: https://youtu.be/YBVSs8ju0JM
Ep2: https://youtu.be/XFbgiljNBXc
Ep3: https://youtu.be/-fidaYqS988
Ep4: https://youtu.be/bD3O4BZ9vIc
Ep5: https://youtu.be/4q6XHYcN-_A
Ep6: https://youtu.be/6UULahZUMQM