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The heart rate variability analysis (HRV) test utilises the Nerve-Express system which depicts a visual representation of heart rhythm called rhythmography. 

Rhythmography can graph the relationship between the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS). 

This reflects the body's regulating process and can help determine one's overall degree of physical fitness, and current psychological and emotional states. the key advantage is the method's ability to detect early signs of the development of pathological processes or the presence of some functional disorder which may not be revealed by an ordinary physical examination.