Heart Rate Variability: I should have listened to my body.

Heart rate variability (HRV) basically tracks the differences in length of time between each of your heart beats. I will not get down to the exact science of how it is measured. A Google search will give better explanation than I would. The gist of how HRV works I obtained from here. 

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Heart rate and rhythm are largely under the control of your autonomic nervous system, which is split into two branches, your “rest and digest” parasympathetic nervous system and your “fight and flight” sympathetic nervous system. 

Your parasympathetic nervous system influences heart rate via the release of acetylcholine by the vagus nerve, which can inhibit activation of SA node activity and decrease heart rate variability. In contrast, your sympathetic nervous system influences heart rate by release of epinephrine and norepinephrine, and generally increases activation of the SA node and increases heart rate variability. 

If you’re well rested and haven’t been overtraining aerobically, your parasympathetic nervous system interacts cooperatively with your sympathetic nervous system to produce responses in your heart rate variability to respiration, temperature, blood pressure, stress, etc. 

But if you’re not well rested (overtrained or inadequately recovered), the normally healthy beat-to-beat variation in your heart rhythm begins to diminish. This variability indicates sympathetic and parasympathetic nervous system balance, and proper regulation of your heartbeat by your nervous system.

In other words, the delicate see-saw balance of your sympathetic and parasympathetic nervous system no longer works. 

In a strength or speed athlete, you typically see more sympathetic nervous system overtraining, and symptoms such as:
 - High cortisol & insulin
 - Low testosterone & DHEA
 - Fluid retention and swelling
 - Highly variable HRV (heart rate variability number bounces around from day to day) 

Comparatively, in endurance athletes, you typically see more parasympathetic nervous system overtraining, and symptoms such as:
 - Adrenal exhaustion (low cortisol, low insulin, low testosterone, low DHEA)
 - Weight loss
 - “Rest & Digest” overstimulation
 - Low HRV (the heart rate variability number stays consistently low)
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This was my HRV on the morning of 10th November 2013. Though I felt a little tired, I foolishly went for a "short" 1hr training run. 

The next day I promptly fell ill with a chest infection, runny nose and fever. The above shows my attempt to rest and recover. However as of Sat afternoon, 16th November 2013, a low reading together with lingering feeling of fatigue, cough and itchiness of lungs, I decided to rest and forgo racing 42km at the PBIM. 

Whether the chest infection would have flared up in any case even if i had rested? I do not know. Perhaps. But weakening the immune system further certainly would not help. I will keep a closer eye on HRV in the next year.

I use Azumio's Heart Rate iOS App with Stand-up Test to record my HRV.  A more accurate reading would be to use an ECG accurate chest strap with systems from iThlete or Sweetbeat or similar.

(Disclaimer: I am neither a qualified medical practitioner, certified dietitian, nutritionist or fitness/ sports coach. Above views are my own from personal experiences and from what I've read, seen or heard.)