HRV For Training

The below experiment from early 2013 compares the accuracy of a Mio Alpha,  Armour39 and Polar H7 for tracking HRV.  The Polar H7 was the most accurate, which is what I would expect.  Chest straps are still the gold standard, although patches placed on the chest, such as VitalConnect HealthPatch work just as well. There is also a lot of development effort being put into optical sensors for use in watches/wrist bands to support continuous HRV monitoring.

http://www.marcoaltini.com/blog/heart-rate-variability

Also, a short article on why monitoring HRV trends is so important.

http://www.220triathlon.com/training/heart-rate-variability-what-it-is-and-why-you-should-measure-it/8547.html