Are heart rate monitors on fitness trackers accurate?

Several fitness trackers claim the ability to measure your heart rate, but are they that accurate or reliable?

Chek out CNET's Sharon Profis with help from Kaiser Permanente cardiologist Dr. Jon Zaroff put the Garmin VivoFit, Withings Pulse O2, Basis Carbon Steel, Samsung Gear Fit, and Samsung Galaxy S5 to the test.