I've been running a lot this year. I ran in two half marathon races (one in the spring and one in the fall) so all this exercising has led to a condition called athletic heart syndrome, also known as athletic bradycardia. By definition, bradycardia is when your heart rate falls below 60 (in an adult). My resting heart rate lately has been in the 40s. Here's a picture of my ECG captured using the AliveCor heart monitor for iPhone:
Sinus brady at 45. Aymptomatic. How low will it go?