Bradycardia is usually caused by an electrical problem and I don't believe that there is much training/diet wise that you can do for it. My wife had always had a low heart rate but being an endurance cyclist it was something that never caused concern, until the day it got down to the low 20s. She still had a low normal BP and her oxygen levels were fine at that rate, but when the cardiologist saw her ECHO she was moved to the front of the pacemaker schedule as she was in complete heart block. If you are having symptoms you should discuss it with your Dr- it's better to get a test that tells you that you just have a normal athletes heart than to not get one and discover that your resting pulse wasn't just your great fitness. It's possible to be in great shape but still have an issue.