From an aviation medicine point of view a heart rate in the 'normal range' roughly 50-90 beats per minute, depending on aerobic fitness, is what an aviation medicine examiner would be happy with. A pulse rate of 20 would certainly get you grounded straight away, and quite rightly, since levels this low are associated with disease and sudden death. I agree with previous post that it is very unlikely that Steve Redgraves pulse was ever that low, but I don't know for sure.