The old rule has been discredited. There is way too much variation in the human population. The only way to gauge HRmax is in a laboratory. Many ahtletic people figure their HRmax based on what they see on the heart rate monitors at peak exertion -- such as climbing a hill.

I really think that anyone who has questions about the behaviour of their heart should consult a doctor and at least get an ECG done.

I've had one heart attack around the age of 42, and I am cautious about just how far I push myself. I also occasionally get what effectively is a double heart beat tht makes it appear as though I have missed a beat. My GP said it is a double contraction right on top of each other; he was familiar with my cycling, and said it shouldn't be a significant issue.

I am on a daily dose of enteric-coated aspirin, and that's it. I get warned sometimes that my cholesterol levels might be a bit high, but I try to control that with diet, and reduce the amount of red meat I might be eating.

As to heart spasm, I have a theory: The heart is yet another muscle in the body. When we exercise and sweat and become dehydrated, I cannot see why the heart should not react in the same way as any other muscles in the body -- if the salts (sodium and potassium) are depleted along with the fluids, it might just cramp, which translates in to a form of heart attack.

I am not medically qualified in any way, so consider what I say as hogwash, if you wish.