Carbonfiberboy 

Personally, I ignore the medical studies because all them which I've read contain either implicit or explicit bias in the direction of "more is better." I just look around and have a lot of serious riders to look at. I've been a member of a fairly serious invitation-only recreational riding group for 25 years with over 120 members. Most of them don't show up for rides anymore.

I've talked to many of those who no longer show up and the usual response is, "I'm not fit enough to ride with you folks anymore." At 52, I was one of the older folks when I joined, and one of the least talented. I trained and rode hard enough to get to be the A group leader for a few years and as soon as I wasn't on the front anymore and able to easily run the paceline up and down, I backed off on the intensity volume. Lucky boy.

I've never had an irregular heart beat problem. However some people in the group have had Afib. Some dropped out and others got a ablation which enabled them to continue. So who got Afib? The fastest climbers in the group. Why? They either did more high intensity riding than anyone else or they were k-hounds and they aged out of being able to keep that up without damage. It's rather a time-in-grade issue. And then there was the wonderful person, fastest current climber, who went BC skiing, came home, cooked dinner for his wife, watched a movie, went to bed and had a fatal heart attack with never a symptom That's not good, either. I organized his memorial ride.

The high intensity folks would go out and do long, steep climbs several days a week. The k-hounds had a great pile of distance medals, starting with 10,000 k/year of strictly brevet riding. Brevet organizers publish rider times on the web, so it's not just riding long distances, it's racing to see who can ride a brevet, especially PBP, the fastest, etc. All that takes a toll over the decades. I'm not aware of anything of this nature being investigated in the literature, but it's obvious if one rides with a lot a very fast long distance riders who've been at it a long time.

Yesterday, at 77, I rode RAMROD, 162 miles and 10,000' this year. It was one of the hotter RAMRODs I've ridden, with temps up to 100°. Nobody died, but a rider I tried to help was sagged off the course and had to get IV rehydration at an ER. I had bib #7, the 7th oldest ride to enter. Rider #1 had to be sagged off the course this year, not uncommon.

Basically, my guess is that there's a mathematical model which could be constructed using total time, time at the various intensities, and longest ride length which might give a reasonable result for Afib risk. I don't think total time alone is a risk, because we had a group member who did 20K-30K yearly mileage totals and up to 1.5M feet climbing. He's fine. Percentage of time spent at high intensity X total time looks like a risk factor. Very long rides with a large percentage of time spent above some unknown limit looks like a risk factor.

I think if one did enough HIIT to be able to ride PBP in under say 70 hours, that might just do it, and probably before that target date - or maybe one has to do that for a few years for enough damage to occur. As far as I can tell, no one really knows what's too much of what. There's some chance it might be individual, but I rather doubt that. The Afib-ers I know are a rather diverse lot. They just have that one thing in common, too much total time at high intensity.

BTW, the more total time one spends at high intensity, the faster one gets - for a while. It's quite seductive.