There's a sub-branch of philosophy called "trolleyology" that deals with moral choices, named for a thought experiment devised in 1967:
You are walking near a trolley-car track when you notice five people tied to it in a row. The next instant, you see a trolley hurtling toward them, out of control. A signal lever is within your reach; if you pull it, you can divert the runaway trolley down a side track, saving the five — but killing another person, who is tied to that spur. What do you do?
Subsequent iterations tweak the problem to change how actively you have to kill one person to save five -- e.g. the "Fat Man" version of the problem requires you to throw an unsuspecting bystander off a bridge to stop the trolley and save the other five people. The way people answer these questions tells us a lot about how we view right and wrong.
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I couldn't help thinking of these dilemmas yesterday when a doctor friend asked about the message given to a group of GPs by a cardiologist at a continuing medical education course -- that for people over 40, the risks of interval training outweigh the benefits. The common thread: while it may not seem rational, most humans seem to prefer taking a passive role in a greater harm than an active role in a lesser harm.
The central trade-off here is the same one that applies to all exercise: you have a slightly increased risk of a cardiac event during exercise, and in exchange you get a slightly decreased risk of a cardiac event during all the hours you're not exercising. This is true for marathons, sprints, moderate continuous exercise, intense interval exercise, you name it. That's why doctors used to discourage heart-attack survivors from doing any exercise -- until they realized that avoiding exercise ultimately made a recurrence more likely rather than less likely. This caution is understandable: if you're a doctor and you say "Do this exercise program," and then the patient dies while doing the program, you feel responsible. On the other hand, if you say "Don't exercise," and the patients later dies while watching TV on the couch, you don't feel quite the same responsibility.
But leaving aside the trolleying, what we really want to know is: what gives you the best overall odds of surviving, regardless of who's "responsible"? Is the cardiologist I mentioned above correct in saying that the risks of intense exercise outweigh the benefits for people over 40?
It's not even close, as far as I can tell. It's so far from being debatable that no one even bothers to study it. Instead, researchers look at much more high-risk groups, like older people recovering from cardiac events such as heart attacks, heart surgery, and heart failure. The best study I can find looking at the risks of high-intensity interval training is from a Norwegian group in 2012, published in the journal Circulation (and freely available online). The cohort in that group was 4,846 cardiac patients with an average age of 57.8 (i.e. far higher risk than the "average" 40-year-old). The results:
- During and after 129,456 hours of moderate-intensity exercise, there was 1 fatal cardiac arrest;
- During and after 46,364 hours of high-intensity interval exercise, there were 2 non-fatal cardiac arrests.
These numbers are so small that you can't make any real comparisons between them. The overall conclusion, the authors say, is that the risk of cardiovascular events during exercise for cardiac rehab -- whether traditional moderate continuous or high-intensity interval -- is very low.
What about the benefits? The authors cite a whole pile of research on the cardiovascular benefits of high-intensity exercise: "[E]pidemiological studies consistently found a greater reduction in risk of cardiovascular disease with high- (>6 metabolic equivalents) in comparison with moderate-intensity physical activity." This holds true even if the total amount of work is held constant (i.e. you spend longer at lower intensity versus shorter at higher intensity). For example, there's some research suggesting that only high-intensity exercise produces beneficial adaptations to the left ventricle of the heart.
That's not to say I'm dismissing the benefits of plain old continuous moderate exercise. The ideal approach, in my view, is to exercise in a way that incorporates a wide variety of exercise intensities, from relaxed all the way through to very hard. Is there such a thing as "too hard"? Probably in some contexts -- but it's all relative. If the cardiologist had said "The risks of interval training outweigh the benefits if you're a 70-year-old with high blood pressure and a history of heart disease who hasn't exercised in 40 years," then he might have a point. The right (and safest) exercise intensity depends on the context and the individual, and I'd certainly recommend proceeding cautiously and increasing gradually, especially if you're starting from scratch. But speaking as a 38.5-year-old, if I had a cardiologist who told me that starting in 1.5 years I should stick to croquet and make sure not to watch any exciting TV shows for the safety of my heart... well, I'd be looking for another cardiologist.
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