To compare different workouts, consider time, distance, pace… and effort.
Over the last decade, high-intensity interval workouts have bubbled to the top of the fitness list. They’re quicker, more time-efficient and just as effective at boosting fitness and health – or more effective, by some measures – compared to traditional “continuous” workouts.
Of course, this isn’t earth-shattering news to runners, who have been relying on high-intensity interval workouts for more than half a century. Roger Bannister’s staple workout in the 1950s was 10 x 400 metres; Paavo Nurmi ran short sprints in the 1920s.
But are interval workouts really better than the alternative, or are they just different?
To answer that question, you have to think carefully about how you compare different workouts, to ensure that you’re pitting apples against apples. In a comparison of interval and continuous workouts, should the workouts last the same amount of time? Cover the same distance? Require the same amount of effort?
None of these comparisons is perfect, because the workouts are inherently different. Interval workouts are, by their nature, shorter and more intense. That’s the fundamental trade-off they offer: work harder, and your workout can be over much sooner, while still providing the same benefit as a longer workout.
The way most interval-vs.-continuous studies are designed is by matching the total amount of work. (That’s most easily done with exercise bikes, where you can fix the power required to turn the wheels, but similar principles can apply to any workout.)
For example, a few months ago I wrote about a study  that found greater mitochondrial adaptations after interval training compared to continuous training. The two training protocols were “work-matched” and “time-matched”: either 30 continuous minutes at 50 per cent of peak power; or an interval workout alternating 5:00 at 65 per cent of peak power with a recovery period of 2:30 at 20 per cent of peak power, repeated four times.
If you do the maths, you find both workouts last 30 minutes and have an average power of 50 per cent. And sure enough, the interval workout stimulated greater mitochondrial growth.
But if you look more closely, you see that the extra benefits don’t come for free. By the end of the workouts, the subjects reported perceived exertion (on a scale of 1 to 10) of 3.9 after the continuous workout, and 5.6 after the final hard push of the interval workout. Even though the total amount of work was the same, the interval workout felt harder.
That’s the point that researchers at Foro Italico University in Rome investigated in a study published a couple of years ago in PLoS ONE, which I just saw thanks to a tweet from interval-training researcher Martin Gibala.
The researchers decided to compare interval to continuous workouts using an “isoeffort” approach – a concept suggested by Norway-based researcher Stephen Seiler in which the goal is to expend the same amount of effort in the two cases. They compared 30 continuous minutes with three different 30-minute intervals workouts (hard sections of 20, 30, or 40 seconds repeating every minute).
In each case, to match effort, the instruction to volunteers was simply to do the overall workout as hard as possible, which is similar to how people generally approach hard workouts in the real world.
The results: the continuous workout produced higher average values of heart rate, oxygen uptake and total work compared to the interval workouts. This was a short-term study, so there’s no data on how fitness adaptations would have added up over weeks and months. Still, it’s reasonable to wonder whether the apparent superiority of interval workouts for adaptations like mitochondrial growth would still show up in this type of effort-matched comparison.
Does this mean interval workouts are better or worse than hyped? Not really. It’s just worth clearly understanding what the pros and cons are. If you’re constantly crunched for time, then “more effort in less time” might well be a great trade-off for you to get the same benefits. The big surprise of the last decade or so has been that you really can replicate many of the benefits of prolonged aerobic exercise with short interval workouts – with the very important but sometimes neglected caveat that you really do have to try harder!
In the bigger picture, of course, framing the debate as a “which is better” question between intervals and continuous runs is the wrong way to think about it. Runners have relied on interval workouts for decades, but (with a few notable exceptions) always as part of a varied program that includes long runs, easy runs, tempo runs and so on.
A steady diet of any one of these workouts would result in some improvements, but each is subtly different, so mixing them together is even better. Having workouts with different durations, structures, intensities, and efforts is probably good physiology – and perhaps more importantly, it keeps things interesting.