Distance running trains the heart, intervals train the muscles


As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)


“High-intensity interval training” (HIIT) has been receiving lots of research attention recently as a time-efficient way to get in shape. An interesting pre-print has just appeared in Medicine & Science in Sports & Exercise (thanks to Amby Burfoot for noticing it) that adds a couple of interesting wrinkles.

First of all, the study used running rather than the usual stationary biking — so it provides some evidence that the same kinds of protocols that have been extensively studied in cycling also apply to running. Researchers at the University of Western Ontario had 20 (untrained) volunteers perform six weeks of training, three times a week. One group ran steadily at 65% VO2max, starting with 30-minute runs and building up to 60 minutes; the other group did 30-second sprints with four-minute recovery, starting with four repetitions and building up to six.

As expected, the sprinters improved almost exactly as much in a variety of outcomes as the subjects doing long, sustained runs. They both increased VO2max by about 12%; they both increased 2,000-metre time trial performance by about 5%; they both lost fat (the sprinters lost 1.7 kg while the long-runners lost 0.8 kg). So yes, the HIIT paradigm is applicable to running.

But the study offers one more twist. They measured maximal cardiac output (Qmax), which is the biggest volume of blood your heart can pump in a given amount of time. In this case, the long-runners increased Qmax by 9.5%, while the sprinters didn’t improve at all.

To understand what this means, consider that VO2max (the maximal amount of oxygen you can deliver to your muscles in a given amount of time) is the product of two quantities: Qmax (how much blood your heart can send to the muscles) and “maximal arterial-mixed venous oxygen difference” (how much of the oxygen sent to your muscles is actually extracted from the blood and used by the muscles before the blood heads back to your heart). This latter quantity (the researchers write) depends on “O2 delivery to active muscle fibres (blood flow distribution, capillary density, and arterial O2 content), local enzymatic adaptations, and mitochondrial density/volume.”

So what the study tells us is that short sprints and long, steady runs both increase endurance, but they do it in different ways. Sprints act peripherally (i.e. the muscles), while long runs act centrally (i.e. the heart).

Of course, nothing is quite so black-and-white in real life, and many types of training session will stimulate both types of adaptation. Still, it’s a good reminder that the best training programs will balance different types of stimulus — which is, of course, what every elite runner already does. But those looking to HIIT as a way of getting fit should ideally try to also make time for at least one more sustained session each week.

6 Replies to “Distance running trains the heart, intervals train the muscles”

  1. @barnee
    Not only that, but I knew (of) Amby Burfoot for at least a decade before I met Amby S. — so his name never struck me as unusual!

  2. Alex, this study is really interesting for me. The ‘research’ I’ve read and the general knowledge ‘out there’ on the two types of training (65% VO2/SS vs. HIIT) says that HIIT improves the heart (higher stroke volume) and SS increases capillary density and the number of mitochondria in the cells (among other things). One grey area for me is whether SS workouts are a type of recovery or whether in and of themselves they represent something equally as important as HIIT if not more important (I am leaning very much towards the latter). The current knowledge seems to indicate that runners/cyclists/rowers/swimmers should have a training program that includes 3-5 HIIT workouts per week with SS workouts surrounding the HIIT work (a 80/20 split of SS and HIIT). The SS sessions are essentially recovery FROM the HIIT work (with the emphasis on the HIIT sessions being the work that stimulates the adaptations).
    I guess I haven’t really asked a question but my impression is that HIIT leaves the body sore/tired/whatever, while SS usually leaves one feeling as though they could manage another hour or so of continued exercise. Because of theses differences in how the body feels, people have associated SS with recovery because it doesn’t push the limbs or ‘smash the legs’ like HIIT does.
    Anyways, a big strong heart with high stroke volume is what I want, so I’ll be focusing on my 65% VO2/1.5-2mm lactate level work.

  3. Interesting stuff, Marc. These are good questions, and I don’t think anyone really knows the answers — but I agree that it definitely hints at the fact that there important adaptations to both kinds of training, so steady-state training is NOT simply recovery.

    One caveat for studies like this one, once again, is that they’re very much a test of different extremes. Here, the HIIT was 4-6 x 30 seconds. If we’re talking instead about an interval session of, say, 5 x 5:00, then we’re likely looking at something in between. Rather than workouts being type A or type B, most probably lie somewhere along a continuum — and my naive intuition is that there are benefits to hitting all parts of that continuum, both the extremes and the middle regions. The art, I guess, is getting the right mix of those ingredients.

    Bottom line: I agree that we shouldn’t make the mistake of thinking that only the workouts that leave us feeling smashed have real benefits.

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