Distance running trains the heart, intervals train the muscles
“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.