Quick note about a recent study that addresses a very common question, though it doesn't settle anything definitively. The question is: should you walk or jog during the recovery periods in an interval workout? A group of researchers in France and Tunisia tackled this question in a study published in the European Journal of Applied Physiology. You can read the abstract here, but the gist is as follows.
A group of 18 volunteers were assigned to do a seven-week interval training program, doing workouts three times a week. Each workout consisted of repetitions of 30 seconds "hard" and 30 seconds "easy." The hard sections were at 100 to 110% of "maximal aerobic velocity," which is basically the speed you reach during a VO2max test just before you fall off the treadmill. The easy sections were either jogging at 50% of maximal aerobic velocity, or doing nothing.
To balance out the overall training load, the group taking passive rest did 50% more hard repetitions -- for example, if the active recovery group did 10 reps, the passive recovery group did 15 reps. I understand why they did this (to balance the training loads), but this means that the study isn't really tackling the question I'm most interested in -- which is, if I do a given workout (e.g. 10 x 400m with 1:00 recovery), what are the pros and cons of active versus passive recovery?
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Anyway, the researchers did a whole battery of tests on the runners before and after the training period, but there's really only one result that matters: the active recovery group got a big boost in VO2max, but the passive recovery group didn't. This is despite the fact that both groups spent roughly the same amount of time overall at high intensities (greater than 90% and 95% of VO2max). Why is this? One effect they discuss is that passive recovery allows you to regenerate your stores of phosphocreatine -- the "instant energy" stores that you use for the first 10 seconds or so of a sprint -- more effectively. That means in each sprint, you're getting an extra boost from phosphocreatine, whereas with the active recovery, you have to rely more fully on the standard aerobic and anaerobic energy sources that matter to endurance athletes.
The takeaway? On the surface, it's that if you're interested in endurance you should jog (or whatever sport-specific intensity applies in your activity of choice) your recoveries. Of course, in the real world, it's a bit more complicated. If you're a middle-distance runner, you'll sometimes be doing workouts where the focus is on speed, not endurance. And there's not always a black-or-white distinction between different types of workouts -- so when should you walk and when should you jog? For now, that remains art, not science.
