Thanks to outfits like CrossFit, high-intensity interval training, or HIIT, is now a household phrase.

Before getting into the benefits of HIIT and how to transform your treadmill workout, here’s a little background: Exercise intensity and energy systems are unavoidably linked. You have one energy system — the aerobic system — which can deliver all the energy you need for minutes to hours of low- to moderately high-intensity activity. You have other systems — the anaerobic systems — which kick in for those times when you need to work harder or faster and require more energy than your aerobic system can provide. The drawback of these anaerobic systems is that they produce byproducts such as lactic acid that cause discomfort and, with very high intensity effort, ultimately accumulate to levels that prevent you from going any further at such high workloads.

You might think it’s a bad idea to work so hard that you produce substances that inhibit further exercise. Actually, the opposite is true. Many of the benefits we associate with exercise correlate with very high workout intensities and correspondingly high lactic acid levels. In fact, maximal or near-maximal bouts of activity lasting up to 60 seconds provide benefits that simple “high-intensity intervals” of longer duration and sub-maximal effort do not, especially for the amount of time spent.

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