For many years, we were told that if we wanted to properly build our base fitness, we needed to spend 12 to 16 weeks riding long, steady, low-intensity miles to strengthen our aerobic systems, so they could eventually handle harder training rides and races. Well, this method works great if it’s your job to get up and ride your bike four to six hours a day, but for the rest of us without many free hours, a schedule-friendly method called polarized training presents a practical way to build endurance on a time budget.
“Ultimately, your ‘base’ comes down to your mitochondrial capacity,” says exercise physiologist Paul Laursen, “Research shows that while longer, lower-intensity exercise increases the number of mitochondria in your cells, high-intensity training makes those mitochondria more powerful.” (Some studies show high-intensity exercise performed regularly can stimulate the production of mitochondria, too.)
Plus, when you do a set (or especially multiple sets) of high-intensity intervals, your heart rate stays elevated during your “recovery” periods, which benefits your aerobic energy systems—especially as the session progresses, says Laursen.
However you slice it, interval training undoubtedly improves endurance, even if you’re already pretty fit. “Our research has found that when well-trained cyclists performed two interval sessions a week for three to six weeks, their VO2 max, peak aerobic-power output and endurance performance improved by two to four percent,” he says.
To that end, the best recipe for building endurance is blending the distribution of your training so about 80 percent of your rides are in those aerobic ‘zone 2’ intensities (in terms of heart-rate zones) and about 20 percent are performed at high and very-high intensities, or a blend of zones 3 to 5 throughout the week, says Laursen.