University of Wisconsin-La Crosse physiologist Carl Foster once monitored the school's track team for five weeks. Each day he asked the coaches how hard that day's run was meant to be; then, after the session, he asked the athletes how hard they had actually run. They were consistently pushing harder than their coaches intended on easy days, and not hard enough on hard days. This tendency to drift toward midlevel efforts is all too common. To get the most out of your training, you need to fight it.

A better approach is polarized training, in which most workouts are either very hard or very easy. In one study, runners using a program like this improved their 10-K times by five percent, while those who did more running in the middle—and trained harder overall—improved by just 3.6 percent. Here's how to polarize your plan.

Split it up (Unevenly): Elite runners typically follow a lopsided polarized plan, in which they devote about 75 percent of their training time to easy running, 10 percent to threshold work, and 15 percent to very hard efforts. Tempo runs are important, but that middle-intensity zone is still the smallest.

Use a heart-rate monitor to stay in the zone. On easy days, your heart rate should always be below 80 percent of maximum; on hard days, it should get above 90 percent. If you're spending long stretches between 80 and 90 percent, then you're going too fast for a recovery day and too slow for a truly hard workout.

Keep the Easy Easy: Olympic Trials marathoner Camille Herron was perennially injured in high school and college. When she slowed her easy-run pace from 7:00—7:30 to 8:30—9:00 per mile, she was able to increase her mileage while staying healthy, and to run faster on hard days.

Herron emphasizes keeping a rapid turnover to avoid feeling like you're plodding. Use a metronome app to keep your cadence within five percent of what it is at tempo pace, and keep your heart rate well below the border of the tempo zone—between 60 and 70 percent of max.

Make the Hard Hard: As an athlete, I dreaded workouts with lots of rest and not many intervals—less rest was an excuse to run slower. Don't think more is always better: By cutting back volume and adding rest to your usual workouts, you can boost your intensity.

Instead of running 6 x 1000 meters with 2:00 rest, try 4 x 1000 with 3:00 rest. Do each rep at least five seconds faster than usual. Repeat every two weeks, and either shorten the rest or add a repeat until you're back to the original workout—but now at a higher intensity.

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