Before embarking on one of the following workouts, you must establish some working numbers. Most heart-rate training programs ask you to find your maximum heart rate and calculate the percentages for your four training zones from there. Typically, you subtract your age from 220 to establish your MHR, but such calculations are often inaccurate, failing to consider the differences among age groups and fitness levels—even differences within an individual moving from one sport to another. And maximum heart rate has more to do with genetic traits, such as the size of your heart, than with your level of fitness. A better solution, says fitness expert Jay Blahnik, is to learn to recognize the four workload zones—recovery, aerobic, threshold, anaerobic—and then glance at your HRM to find the corresponding heart rates.

To find your targets, strap on an HRM and warm up as you normally would for your ski, ride, or run. Gradually increase your pace, evaluating how your body feels after three to five minutes at each level of intensity. When you reach each of the zones below, note your heart rate.
Recovery: You feel you could carry this pace for 60 minutes or more.
Aerobic: If you had to, you could maintain this exertion level for 20 to 40 minutes.
Threshold: You could do this for no more than ten minutes.
Anaerobic: This hurts. You feel like you can do it for a minute, maybe two at most.
Do the sport on the same terrain two or three times over a few weeks, says Blahnik, and when some common numbers begin to appear, assign them the following values: recovery, 60 percent of your max heart rate; aerobic, 70 percent; threshold, 80 percent; anaerobic, 80 to 90 percent.

SkiTwo elements are critical for a recreational racer who wants to improve his performance, says Chris Grover, assistant coach of the U.S. Cross-Country Ski Team. "If you only get out twice a week, you want to spend one session working on straight distance at 60 percent of your max heart rate or lower, ideally for twice your standard race distance. The other day should be spent doing intervals." Gradually, you'll increase the amount of time you can work at high intensity without rest and without going anaerobic. You'll also begin to intuitively measure your own intensity levels. "Our guys use HRMs so much," says Grover, "that they can feel the difference between 130 beats per minute and 145."

THE PLAN:
On your first day, begin with four five-minute intervals at 85 percent of your maximum heart rate. Follow each interval with five minutes of recovery at 60 percent. Shoot for three seven-minute sets, and then two ten-minute sets, each followed by equal recovery time. Work in an outing a week at a slower-than-normal pace. On your second day, ski at 60 percent of your max for two to three hours.