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Heart-rate training is the key to gauging your aerobic intensity and building endurance. Here’s how to get started.

1. Buy a Heart-Rate Monitor (HRM)
In order to get the most out of the interval training used in The Shape of Your Life program, a midlevel HRM that can calculate average heart rate and provide target-zone programming with an audible alarm will be the most effective. With those functions, you’ll be able to bump into higher and lower heart-rate zones (see step 3, below) without looking at the watch face. Models we like are created by , , and .

2. Calculate Your Maximum Heart Rate (MHR)
Your MHR will determine the numbers that define your training zones. Use the following formula to determine your baseline MHR.

217 – (your age x 0.85) = MHR (in beats per minute)

Example: If you’re 35, that comes out to 217 – 30 = 187 bpm. For rowing, subtract another 3 bpm. For cycling, subtract another 5 bpm. For swimming, subtract another 14 bpm.

3. Establish Your Four Heart-Rate Zones
A little more math and you’re done. Using your MHR as a baseline, write down the corresponding heart rates for the following zones: recovery (60 percent of your MHR); aerobic (60-75 percent of MHR); lactate threshold, or LT (75-90 percent of MHR); and anaerobic (90 percent of MHR and above). You’ll use these numbers to work out at prescribed intensities during each month’s regimen.

Individual lactate thresholds vary widely among athletes. If you’ve let fitness slip for a while, your LT probably falls at the low end of Zone 3 (maybe 75 to 80 percent of MHR); if you’re in good shape already, LT may hover closer to 80 or 85 percent. On Friday of week two of The Shape of Your Life program, you’ll perform a workout designed to determine your LT more accurately for the upcoming intervals. At the end of each month you’ll take a one-mile LT test to see if you’ve pushed it back.