Combine resistance and high-intensity interval training to get MRT.

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Metabolic resistance training (MRT) is nothing more than loaded HIIT (high-intensity interval training), or, put another way, HIIT with resistance. HIIT is traditionally performed using cardiovascular activities: running/sprinting, cycling, stair stepping, rowing, using elliptical trainers, etc. But this neglects muscular fitness and the maintenance of lean body mass. (My new book, 60 Second Sweat, teaches you how to get a rock-hard body one minute at a time with HIIT and MRT.)

After age 30, if you don’t do something to prevent it, you will gradually lose muscle mass each year you age. This is a big problem. Your muscles, via your tendons, pull on your bones to move you. If you lose strength and muscle mass as you age, you cannot produce as much force, which means it will become harder and harder to move, much less live an active life and enjoy activities. Ever see an elderly person shuffle? Or someone unable to get up off the floor? This is due to a lack of muscular fitness and overall stability. (Here are more ways to anti-age your muscles.)

To prevent muscle loss, you need strength training (also sometimes called weight or resistance training). Strength training also increases your resting metabolic rate (the number of calories you burn at rest), improves insulin sensitivity, helps prevent type 2 diabetes, lowers blood pressure and cholesterol levels, increases bone mineral density, decreases lower back pain, and maintains and improves strength (obviously!). And, of course, it helps you look toned and trim.

With MRT, you stay true to the fundamental principles of HIIT—alternating brief bouts of hard work with periods of recovery—but you add in resistance with strength-training exercises. The synergy of MRT and HIIT develops both cardiovascular AND muscular fitness in a single, efficient, no-nonsense workout.

There is a big misconception out there that strength training cannot build cardiovascular fitness. (These are other important fitness myths to ignore.) It certainly can if it is programmed correctly. Anything that increases your heart rate and respiratory rate can improve your cardiovascular fitness. While strength training is not aerobic (meaning it does not rely on your aerobic energy system, which uses oxygen), it is cardiovascular. Do 20 loaded squats with a challenging weight and tell me you are not breathing heavier and your heart rate isn’t up!

Research proves that strength-training exercises can boost cardiovascular fitness. In a 2010 study, after completing a treadmill test to determine VO2max, 10 college-aged men did as many two-handed kettlebell swings as they could in 12 minutes, using a 16-kilogram kettlebell (about 35 pounds). This is a loaded exercise, a perfect example of metabolic resistance training. The average percentage of VO2 (65.3 percent) seen in this study was within the training range (60 percent to 85 percent) recommended to improve cardiorespiratory endurance, so the authors concluded that this protocol “provided a metabolic challenge of sufficient intensity to increase VO2max.”

Unless you are a current or an aspiring endurance athlete, you don’t need to devote much time toward low-intensity (below 70 percent of your maximum heart rate), prolonged (exceeding 20 minutes) aerobic training. If your only goals are to feel better, move better, improve the tone of your entire or specific areas of your body, and look better naked . . . you can develop total -fitness—cardiovascular and muscular—in a fraction of the time using MRT and HIIT.

For my exclusive workout plans that incorporate MRT to help you sculpt the body you want and lose weight with less time in the gym, check out my new book 60 Second Sweat.