What is it: An active metabolic assessment aims to help you make the most of your workout and kick-start weight loss if you've hit a plateau. This cardio-based assessment determines your specific heart-rate zones and how well your body burns carbohydrates and fats to fuel your exercise.

The back story: As your heart rate increases, you burn more calories, but fewer calories from fat, explained Dan Hubley, senior program manager of metabolic assessments and health technology for Life Time Fitness. Since our bodies are very effective at storing fat, you want to find the "sweet spot" where your body burns the most calories from fat.

While many fitness trackers can measure your heart rate (and some exercise machines give you a ballpark figure), a metabolic assessment can determine personalized heart-rate zones at which you burn the most fat. As Hubley said, "If you're not assessing, you're guessing" — guessing how often you should exercise and at what level.

The test: I wore a heart-rate monitor and was fitted with a mask hooked up to a machine that measured my VO2 (my body's ability to use oxygen). After a 10-minute warmup walking on a treadmill, there was a 20-minute assessment, in which the speed and incline of the treadmill were increased until I reached the point of exhaustion. (I chose walking on an incline, but the test also can be done on a bike, elliptical, stair climber or rowing machine.) It ended up being a tough workout, but my biggest challenge was remembering to breathe through my mouth.

Workout changeup: Instead of doing primarily shorter, high-intensity workouts, Hubley said my results showed that I should slow it down. In fact, he recommended that 75 to 80 percent of my cardio workouts should be 30 to 60 minutes long at a lower heart rate. (Between 83 and 154 beats per minute for me.) For a fan of hard-and-fast workouts, that's easier said than done. He also recommended I do a 10-minute warmup before any exercise to improve calorie burn, limit my risk of injury and increase my endurance.

Cost: The $259 active metabolic assessment includes target heart-rate zones, peak VO2 score and the percentage of fat and carbs burned. It also includes a customized workout plan. The assessments are available at Life Time Fitness, to nonmembers as well as members. For more info, go to bit.ly/AMADescription.

Dan Hubley filled out an assessment form. “If you’re not assessing, you’re guessing,” he said.

My takeaway: I work out four to five days a week (what I consider damage control for a fairly fit 56-year-old), but it's definitely time to change things up. I've had the same 10 pounds to lose for a couple of years. So while I'm still doing high-intensity workouts, I'm subbing some for longer, lower intensity workouts. I'm also adjusting my diet by trying to eat some protein and good fat at breakfast. Goodbye, unadorned oatmeal; hello, nuts and almond milk.

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