Some trainers want you to believe there’s a “best” way to train for fat loss. That their approach is superior to the next guy’s. 

But there are only two factors that really matter: intensity—how hard you work—and density—how much you work in a given amount of time.
Here’s how.

If you want to burn fat for longer, intensity trumps the length of your workout. In fact, research has found that an all-out, push-your-limits 10-minute circuit will rev your internal calorie furnace the same amount as three of those 10-minute circuits done back-to-back.

There are 3 primary ways to achieve maximum intensity.

1. Absolutely Heavy Loads
This pretty much means lifting heavy shit, or at least heavy shit for your current fitness level.

 For example, during classic lifts like the bench press, squat, or deadlift, you’d choose a load that challenges you in a 1 to 5-rep range. That means your muscles are quaking during those reps.

2. Relatively Heavy Loads
This category applies more to bodyweight training in which there is no real number to describe how much you’re lifting. But it’s clear that a single-arm pushup is much harder than a regular pushup done on two arms.

The relative load is much heavier, and therefore, so is the overall intensity.

3. Rate of Perceived Exertion (RPE)
This is basically a subjective assessment of how hard you’re working on a scale from 1 to 10 (1 being easiest and 10 being hardest).

If you’re doing a high-intensity workout, you typically you want to stay within a range of 7 to 10.

When lifting weights, that equates to loads in a 70 to 100-percent range of your 1-rep max.

Or this could apply to how hard you work with any size load. So using a 90-percent effort at any given load would mean you’d leave a single rep in the tank. At 80 percent, you’d leave 2 reps in the tank, and so forth.

RPE can also be expressed by doing a move as explosively as you can (think: vertical jump) or as fast as you can (think: sprinting 100 meters).

Another option is doing something repeatedly for time or reps as quickly as you can. For example, doing AMRAP (as many reps as possible) of pushups in 60 seconds. Or timing how long it takes you to do 100 pushups.

Though intensity is important, you simply cannot maximize results without enough training volume.

This is where the evil cousin of intensity, density, comes into play.

Density describes how much work you accomplish in a given period of time. The more you complete in the same amount of time or less, the more fat you’ll burn, and the greater muscle-building and metabolism-boosting response you’ll achieve.

To get maximum training density, you must strategically order exercises—like alternating between a lower-body move and an upper-body move in a total-body metabolic circuit format. While one body part works, the other body part recovers.

This allows you to continue working and to take shorter rest periods. You can do more work on the minute every minute without sacrificing intensity.

How short of a rest am I talking about? Sixty seconds or less is a good goal, but 30 seconds or less is ideal.

We also know that work periods of 30 to 60 seconds burn the most muscle glycogen (the sugar stored in your muscles), which enhances the fat-burning process during rest periods and between workouts. This is especially effective when combined with a diet that is lower in overall carbohydrates and higher in protein and produce.

The Power Duo: Intensity + Density 
Since intensity and density tend to have an inverse relationship—as density increases, intensity decreases—it’s critical to find that sweet spot between the two.

Where is that sweet spot? I found it in the Density Doomsday workout, shown in the video above. You can find this workout in THE 21-DAY METASHRED, my all-new body-shredding DVD plan. (You’ll also get 8 other 30-minute workouts that torch fat fast. One guy lost 25 pounds in just 6 weeks.)

Throughout the 30-minute workout, you’ll progressively increase the length of your work periods (from 40 to 60 seconds) while decreasing the length of your rest periods (from 20 to 0 seconds).

This escalating density formula allows you to maximize your intensity at the start of the workout. And since there’s a cumulative fatigue, you’ll develop extreme endurance.

The result: the greatest possible afterburn.

How to do it: Do each exercise below in order, using the prescribed work to rest ratio for each cycle.

1.    Squat press
2.    Dead-stop pushup
3.    Low-lateral squat switch
4.    Pull complex (2 rows, 1 curl)
5.    Butterfly situp
6.    Run in place

Complete all 6 moves in the circuit before starting the next cycle.

Cycle 1: 40 seconds of work, 20 seconds of rest
Cycle 2: 45 seconds of work, 15 seconds of rest
Cycle 3: 50 seconds of work, 10 seconds of rest
Cycle 4: 55 seconds of work, 5 seconds of rest
Cycle 5: 60 seconds of work, 0 seconds of rest

I can promise you this: If you don’t burn fat from this workout, you never will.

Source: Men’s Health