met·a·bol·ic  adj.
1.  Of, relating to, or resulting from metabolism.
2.  Undergoing metamorphosis.  (1. A transformation, as by magic. A marked change in appearance, character, condition, or function.)

The two best undisputed training methods to improve your physique are weight training and HIIT (high intensity interval training).

Enter the Metabolic Circuit.
A Metabolic Circuit is an intense combination of resistance training and HIIT.  These are fantastic from a conditioning standpoint, with the added benefit of creating an oxygen deficit. What does this mean for you? It means you'll be burning calories long after your workout is complete. It will also improve your conditioning, all while becoming stronger and faster.
I find this style of training incredibly challenging, fun and effective. I'm constantly brewing up new ideas or combinations to try out. The key to success with these circuits is to have a plan and set up any necessary equipment so you can transition as quickly as possible from one move to the next.

There are a couple of different ways to do Metabolic Circuits:

  • For reps.
  • For time.

Metabolic Circuit for reps.
If you are using a metabolic circuit for reps, you are going to pick a few exercises and complete as many reps in the allotted time frame as possible. Then, with little or no rest you will move on to the next exercise.
You will complete the exercises in order for the time instructed and do as many reps as possible (AMRAP). Go all out and do not hold back. Count your reps for each exercise, and the next time through, try and get more reps than you did the previous set. In short, you are going for a rep PR (personal record).

Circuit example
Perform each exercise for 20 seconds
Kettle bell swings
Plyometric split squats
Squat jumps
Mountain climbers

Rest for 1-2 minutes.  Repeat this two or three more times, and then move on to another circuit. The beauty of this style of training is that you can use any exercise you please for the amount of time you choose. Of course bigger compound movements are great, as they recruit more muscle. For example, do not choose a tricep kickback for one of the exercises. Actually, don't choose a tricep kickback for anything, ever.
If you are just starting out, I'd suggest starting with just two or three body weight circuits, and maybe run through each one twice. As you become more advanced, you can choose weighted exercises, or you can set up three or four circuits and run through them all two or three times. There really are no rules.

Metabolic Circuit for time.
If you decide to use a Metabolic Circuit for time, I must warn you, these are brutal. I've attached a video below so you can see exactly how intense this can (and should) be. Here's an example of something you could do after you've warmed up. You will need a stopwatch handy or somebody to time you from start to finish. With this style of Metabolic Circuit, you are striving for a time PR.

Circuit example
Squat jumps x 8
Medicine ball slams x 8
30 yd Sprint
30 yd Prowler push back
Push-ups x 8
Mountain climbers x 15

Again, you will complete all of the reps and move into the next exercise as quickly as possible. After you've completed this entire circuit, rest a minute or two and then run through it again and attempt to beat your best time. You can run through this once, twice, or three times, and then move onto another circuit. This type of training is for advanced people that have a good conditioning and strength base, and are able to complete the reps safely but quickly, without getting sloppy and dangerous. These types of circuits are also great finishers to a lifting session, or a great conditioning option if you don't have access to a hill for sprints. You could also set something like this up and do it to the best of your ability without timing it, and just blaze through it as fast as you can.

Remember, a great level of conditioning is important!