I like to work in circuits with my workouts because I find that you get more bang for your buck. It is far more effective to incorporate higher intensity cardio for a shorter amount of time in order to burn fat and retain muscle. Many of you have asked, so here's a little post on my thoughts on cardio training:
There are two types of muscle fibers. One involved in endurance training (such as marathons) and one involved in short-lived high intensity training (such as sprints and weight-lifting). You can especially see the difference in these two types of fibers when watching the olympics; compare the type of muscle mass on the sprinters to that on the distance runners.
Those of us who spend our days in the weight room isolating muscle groups, we do it for the tone that is derived from the muscles built from the short-lived high intensity training as mentioned above. Therefore it would be counterproductive to build these muscle fibers lifting weights then run on the treadmill for an extended period of time. This causes your body to break down the muscle in order to fuel it for endurance (yes, unfortunately you will begin to burn muscle before you burn fat when entering endurance training thresholds). That is why I, personally, do higher intensity cardio (after I lift! ..not before, you want maximum strength out of your muscles in the weightroom) that utilizes large muscle groups and also strive to incorporate them in my weight sets to keep my heart rate up.
My post-weight training cardio exercise, is spent doing intervals on the elliptical (trainers have advised me to not use the incline but to strictly use resistance intervals alternating between easy to near-impossible), running stairs (you sprint as you ascend and rest as you descend), or circuit training (the equipment opportunities are endless: jump ropes, kettlebells, rowing machines, body weight exercises such as burpees, and more that I will outline in the weeks to come)!
Many people get confused on how much and what type of cardio to do, and my answer is is that it's entirely up to you and the type of muscle definition that you want. Ultimately you need to find a balance that gets you to GO the gym because chances are if you dread it, you won't do it!
Here's a little taste of what I did today:
(I marked cardio-incorporating workouts with an *)
Warm-up*
Strength circuit (for time, 8 minutes)
Bent dumbbell rows, 10 reps
Alternating dumbbell overhead press, 10 reps
Dumbbell squat clean press*, 10 reps
Superset #1 (shoulders)
Dumbbell arnold press
Dumbbell lateral side raise
Weighted ball squat press throw*
Superset #2 (back)
Reverse dumbbell flyes
Lat pull downs (behind the neck)
T-bar rows
Cardio Circuit (for time, 25 minutes)
Jumprope*, 15 reps
Rowing machine*, 500 yds