I reposted this from an egroup, and it is personally what I wish to always follow, for maximal muscle pump and maximal gym time:
It is said that Cardio makes you lose muscle, sad as it is, it is true and scientifically proven, what happens is that muscle proteins are broken down and used for energy during aerobic exercise, but the thing is that you are constantly breaking down and re-building muscle tissue as you do your cardio workout, this process is called "protein turnover" what's going on is that your body is constantly alternating back and forth among anabolic (building) and catabolic (breaking down) cycles.
Reducing the catabolic side just enough so that you stay on the anabolic side will help you to gain muscle or at least will help you maintain muscle. But if you'd like to do cardio but you'd also like to build your muscles or maintain them at least as they are I recommend Circuit Training, because this is sort of aerobic weight training.
And since you use weights, although lighter weights, muscle is not lost, sometimes some muscle is gained. So remember if you'd liketo do cardio but not lose muscle or gain a bit of muscle, do a circuit training workout program.
A Sample Circuit Training Workout
1. Bench press
2. Squats (or leg press)
3. Seated rowing
4. Hamstring curls
5. Lat pull downs
6. Calf raises
7. Triceps extensions
8. Barbell curls
9. Ab Crunches
Move quickly from one exercise to the next, so as to maintain your target heart rate.
Begin by going through the complete circuit (all 9 exercises) twice.
You can build up to four times as you improve.