There is much debate and controversy on the subject of doing cardio while building muscle. Once and for all I am going to set the record straight. So without further adieu, here's the real deal on doing cardio while trying to gain size and strength...
If you are a beginner who also happens to be a ripped ectomorph who has to fight for every ounce he gains (e.g. a classic hardgainer), I suggest that you lay off cardio almost entirely for at least 8-12 weeks. Get your training and diet down and pack on some size.
In that time you should be able to gain at least 15lbs of muscle if not 20+. After you have done that you can add in some cardio. I would start with three weekly sessions of twenty minutes of moderate intensity cardio; no intervals. Use a bike to limit the amount of eccentric stress or pounding on the joints.
And remember there are actually things known as real bikes that go outside, not just stationary bikes that people park themselves on to watch Oprah. Although, if you choose that route, get one with a well padded seat that will not lead to the death of your sex life.
If you are beyond the beginner level you should always be doing some kind of cardio on a regular basis, be it intervals, moderate intensity steady state, or low intensity, long duration steady state. Again, don't limit yourself to machines indoors; get outside and drag a sled, run sprints, jump rope or play a sport. That's a lot more fun anyway.
I think everyone should be doing something like this at least three days per week for at least 30 minutes. It's healthy and prevents a host of health problems, not to mention that it keeps you in shape and looking good.
Contrary to what many people believe, cardio can actually be of great benefit to those looking to get bigger and stronger. Not only does it improve the cardiovascular system and thus improve the quality of your weight training workouts but it allows you to eat more muscle building calories while staying lean. To pack on 20-30 pounds of muscle you have to eat an inordinate amount of food. Doing some cardio will help ensure that you don't get fat from all the excessive eating.
The bottom line is that everyone but absolute beginners should be doing some kind of cardio type activity at least three times per week for thirty minutes. This will not inhibit size or strength gains in the least but may actually enhance them. You should vary your activities and intensities as much as possible.
You can do cardio immediately after you train, although I prefer to do it on non weight training days or later in the day after training because I am usually too spent after lifting to give it my all on the cardio. Doing it on off days is usually a better option anyway because it serves as an active recovery activity and also gets you burning some calories on those days.
The purpose of doing cardio when trying to get bigger and stronger is to keep you lean, improve your insulin sensitivity and allow you to eat more calories. What kind of cardio will have the least negative effect on your size and strength gains? That's a no brainer; walking.
The great thing about walking is it will not impede your progress in the least, the bad thing is that you have to walk for a bare minimum of 45 minutes to really burn a decent amount of calories and you will not elevate your metabolism much after walking. That is the great thing about interval training; it elevates your metabolism dramatically for long after you have finished your workout.
With that being said, I would still choose a good fast paced hour long walk on the beach over sitting on a stationary bike inside while pounding away on some brutal intervals. I despise intervals on a bike with a passion.
My ass goes numb, my balls go numb and I get a splitting headache. Not to mention that I am bored to tears within three minutes. Plus, I think we all do more than enough sitting and the last thing I want to do is sit some more while I'm supposedly "working out;" my hip flexors are tight enough already, thanks.
If you love to ride a bike then that is fine and you should do what you love. But for God's sake, go outside and do it. People will actually get in their cars and drive ten minutes across town to walk inside a gym and sit on a stationary bike and ride it for twenty minutes and then drive home. Why not just ride your bike across town? I don't get it.
So we have established that low intensity, long duration cardio (walking) is the best option for those who are concerned with any losses in size and strength whatsoever. This method was a favorite of many great bodybuilders such as Dorian Yates. Another option here, is to just go for a long slow/low intensity bike ride. Either one works great; but again I warn you to protect your nuts, aka get a good seat.
Next on the list is medium/moderate/high intensity steady state cardio. This kind of cardio is a little tricky because it can elevate cortisol and lead to losses in size and strength. To prevent this you need to be sure to limit the time spent doing this to 30 minutes, max.
Two days a week should be safe and three days would probably be ok for most people as well. It's when you get into the 4-7 day per week, 45-60 minute marathon sessions that you see people at the gym doing (whose bodies never change in the least from one year to the next)all the time, that you get into trouble. If you limit your use of this method you should be ok.
The good - intervals allow you to burn a lot of calories in a short amount of time and keep your metabolism elevated long after you finish doing them.
The bad - if you are training legs two or even three times per week, you can not do intervals more than once a week without overtraining. Let me rephrase that; you can but eventually it will lead to overtraining or at the very least slow down your strength gains.
You can negate this slightly by keeping your leg training volume extremely low and doing your intervals on the same day as your weight training. You can't do five to eight sets of legs two or three days a week and 30 minutes of intervals on top of it. That's a dead end road.
You also have to remember to do your intervals on your training days and not on off days like you might do with other forms of cardio because that will lead to overtraining much quicker.
The ugly - if you choose sprinting as your form of interval training you could get hurt; it's an ugly truth that has to be faced. The thing that will lead to even more injuries is following faulty interval protocol advice.
Normally it is recommended to do 30-60 second intervals when they are being performed on a stationary bike. A lot of people take these recommendations and apply them to sprinting. This is a huge mistake! Nobody can sprint for 30-60 seconds. Ok, not nobody; but most average people can't do it. World class athletes can sprint for that long, but not everyone else.
Don't believe me?
Go try it. Warm up thoroughly and try to sprint for 60 seconds straight. Let me know what happens. We have all seen the Olympics and how winded guys are after sprinting the 100 which happens to last all of ten seconds. Most of us have seen guys run the 40 and not be able to catch their breath for at least a few minutes afterwards. And that takes five seconds or less. Not only is sprinting for 30-60 seconds impossible for most people but it also greatly increases the risk of injury.
When you keep your sprint distances and times very short, you decrease the risk for injury because you never hit top speed and instead spend most of your time in the acceleration phase. This phase has the least potential for injury.
For that reason, most people should be running 20-50 yard sprints. This keeps you at top speed for a very short period of time; usually little enough time to maintain form and not suffer an injury. When you run at top speed for too long the chance for a break down in form and thus an injury is greatly increased.
I would never recommend that a non athlete ever try to sprint for 30-60 seconds straight and you should never take that advice from anyone. It is faulty and dangerous. To further reduce your injury while sprinting, use adequate rest periods between sets. Also, running with a sled slows you down enough to avoid top speeds and makes sprinting much safer.
Bottom Line - Intervals are a great tool for getting ripped, however when your main goal is to get big and strong and just keep fat gain to a minimum, they should be used sparingly if at all. I would recommend sprints above intervals on a bike and even then I wouldn't do them in true interval fashion but more of a traditional speed workout with short sprints and adequate rest periods.
This will still elevate your metabolism greatly and keep you lean. Just look at the physiques of Olympic sprinters for proof of this; that his how they train. Sprint, rest... no intervals.