Why you wouldn't mix cardio and strenght training

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There are several factors that play into how people should approach their training, and it's not just a simple "You need to cut out cardio if you want more muscle".

The twitch fibers play a role in how your muscles will respond, but every person is going to have a different ratio of those fibers in their body, and those don't change over time. At all. You have what you were born with, which is why certain people are sprinters, and certain people are distance racers, and rarely do they mix and create superior athletes in both. Distance racers have more aerobic muscle fibers, while sprinters require more aerobic. Some people build more muscle easily, and that's because they have more of one fiber, and they're usually at a more optimal length that creates larger diameters.

But ANY exercise that puts those muscles into overload and fatigue, where the myosin and actin heads cannot connect any longer, will cause muscle growth, in either fiber. Even the cardio exercises send slow twitch fibers into hypertrophy (getting larger), if you overload them to a point where the sarcomeres experience fatigue and need to rest. The main thing is that ONLY doing activities that require certain fibers, like aerobic, tends to send the other fibers into atrophy (where they shrink), because they aren't needed as much. Only after an extended period of time, or if someone has extremely low body fat or poor nutrition, do those fibers actually break down for energy or protein, because the muscle will simply shrink first before actually breaking down.

So saying "If you do cardio and strength training, you're going to catabolize muscle" is up for serious debate, because the only way that you break down any muscle tissue is if you stop overloading it for long periods of time, and you aren't getting sufficient nutrition to supply energy for the muscle repair that creates larger diameters. It's not just going to happen because your body is introduced to another activity for a little while. The human body would not have managed to get through the evolutionary chain if it broke down muscle all the time on a dime, because once broken down, those fibers don't come back. Literally, you cannot grow more muscle fibers once they're gone.The only way to improve muscle function would be to work the surrounding fibers harder and cause them to grow larger, which wouldn't be efficient if you had to do that every time you started a new activity.

The main benefit to doing both cardio and strength training is that every fiber gets used, and overloaded to the point where they need rest. You can do cardio, target one set of muscles, and then do strength training, which targets different fibers. You can do more of one than the other, and experience more growth in one area while doing so. The thing that is detrimental to seeing more muscle growth is not allowing enough time for rest. That's all. You're not going to experience degradation of muscle if you allow for rest and have proper nutrition to maintain that exercise. The only thing that might happen is that hypertrophy would probably be slower in general. But cross-training will keep your muscles more fit, because they're consistently exposed to new stresses.

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If there is roughly total 2000cals of glycogen stores in my muscles + liver, presuming I do eat carbs to replenish these and am not in a ridiculously low deficit, why then can we not do say, for example, a 300cal strength session followed by a 400cal cardio session? are there still 1300cals of glycogen sitting in the muscles ready to be used for repair + general movement?

If you eat properly, you will replace all your glycogen stores within 24 hours, even after a very hard exercise session.

If you do not eat properly then you will gradually deplete them until there is nothing left.

Also, after a 400 cal cardio session it is hard to say how much glycogen you used since there is always a combination of glycogen and fat being burned. The more aerobiclly fit one is the more fat will be burned as opposed to glycogen.

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Razique, you are holding out on me man! You send me links when you put up good stuff like this and you left me out in this one! I had to find it all by myself!

I agree that both are useful but not on the same day. Both intense lifting and higher intensity cardio cause a need for recovery and both on the same day overloads the CNS and causes competiion for recovery resources. Lower intensity is fine as is 5 to 10 minutes of warm up/ cool down for lifting.

Interesting stuff - while I get that that strength training and cardio fitness are somewhat in conflict, I'm confused as to how you'd recommend applying that if you want to improve both (over the long term). If one starts as an active, but overweight person who wants overall fitness and versatility, what would you do?

I'm doing P90X, which seems to mix the two and I'm already seeing performance results (for example, strength when I sling bags of mulch in working in the yard) and stamina (running with the kids).

I want to be someone who could do a 5K, play tennis, hike/climb, play sand v-ball, bike, play tennis, play soccer, cut wood all day, swim/surf all day, etc. -- well rounded, I guess. While as, an older fat guy, it seems a stretch - aesthetically, I'd like to imagine working towards looking like the people who play gladiators on film versus the more gaunt looking distance runners OR the thicker looking body-builders.

It seems like P90X, boot camps (military and gym based), training for actors playing those physical/buff looking action roles mix strength and cardio - does this only become an issue once you get far enough along in fitness? And once at that stage, do you go in cycles - strength train and diet for muscle mass for a few months and then shift back to cardio/circuit based training for a couple of months -- does this up the bar in both areas over time?

If there is roughly total 2000cals of glycogen stores in my muscles + liver, presuming I do eat carbs to replenish these and am not in a ridiculously low deficit, why then can we not do say, for example, a 300cal strength session followed by a 400cal cardio session? are there still 1300cals of glycogen sitting in the muscles ready to be used for repair + general movement?

If you eat properly, you will replace all your glycogen stores within 24 hours, even after a very hard exercise session.

If you do not eat properly then you will gradually deplete them until there is nothing left.

Also, after a 400 cal cardio session it is hard to say how much glycogen you used since there is always a combination of glycogen and fat being burned. The more aerobiclly fit one is the more fat will be burned as opposed to glycogen.

From my understanding we can only replenish glycogen at 5% per hour, doesn't matter how many carbs you eat. Elite athletes can replenish it at 10% per hour.

Well, at 5% per hour you will back at 100% in 20 hours or less depending on how depleted you are.

Oooh. Arithmetic!

If there is roughly total 2000cals of glycogen stores in my muscles + liver, presuming I do eat carbs to replenish these and am not in a ridiculously low deficit, why then can we not do say, for example, a 300cal strength session followed by a 400cal cardio session? are there still 1300cals of glycogen sitting in the muscles ready to be used for repair + general movement?

If you eat properly, you will replace all your glycogen stores within 24 hours, even after a very hard exercise session.

If you do not eat properly then you will gradually deplete them until there is nothing left.

Also, after a 400 cal cardio session it is hard to say how much glycogen you used since there is always a combination of glycogen and fat being burned. The more aerobiclly fit one is the more fat will be burned as opposed to glycogen.

From my understanding we can only replenish glycogen at 5% per hour, doesn't matter how many carbs you eat. Elite athletes can replenish it at 10% per hour.

Well, at 5% per hour you will back at 100% in 20 hours or less depending on how depleted you are.

Oooh. Arithmetic!

lol......soooo, the maths seems say strength + cardio + eating enough carbs = no problem.....? agree? disagree?

lolli, that would be my conclusion. But I'm just a girly girl, and had to seek advice from a knowledgeable friend to cope with all the bro-science in this thread. :-)

The funny thing about knowledgeable friends, is depending on WHERE they learned what they learned...

They can be completely wrong.

Seriously, there's so many conflicting viewpoints even among the 'knowledgeable professionals', the common person doesn't have a chance. The minimum you can do is pick someone who has already achieved the results you're looking for, and try to emulate. If you need more control...research for yourself and experiment on your own.

But I will say this...Raz and his results fall within methods used by some pretty well known and respected people in the fitness industry, and oddly enough, they're not the ones out selling their snake oil to make a buck. Bro-science or not, it's clearly working.

Hmmm...

When I go to the gym I warm up on the elliptical for 10 minutes then do about 45 minutes of strength training. I then finish off with about 15-20 mins on the stationary bike. In terms of calories burned doing cardio, it works out at around 200 cals.

So does this mean i'm ok to keep doing these on the same day as weights? Or have I just got confused?

Interesting information. Bump for later!!