For the bodybuilder, glycogen stores are one of the most critical factors in muscle growth. However, most aspiring bodybuilders seem to have an unjustified fear of eating carbohydrates in the fear of becoming fat. This is a direct contradiction to the professed goal of building muscle. Glycogen is the dominant fuel during exercise. Only a few sets using 65% of your 1RM could deplete your glycogen levels by as much as 70% of their previous levels [1,2]. Continual workouts and avoidance of carbohydrates will lead to rapid depletion and this will reduce the intensity are which you can work out at . And that means fewer stimuli for growth!
How It All Works
There are numerous different classifications of muscle fiber types. For general purposes, they are broken down into three primary categories. Type I, which is often referred to as the "slow-twitch". These muscle fibers have the highest concentration of an enzyme called pyruvate dehydrogenase. This enzyme takes the 3-carbon atom pyruvate, which was formed by splitting Glucose (6 carbons) in to two halves, and puts it into the KREBS cycle and electron transport chain. In other words, these muscle fibers are much better doing oxidative (AKA: Aerobic) type work.
On the other end of the spectrum you have the Type II-b fibers, or what are known as the "fast-twitch" fibers. The have an abundance of an enzyme called Lactate Dehydrogenase. This enzyme will take pyruvate and convert it into Lactic acid. In doing so, it liberates a Hydrogen (H+) ion, which can be used for energy. This process is fast but very inefficient. Now, when you lift weights, these Type II-b guys become more and more involved as he loads get heavier and heavier. This is why, there is an inverse relationship between the weight you can lift and the amount of reps you can perform. Lactic Acid gives up hydrogen ions very easily (That's what "The Burn" is) and it is these Hydrogen ions that inhibit muscular contraction. All other muscle fiber types are lumped into the class of Type II-a. These are the ones that have the ability to change with specific modes of training.
Now, because of all of the above information, many bodybuilders seem to think that weight lifting and high intensity exercise depletes the Type II-b fibers. And conversely that only the long drawn out low intensity exercise depletes Type I muscle fibers of glycogen. Well this isn't exactly the case. Muscles are "smart". They know when they need to work hard and when they don't. They also know how to produce the amount of force needed to complete a job. So if we have these three different types of fibers, each with its own force production capability, wouldn't it makes sense to use ALL of them for the heaviest workloads? For both oxidative and non-oxidative (Because oxygen is ALWAYS present in our body, we know that it isn't correct to say anaerobic...Right?).
Muscle fibers have a pattern of involvement. For the very low intensities, the Type I fibers can do the job. As we delve into the moderate intensity, the Type II-b fibers kick in. BUT, the type I fibers do not drop out of the picture. If they did, they would only be lowering total force and power outputs. Now when we get to about 80% of our 1RM, or VO2 max, those Type II-b fibers come in and start kicking some ass. The speed of the contraction also comes into play. When we have to contract excessively fast, the Type II-b fibers will be activated as well. But this doesn't mean that lifting lightweights really fast will do you any good. So with the heaviest workloads, we effectively recruit ALL muscle fibers. With that being said, weightlifting is not limited to depleting only type II fibers. And cardio isn't limited to only depleting type I fibers.
Now, if we want to get the most out of the carbohydrates we eat, we want to ensure uptake by all muscle fibers. We pretty well know that over compensation of muscle glycogen is seen when glycogen has been depleted . But the protocol to achieve those states is far and beyond what most of us would ever do. However, recent research has found that after engaging in near maximal exercise for a short time, that we do in fact see the transient glycogen "super compensation" in ALL muscle fiber types .
Now with in 20 minutes of this brief work out the subjects were downing carbs like there was no tomorrow and continued to do so for the next 24 hours. How brief is brief you ask? Would you believe it was only three minutes! After the short carb-load (traditionally it is for 3-6 days with-out training) they saw an 82% increase in glycogen content. That's a lot of glycogen! Even more glycogen accumulated after the short period than has ever been seen in a 3-6 day carb loading protocol [14,15]. All those carbs were soaked up by muscle tissue like it was going out of style. And we all know that glycogen holds water inside muscle cells, which is a like a light switch for protein synthesis .
Table 1- Characteristics of the Three Muscle Fiber Types
|Fiber Type||Slow Twitch (ST)||Fast Twitch A (FT-A)||Fast Twitch B (FT-B)|
|Contraction time||Slow||Fast||Very fast|
|Size of motor neuron||Small||Large||Very large|
|Resistance to fatigue||High||Intermediate||Low|
|Activity used for||Aerobic||Long term anaerobic||Short term anaerobic|
|Force production||Low||High||Very high|
|Major storage fuel||Triglycerides||CP, Glycogen||CP, Glycogen|
During intense exercise, glycogen and glucose are the main fuel. However, much of the glucose is coming from the breakdown of liver glycogen and this is supplied to the working muscle. It is an attempt to spare muscle glycogen and keep blood glucose from dropping too low. However, the rate rarely meets the demand we place on the liver. So liver glycogen content also becomes a problem. And this has more negative implications than you think. Aside from the fuel source, the liver is one of the chief producers of IGF-I, which is a seriously anabolic hormone [5,6,7,8]. And in my estimation are the main effects on muscle mass that have been accredited to Growth Hormone. Growth Hormone it self has never been shown to increase muscle mass on itself. On the other hand IGF-I has, and it is made largely in part by Growth hormone and insulin at the liver. Why is this important? Well it seems that when liver glycogen content gets low, it starts to produce high amounts of the binding protein for IGF-I . The unbound form is the form that it 'active'. Once the binding protein latches on, that's it. Its over, it's no good! So it would be a very smart Idea to keep liver glycogen levels up by consuming some high fructose foods a few times a day like apples, or even honey .
After reading this research, I stand even more firmly behind my article, Anabolic Aerobics. If a person were to lift M, W, F, they would be more than smart to do a very high intensity cardio session on T, TH and possible Sat and do them early in the day. On those cardio days, you should increase your carbohydrate intake to almost double of what you normally eat, and make sure that the additional carbs are higher on the Glycemic Index  Try to avoid to many fibrous foods. Think of these days as "Accelerated Growth & Recovery Days".
But, as a warning, body fat does in fact come down to caloric intake. So you may need to adjust your fat or protein intake to allow for the added calories. Don't restrict calories too heavily, and most certainly, don't restrict carbs! Trust me, you won't get fat! Remember back to Anabolic Aerobics, doing so is not the goal here. Feed your muscles. A day or two of only 250g of protein won't kill you. However, this isn't a licensee to gorge you in junk food. It is becoming more and more evident all the time that cardio, if performed at a high enough intensity and when you structure your diet right, will only HELP YOU BUILD MUSCLE.
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