The key to muscle gain > activation of muscular growth
Bulletin No. 26, Activating Muscular Growth, Part III
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In the last bulletin, I left you ready to start your last set of the belt squat workout - the dreaded 100 rep set. This set pushes both aerobic and anaerobic energy producing systems in their absolute limits. It elicits an adaptive response in your cardiovascular system to strengthen your heart and increase capillary density in muscle. This increases cardiovascular reserve and anaerobic threshold and improve nutrient and oxygen 
delivery to the muscle. A cell contains enough ATP to supply energy to last for about two seconds (1). So you would use this up during your first rep. Obviously you have to immediately and constantly replenish you ATP supply. Within about 1.2 seconds of maximum contraction, 80% of the ATP is being derived from CP - creatine phosphate (1). CP is the other phosphagen compound, along with ATP, which supplies energy very rapidly. The phosphagen system is always the first energy pathway called into action. Since the machinery of the cell needs ATP for power, it begins by using ATP, and then other energy sources are used to replenish the ATP. CP acts like a buffer to maintain relatively uniform levels of ATP within the cell. As ATP is broken down to release his energy, one of its phosphate groups is split off to form a molecule of ADP (adenosine diphosphate) and a free phosphate group. CP is able to donate its phosphate group to the newly formed ADP to regenerate ATP. The phosphagen system is able to supply energy in rapid bursts, immediately on demand, but it doesn’t last very long (about six seconds at maximal power output).
This is why a typical set with heavy weights doesn’t last very long - you run low on ATP and can’t make it fast enough to continue. Within 2.5 seconds of maximal contraction, 50% of ATP is being supplied by the lactic acid system, also known as the glycolytic pathway (1). Anaerobic glycolysis is the pathway used to make ATP from carbohydrates in the absence of oxygen. This is the second energy producing system called into play and is used to meet short-term energy demands. When the intensity of exercise is too great and the body can’t supply oxygen fast enough, carbs can be partially broken down to yield energy without oxygen. The advantage is this is a very rapid way to produce energy, but the disadvantage is that it’s not as efficient as aerobic energy production. After six seconds of maximal contraction CP levels have fallen to around 65% of their resting level and power output begins to decline. Continuing beyond six seconds of maximal contraction, ATP and CP levels begin to fall and lactic acid begins to accumulate.
These factors severely hinder power output (1). So we can see that after just a few reps we’re using both of our anaerobic pathways: the phosphagen system and the lactic acid system. Back to our belt squats: during the second 20 reps you will need some help. ATP levels may be depleted by as much as 60% of initial values (1). CP levels are nearly exhausted after about 40 seconds of maximal intensity exercise (1). At this point glycolysis is going full speed, but is unable to generate enough ATP to keep up with the demand. Lactic acid production is maximal during exercise of intensity that can be maintained for 1-3 minutes. So one minute into the set lactic acid levels are soaring and your muscles are burning like crazy. The third energy producing system - the aerobic pathway - begins to kick in after about 20 seconds into the set, and becomes the major energy producer after about 90 seconds (see figure).
Notice that there is considerable overlap, with all three energy systems being utilized at the same time. It’s not like you use up one energy source, turn off that system, then turn on the next system. They all blend in together, with different systems playing the major role depending on the intensity and duration of the exercise. If we were talking about low intensity exercise like walking instead of belt squats, the aerobic system could produce energy fast enough to fuel the activity, and the lactic acid system would never be called in. The aerobic pathway is able to supply energy for long-term demands - even for hours. Activities like weight lifting draw mainly from the phosphagen and lactic acid systems, while things like running and biking are fueled mostly by the aerobic system. The advantage of the aerobic system is that it can supply energy for a very long period of time, but the disadvantage is that it cannot produce energy very quickly. For weight lifting you need to supply a tremendous amount of energy immediately, but for endurance activities you need a lower energy level for a longer period of time. In the aerobic pathway, carbohydrates and fat are burned - combined with oxygen - to release energy. The rate of energy production by this pathway is limited by your vascular supply (which limits oxygen delivery) and by the size and number of mitochondria inside cells. Mitochondria are organelles in cells where aerobic metabolism is carried out. Notably, fat can only be used for energy via the aerobic pathway. Fat cannot undergo anaerobic glycolysis, as can carbs.
Therefore, aerobic activity is the only way to burn fat. This is another reason to make aerobic exercise a part of your program, in addition to strengthening your heart and blood vessels. Now, back to our set: after 50 reps the phosphagen system is long gone, and the glycolysis is pretty much shot too. You have continued to expend energy faster than your cells can replace it, and consequently build up an “oxygen debt.” This describes a situation we are all familiar with. You know how you breathe real hard for a few minutes after a set? This extra oxygen is being used to replenish the ATP, CP and glycogen you spend anaerobically during the exercise. The oxygen debt is the difference between the amount oxygen actually consumed and the amount that would have been consumed if the exercise had been fueled entirely aerobically from the beginning (2). By the time the oxygen debt builds up to 3-4 liters of oxygen, you enter a severe level of ATP depletion. Exercise will only continue on a “pay as you go” basis wherein ATP is being continually replenished by aerobic metabolism (2). 
Power output decreases and your heart and lungs are working at absolute maximum. You will be sweating profusely. All energy systems are either maxed out or have already failed. By 60 reps you will beg to quit. At this point you will be beyond positive failure and it will be difficult even to resist the weight on the way down. From here on out your life is in the spotter’s hands. By 70 reps you get that adrenaline rush that comes from the fear of eminent death. By 80 reps you can’t feel your legs anymore and your mind enters a strange trance-like state. You kind of lose touch for a while.
You probably won’t have much energy left to groan or scream and your body gets limp. By 90 reps you’re just along for the ride, with the spotters doing almost all the work. The universal reaction after this experience is to lay down on the floor. Some people have to throw up. Loss of consciousness may occur as the result of acute metabolic acidosis - the lactic acid builds up and acidifies the blood. This corrects itself in a few minutes after you blow off some carbon dioxide. Most people lie down for five minutes or so before pulling themselves over to a bench. You’ll be breathing hard and sweating for about 20 minutes after this. One other energy system deserves mention, but probably doesn’t play much of a role in belt squats. That’s the glucose-alanine cycle (2). During long term aerobic exercise after blood glucose and glycogen are depleted, the body begins to break down muscle tissue into free amino acids. The amino groups are removed from the amino acids and added to pyruvate to form alanine. Alanine is transported by the bloodstream to the liver where it can be converted to glucose. The glucose is carried back to the muscle to be used as fuel (2). In addition, the branched chain amino acids (BCAA’s) leucine, isoleucine and valine can be used directly as fuel by the muscles. So in ultra-endurance activities, such as a marathon, muscle is actually broken down and used as fuel. This is one reason why endurance athletes have small muscles and one reason why bodybuilders don’t run marathons. Amino acids may supply 15% of energy used in endurance activities (2).
In summary, we find that the belt squat boosts energy producing capabilities of the entire body. Cardiovascular reserve and anaerobic threshold are increased. Training at this level of intensity carries over into your other exercises and allows you to perform them more intensely also. Belt squats tax all three major energy producing systems at maximal levels. While ultra-endurance activities like marathon running are counter-productive for bodybuilders, a certain amount of high intensity aerobic exercise seems to allow increased muscular growth. This may result from increased vascular supply to muscles which may stimulate growth by providing increased nutrient delivery (1-3). Everybody has a dream. Everybody wants to get to that next level. That’s what Parrillo Performance is all about.
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References
1. Hatfield FC. Hardcore Bodybuilding - A Scientific Approach. Contemporary Books, Inc., Chicago, 1991.
2. McArdle WD, Katch FI, and Katch VL. Exercise Physiology - Energy, Nutrition, and Human Performance. Lea & Feiberger, Malvern PA, 1991.
3. Lieber RL. Skeletal Muscle Structure and Function. Williams and Wilkins, Baltimore, 1992.
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