Metabolic Misconceptions: Part 4
by Fred Dimenna, CSCS

Many chemical reactions in the body require the input of energy. These reactions, collectively termed endergonic, cannot occur spontaneously they must be driven by exergonic reactions. An exergonic reaction releases energy. The cellular compound used to transfer energy between exergonic reactions and energy-requiring ones is adenosine triphosphate (ATP). When ATP is "cashed in," work can be done. One of the body’s endergonic reactions is muscle contraction, which involves interaction between the contractile elements housed within muscle fibers. This interaction results in the development of tension, which is a requisite for overcoming opposition and creating movement.

As an energy-requiring process, contraction cannot occur on its own an exergonic reaction must drive it. ATP hydrolysis is an enzyme activated removal of one of the three phosphate groups this molecule contains. When this occurs, energy is released. This serves as the energy that powers contraction. ATP is a critical player, so there must be effective ways for ensuring that an adequate amount is always present. Chemical reactions proceed toward equilibrium, so the only environment that ensures a rapid breakdown of ATP is one in which its concentration far exceeds the product of its hydrolysis (ADP). Consequently, if you want your muscles to respond to your commands, your ATP/ADP ratio better be high (50 to 1, for example).

There are three mechanisms we use to maintain the ATP concentration gradient. Generally speaking, they operate simultaneously, but to dramatically different degrees. Which pathway predominates is important because they derive their ATP from different fuels. All of the energy that is eventually transferred through ATP is contained in the foods we eat. The sun packs chemical energy into plants by virtue of a process known as photosynthesis. This energy is stored in bonds between carbon and hydrogen. We eat the plants (or animals that have eaten the plants) and tap into that potential energy.

Most of the time, we use hydrogens and accompanying electrons from food to power the ATP-building process in association with oxygen uptake. In this case, we dispose of the carbons as carbon dioxide and use the oxygen as the final acceptor of the hydrogens once electron movement has "turned the dynamo." The product of the hydrogen/oxygen association is water. In accordance with the key contribution of oxygen to this process, this pathway is called aerobic.

Because water and carbon dioxide are easily disposed, aerobic ATP production proceeds with no detrimental effect. Even better, different fuels (macronutrients), including both fat and sugar (carbohydrate), can be used. After considering these "user-friendly" features, one might wonder why we don’t utilize this metabolic option all the time. Well, the body tries to, but sometimes we simply don’t give it the chance! The aerobic breakdown of macronutrients takes time. Sure, a lot of ATP will be reformed during the process, but if you’re using it up even quicker (contracting your muscles vigorously, as you would when lifting weights or sprinting), you have to maintain your stores at all costs. In these cases, it will be necessary to sacrifice some of the efficiency of the aerobic system for the rapidity of other ATP-providing pathways.

A small quantity of ATP is always stored within the actual muscle fibers. This provides an immediate source that fills a small portion of the void. In addition, ADP (the byproduct of hydrolysis) can combine with another compound that is present in muscle to reform ATP and keep it readily available a little while longer. This compound is phosphocreatine, the concentration of which typically exceeds that of stored ATP by four to-five times. These immediate energy sources can satisfy very high ATP demands, but only for brief periods. Stored ATP and phosphocreatine are maintained by aerobically produced ATP, so they are always available when needed.

The third metabolic option is glycolysis, the breakdown of stored sugar (glycogen). When aerobic metabolism predominates, glycolysis is active, but sufficient time exists for the hydrogens its reactions liberate to be transferred to the aerobic power-generating section of the cell. When energy demands are high, another route is chosen. By "dumping" hydrogens at an intermediate site, a limited ATP reformation can take place almost instantaneously. This option is both a blessing and a curse. It allows a demanding activity to be continued for longer than would be possible if immediate stores were the only anaerobic option. However, the temporary disposal product (lactic acid) interferes with contraction when its buildup occurs faster than its use. As a result, when you must rely more on this option, you can keep the pace a little while longer, but you’re on borrowed time.

Fred DiMenna, a Certified Strength and Conditioning Specialist and Lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at or email: