Again, protein will only be used at the level of complete muscular starvation and depletion. At
the end stages of glycolysis, the final product of pyruvate is taken to the mitochondria, and when
there are sufficient levels of oxygen in the cells, begins the Kreb’s cycle (Citric Acid Cycle, or
tricarboxylic acid cycle).
The Krebs cycle (also known as the citric acid cycle, tricarboxylic acid cycle or TCA cycle)
is a crucial component of the oxidative system. It is a portion of the oxidative pathway in which
a series of chemical reactions in the presence of oxygen produce energy in the musculoskeletal
system. This cycle does not actually use oxygen per se. The end products are extracted in
the presence of oxygen by oxidative phosphorylation in order to access the potential energy
in storage within the cycle. As we have previously mentioned aerobic glycolysis breaks down
carbohydrates in the form of glucose into pyruvate, which, to enter the Krebs cycle, must move
into the mitochondria where it is then converted into acetyl-CoA. A series of reactions occurs
through the Krebs cycle, and 12 high energy phosphate bonds are produced. The phosphorylation
of one glucose molecule results in the creation of 38 ATP units. The Krebs cycle is the end
process where carbohydrates, fat and protein metabolism are directed. This system is the most
efficient and quantitative producer of ATP for energy, but has limiting factors such as the need for
oxygen, long duration, and minimal intensity.
Lactic Acid
Lactic acid is a chemical byproduct of pyruvic acid, and is produced from the breakdown
of glucose. It is seen as an end product of anaerobic glycolysis. Lactic acid accumulation within
the muscular tissue is said to inhibit the contraction of muscle fiber. This fact is now being
challenged.
The thought of lactic acid being a fatigue producing substance is now being challenged.
Dr. George A Brooks, a Professor in the Department of Integrative Biology at the University
of California Berkeley states, “It’s one of the classic mistakes in the history of science.”More
evidence is mounting that lactic acid is actually fuel for our muscles. Now, the understanding is
that muscle cells convert glucose, or glycogen to lactic acid. The lactic acid is taken up and used
as a fuel by mitochondria, the energy factories in muscle cells. The idea that lactic acid causes
DOMS (Delayed Onset Muscle Soreness) one to two days after training is thoroughly incorrect,
as lactic acid is gone from your muscles within one hour post exercise.
Metabolic acidosis is when the pH is lowered because of exercise. This decrease in pH
can diminish the work rate of the cells’ energy system. This may be what is really responsible for
muscle fatigue.
What is the Most Efficient System?
As stated, the systems work together, and at no resting state or exercise level is there one
system that completes the total energy production. When dealing with energy systems, the