All muscle contractions are initiated by an electrical impulse from the brain. This impulse (action potential) travels along the surface of the muscle and down t-tubules which go into the muscle. As the action potential passes down the t-tubules it triggers the release of Calcium ions (Ca2+) from the sarcoplasmic reticulum, which surrounds bundles of muscle fibers. The Ca2+ spreads through the muscle and attaches to a molecule called troponin, these troponin molecules are situated on a filament called tropomyosin which runs along the length of the actin filament. When Ca2+ binds with troponin it causes the trpomyosin filament to move, exposing myosin binding sites on the actin. In the presence on ATP myosin cross bridges bond with the actin filament and immediately goes through a deformational change to pull against the actin. During this action ATP is converted to ADP and the myosin cross bridges are only broken when more ATP is available. When the electrical impulses cease the concentration of Ca2+ rapidly decreases and Ca2+ moves back into the sarcoplasmic reticulum.
The only energy source that muscles use to contract is adenosinetriphosphate (ATP), as ATP is used it is converted into adenosinediphosphate (ADP) and Phosphate (P) and some energy is released. The body then uses fat, carbohydrate, glucose, protein and phosphocreatine to convert ADP back into ATP. The source used to regenerate ATP depends on the rate of energy production required (i.e. jogging or sprinting). The ATP content of cells cannot be allowed to fall substantially, therefore, for exercise to continue ATP must be resynthesised and fast as it is hydrolysed. If the cells had no ATP a state of rigor mortis would set in.
ANAEROBIC METABOLISM - There are 2 methods of anaerobic ATP resynthesis: Phosphocreatine/CreatinePhosphate (PCr), and Glycolysis:
PCr is a high energy phosphate compound which can be very rapidly broken down to produce the energy required for the resynthesis of ATP. When it is broken down it not only produces energy but also the phosphate group required by ADP to form ATP.
PCr + ADP -----------------> ATP + Cr
For every mole of PCr broken down 1 mole of ATP is produced. PCr provides the highest rate of energy required for the resynthesis of ATP, and with no waste products. However the body has a relatively small store of PCr.
The body's stores of Glycogen (used in glycolysis) are 4 to 5 times greater than the PCr stores. Glycogen is a carbohydrate stored within the muscle cells and is broken down by a chain of steps. Each step is controlled by enzyme activity to produce pyruvic acid.1 mole of glycogen and 1 mole of ATP are used in the resynthesis of 4 moles of ATP. Glucose, which the liver has large stores of, can also be used in this process. In addition to the glucose from the liver which circulates around the blood-stream the muscles have a very small store. To 'lift' glucose into the glycolytic pathway requires the energy from another mole of ATP, so in total 1 mole of glucose requires 2 moles ATP to produce 4 moles of ATP.
In the absence of oxygen or more correctly if the rate of glycolysis exceeds the rate at which pyruvic acid can be oxidised then that pyruvic acid which is not oxidised is converted into lactic acid, which immediately dissociates into lactate and H+.
OXIDATION OF CARBOHYDRATE
The oxidative production of ATP involves 3 processes:-1.Glycolysis, 2.Krebs cycle, 3.Electron transport chain.
The pyruvic acid produced by glycolysis is converted into acetyl Coenzyme A (acetyl CoA) which then enters the Krebs cycle which permits the complete oxidation of acetyl CoA. At the end of the Krebs cycle 2 moles of ATP are resynthesised, producing CO2. The Krebs cycle also produces hydrogen ions which would make the cell too acidic if they were to stay there, so they enter the electron transport chain (ETC). The electron transport chain ultimately produces water and enough energy to resynthesise a further 34 moles of ATP.
In total the energy yield from 1 mole of carbohydrate is 39 moles of ATP (3 from glycolysis, 2 from the Krebs cycle, 34 from the ETC). If the raw product is glucose then 38 moles of ATP are produced.
OXIDATION OF FAT
Triglycerides are stored in fat cells and must be converted to free fatty acids (FFA) by lypolysis before they can be transported to the muscle cells and used for energy production. FFA are broken down (catabolysed) in the mitochondria of muscle cells by a process of beta-oxidation to produce acetyl CoA. The combined reactions of oxidation, the Krebs cycle and the ETC produces 129 moles of ATP from 1 mole of palmittic acid (a very abundant FFA).
Although fat provides more energy per gram than carbohydrate, fat oxidation requires more oxygen than carbohydrate oxidation. Oxygen delivery is limited by the oxygen transport system so carbohydrate is the preferred fuel during high-intensity exercise.
Slow twitch fibers have a larger number of mitochondria than fast twitch fibers and so can utilize more FFA for energy production.
The healthy body utilizes little protein during rest and exercise (under 5% of total energy supply) but you should be aware that the amino acids that make up protein can be converted into forms for entry into the oxidative processes of energy provision.
Why is lactic acid a problem?
Lactic acid dissociates into lactate and Hydrogen ions (H+) and at moderate levels of exercise both begin to build up in the body. The build up of H+ is what you to 'tie up', this is due to H+ having a greater affinity with troponin than Ca2+. As the H+ build up they bind with troponin, preventing Ca2+ from binding, therefore less myosin binding sites are exposed and the muscle has difficulty contracting.
Lactate diffuses slowly into the blood-stream, some is oxidised to produce carbon dioxide (CO2) and water, some is resynthesised back to glycogen by glyconeogenesis.