Introduction to exercise metabolism
 

Human locomotion, and anything as far as maintaining vital bodily processes is only possible through the use of energy. Most often, this energy comes in the form of ATP. This is the most important equation to consider in exercise bioenergetics:

ATP---> ADP + PI + H+ + energy

The above equation is known as ATP hydrolysis, which produces energy. But, the key thing to keep in mind (which is why I bolded it) is that ATP hydrolysis dissociated a proton (a hydrogen ion). This is important because it lowers the pH, which means that the muscles are now in a state of acidity. For muscles to function properly, enzymes are needed to speed up the rate of reactions by up to trillion folds. It is important to keep in mind that enzymes are proteins, and when in a state of acidity, these enzymes will denature (basically, their structure is disrupted), and the muscle doesn't function. For those who want to get technical, there are other aspects involved, but we won't go into that.

So, now as a result of muscle contraction, we have the dissociation of hydrogen ions. In order to maintain function, the body must buffer these hydrogen ions, to maintain a normal pH (level of acidity). It does so in the following main ways:

1) The coenzymes NAD+ and FAD. These coenzymes are reduced by bonding the hydrogen ions to form NADH and FADH2. NADH and FADH2 then ship the hydrogen and electrons out of the mitochondrial matrix (the inner part of the mitochondria) into the part known as the intermembrane space.

This diagram below is complex, but I circled the parts to focus on. Just pay attention to the point that NADH, and FADH2 in the matrix sends hydrogen ions out into the intermembrane space. These hydrogen ions will eventually go back into the mitochondrial matrix through the ATP synthase motor, which by it's name, forms ATP. Each NADH forms around 2.5 ATP, while each FADH2 forms around 1.5 ATP.

2drdmko-1.jpg

2) The bicarbonate buffering system

This works by the following reaction.

H+ + HCO3- ---> H2CO3---> CO2 + H2O

As most are certainly aware, when we exhale, we breathe out CO2. During exercise, obviously we use much more energy than at rest. In turn, we are releasing a lot more hydrogen ions. Therefore, we form much more CO2.

The CO2 formed is a result of bicarbonate buffering of hydrogen ions is the reason we get out of breath during exercise. We don't get out of breath because we need to take more oxygen in, we get out of breath because we need to get CO2 out. CO2 stimulates chemoreceptors in the brain which in turn sends the stimulus to breath.

c) Lactate metabolism.

Typically, the end product of glycolysis is a a 3 carbon sugar known as pyruvate. In turn, pyruvate goes to Acetyl CoA.. which enters the Krebs cycle and goes through all the other steps of metabolism, but we won't worry about that. Originally, it was thought under aerobic conditions, pyruvate was the end product of glycolysis, while under anaerobic (no oxygen) conditions, lactic acid was formed. Relating this to exercise, it was thought that with strenuous exercise, we became short on oxygen, and therefore lactic acid formed which was the cause of the "burning" sensation. This is false. We now know that as mentioned, we breath to get CO2 out, not because we need oxygen in. Hydrogen ions are the cause of the burning sensation.

Lactic acid is formed by the following reaction, catalyzed by the enzyme lactate dehydrogenase

NADH + H+ + Pyruvate --- NAD+ + lactate

From there, lactate has several fates:
1) the main fate is direct oxidation in working muscle
2) it can be shipped to inactive muscles, particularly muscles with slow twitch fibers (because they are abundant in mitochondria).
3) a substrate for energy by something called the Cori Cycle
4) other ways

So, going back to the study by Robergs that Tim posted, the lactate reaction actually prevents fatigue, not causes it. It does so by buffering hydrogen ions, and by providing a substrate for energy

Believe it or not, sitting here right now, we are all producing lactate. But, it gets oxidized before it is spilled over in the blood. So, lactic acid is not produced due to a lack of oxygen.


a good animated video of basic metabolism.

..... this is just a general overview, I could add more and talk about adaptations if requested, though it's not as easy to do on the net.