This will be the first in a series of articles on basic exercise physiology. I had initially just wanted to do an article on lactate. Lactate, as it pertains to training and the lactate threshold comes up all the time. This opened a huge can of worms, because how do you even begin talking about lactate without explaining the basic energy systems and why we need them. Therefore, this article will not be about lacate per se, but instead it’ll be a foundation upon which we can talk about all sorts of things in the future.

Lastly, I'm providing this information without references because I'm bad and lazy. Most of the graphics are straight out of Wikipedia. None of this stuff is cutting edge, and can be found in any intro level biochemistry textbook. I hope we can do an actual journal club style post in the future.

To make sure we’re all on the same page, we need to define some terms.

Definitions

• ATP - Adenosine triphosphate. This is the energy currency of the body. Everything runs on ATP, despite what Dunkin Donuts tells you. As the name implies, it is made up of adenosine three phosphates. Your body has a bunch of different ways of making and recycling ATP. You can make it in the presence of oxygen (aerobically) and in the absence of oxygen (anaerobically).
• ADP - Adenosine diphosphate. This is the broken down form of ATP after it has broken a phosphate bond. It’s that 3rd phosphate that is instrumental in transferring energy. ADP can accept a phosphate and go back to being ATP.
• Glycogen - Glucose is stored in your body as glycogen. The glycogen is stored in your muscles as well as your liver.
• Glycolysis - The process of breaking down glucose to form usable energy (ATP) in the absence of oxygen. The name makes it sound like you’re breaking down glycogen, but that is not the case. You are breaking down glucose. Breaking down glycogen is a different process called glycogenolysis. Don’t confuse them.
• Gluconeogenesis - The process of making glucose out of other stuff. Basically it’s the process by which we all keep our blood sugar from dropping when we’re short on glucose (starvation, extended periods of exercise, etc). For example, your body can break down the protein in your muscles to make glucose. You’ll often see people into weightlifting avoid cardio. This is one of their reasons. The real reason is because they’re soft.
• Lactic acid - In the body, lactic acid is produced during glycolysis, but it is quickly dissociated into lactate and H . The acid form isn’t relevant. That free H is potentially important, and we’ll come back to it in a future article. For the sake of keeping things simple, think of lactate and lactic acid as the same thing for now.

ATP and your muscles

ATP is needed to make muscles contract. At rest, your muscles keep some ATP lying around ready to go. Unfortunately, there is only enough ATP to last a few seconds. Luckily, your body has 3 different ways to get ATP.

1. Creatine phosphate
2. Anaerobic cellular respiration (glycolysis)
3. Aerobic cellular respiration

Phosphocreatine (creatine phosphate)

Your muscles will take any extra ATP that’s laying around and store it in the form of phosphocreatine. The ATP donates a phosphate to creatine, making phosphocreatine. The reaction looks like this.

This reaction works both ways. When your muscles need the ATP back, it breaks down the phosphocreatine back to creatine. This process basically donates a phosphate back to ADP, converting it to useful ATP.

The good news: This pathway makes ATP really quickly. The bad news: There’s only enough creatine in your muscles to last about 15 seconds. This is why you see weightlifters taking creatine supplements. Supplementing with creatine will increase your muscle’s ability to contract. Unfortunately, supplementation of creatine has no clear direct benefit for distance running, though data suggest some benefit for sprinting.

Glycolysis (Anaerobic cellular respiration)

Glycolysis is the next option for your muscles to make ATP. This is the process of taking glucose and converting it to ATP. This reaction is complicated, so don’t worry about the exact steps. The main point is that glucose gets converted to ATP. A simplified version looks like this:

• Glucose -----------> 2 ATP 2 pyruvate

Where does the glucose come from? There are two major sources. First, there is some glucose floating around in your blood at all times, simply referred to as blood glucose. The second source is from glycogen. Think of glycogen as a long string of glucose molecules all linked together. In the liver, glycogen can make up about 5% of the organ's weight, coming out to about 100–120 grams of glycogen. In skeletal muscle, glycogen is found in much lower concentrations (1–2% of the muscle mass). The skeletal muscle of an adult weighing 70 kg can store roughly 400 grams of glycogen. With training, your body can become better at storing and using glycogen.

The bad news about glycolysis: This process is slower than phosphocreatine. Also, it only makes 2 ATP from 1 glucose molecule. It’s not very efficient at all. This pathway will give you enough energy for about 60 seconds.

Lastly, you’ll notice that the reaction makes something called pyruvate. Pyruvate is pretty amazing, and is a great energy source. There are two ways your body can use pyruvate. If there is no oxygen around, the pyruvate gets converted to lactate in a process known as the Cori cycle. I will elaborate on that in a separate article about lactate. If there is oxygen around then pyruvate gets converted to ATP through a process known as aerobic cellular respiration, which we will discuss below.

Of note, pyruvate is commercially sold as a supplement, and is easily purchased on Amazon. There’s no good data to suggest that it actually helps with performance. It’s not absorbed well, and high doses of it cause GI problems.

Aerobic cellular respiration

Unlike glycolysis, which occurs anaerobically, this process uses oxygen. Aerobic cellular respiration refers to the conversion of glucose into ATP in the presence of oxygen. It takes place in the mitochondria, which is the powerhouse of the cell. The exact reaction is complicated and involves something called the Citric acid cycle and the electron transport chain. Again, these diagrams are complicated, but I provide them for the nerds among us. I won’t go into the details, but basically pyruvate is shuttled into the mitochondria and converted into ATP. Aerobic cellular respiration is very efficient and can convert 1 glucose into 36 ATP. 36! This is much better than the crappy 2 ATP you get from anaerobic respiration.

• 1 Glucose O2 ---> 36 ATP

Where does the oxygen come from? So, there are two sources: hemoglobin and myoglobin. Myoglobin is related to hemoglobin, with which you may already be familiar. Hemoglobin carries oxygen in our blood. Myoglobin delivers oxygen to our muscles. Myoglobin requires iron for its production, so iron deficiency can cause decreases in both hemoglobin and myoglobin. The red color of raw beef comes from myoglobin, not from blood, as is commonly thought.

Endurance training has several benefits that improve the efficiency of this pathway.

• Increases the size and density of mitochondria in your cells, which allows more sites for this reaction to occur.
• Increases the production of the mitochondrial enzymes involved in the citric acid and electron transport chain.
• Increases capillary production, allowing for increased blood flow to the muscles.
  
• Increases myoglobin production, which helps diffuse more oxygen to the mitochondria.

These factors increase ATP production several fold. The overall result is improved endurance performance.

Although I’ve focused on glucose, aerobic respiration can also convert fat into ATP. While 1 glucose leading to 36 ATP may sound impressive, it can take 1 fatty acid molecule and convert it into >100 ATP. Aerobic respiration is efficient, but slow and needs a constant stream of oxygen. As such, it is ideal for distance running.

This graph demonstrates the importance of the aerobic and anaerobic systems as the duration of exercise continues. As you can see, the anaerobic system is very quickly replaced as the main energy source, but never really stops contributing.

Summary

In summary, your muscles have 3 major ways to get ATP: phosphocreatine, glycolysis, and aerobic respiration. Although all 3 are used in running, aerobic respiration is king with the highest level of efficiency.