Adenosine triphosphate (ATP) is the biochemical way to store and use energy and it is used by every cell in your body.
ATP is an adenine nucleotide bound to three phosphates. The bond between the second and third phosphate groups holds a lot of energy which can be used to fuel chemical reactions within the body.
When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule and when it has excess energy it stores this energy by forming ATP from ADP and phosphate.
ATP is required for the biochemical reactions involved in any muscle contraction. Muscle contractions are biochemical reactions which use ATP and as the muscle continues to work harder it needs more ATP, as ATP gets consumed and must be replaced in order for the muscle to continue working.
This means ATP is critical to everyone, but especially athletes and so the body has several systems designed to create ATP. Interestingly though different types of athletes use different systems in order to get ATP. Meaning a sprinter like Usain Bolt will get his ATP in completely different way than someone like Matthew Centrowitz who won gold for the USA in the 1500m in Rio.
Where Does ATP Come From?
ATP comes from three different biochemical systems in the muscle, Phosphagen system, Glycogen-lactic acid system, Aerobic respiration.
? Phosphagen System
In a muscle cell there is a certain amount of ATP floating around that it can use immediately at any given time, however there is only enough to last for about three seconds. To restore ATP levels quickly, muscle cells contain a compound called creatine phosphate which is a high energy compound.
An enzyme called creatine kinase removes the phosphate group from the creatine phosphate, it then uses it to change ADP into ATP. The cell turns ATP into ADP, and the phosphagen turns the ADP back into ATP. As the muscle continues to work, the creatine phosphate levels will drop. Together, the ATP levels and creatine phosphate levels are called the phosphagen system. The phosphagen system will supply energy to a muscle working at a high rate however it can only do this for around 8-10 seconds which is a very short sprint.
? Glycogen Lactic Acid System
Muscles also have larger reserves of glycogen which is a complex carbohydrate. Glycogen is a chain of glucose molecules and a cell splits glycogen into the individual molecules of glucose. Then the cell uses anaerobic metabolism to make ATP and a byproduct known as lactic acid which I’m sure you have heard of before.
ATP under this system is created by around 12 different chemical reactions and thus it is supplied at a slower rate than the phosphagen system. The system can work fairly quick and produces enough ATP to last about 90 seconds. Oxygen is not required by this system which means it does not need to wait for the heart and lungs to be ready and another reason this is good is because the contraction of the muscle squeezes off its own blood vessels which deprives it of oxygen-rich blood.
The limiting factor in anaerobic respiration is lactic acid, this acid is what causes pain in your muscles when performing strenuous activity. It builds up in the muscle tissue and causes fatigue and soreness. Many of you will be familiar with the term the lactic wall where you reach a level so painful your muscles pretty much cease to function and you either rapidly slow or have to stop.
? Aerobic Respiration
After 2 minutes of working a muscle the body responds to supply working muscles with oxygen. When oxygen is present the process of aerobic respiration occurs breaking down glucose into carbon dioxide and water.
The three different places glucose is found are; the remaining glycogen supplies in the muscles, by breaking down the liver’s glycogen into glucose and the glucose which is absorbed from food in the intestine which gets to working muscle through the bloodstream.
Aerobic respiration can also use fatty acids from fat reserves in the muscle and the body to produce ATP. In extreme cases (like starvation), proteins can also be broken down into amino acids and used to make ATP. Aerobic respiration would use carbohydrates first, then fats and finally proteins, if necessary. This is why people perform fasted cardio in an attempt to use their fat and highlights why it is important to supplement with BCAAs when training.
The Exercise Process
So this is what happens when you exercise:
1. The muscle cells burn off the ATP they have floating around in about 3 seconds
2. The phosphagen system starts working and supplies energy for a further 8 to 10 seconds. This would be the major energy system used by the muscles of a 100-meter sprinter, weight lifter or long jumper, where rapid acceleration, short-duration exercise occurs.
3. If exercise continues then the glycogen-lactic acid system kicks in, this would be for short-distance exercises such as a 200m or 400m runners or 100m swimmers. This is why you feel completely different racing these two types of sprints even if they are both the same activity.
4. Last of all if exercise continues even longer then aerobic respiration takes over. This is how endurance athletes produce their ATP. Events such as an 800m, marathon, rowing, cross-country skiing, Triathlons and new obstacle races that range from 3 miles to 24 hour events.
Take Home Message
You may well have heard of ATP, but maybe you didn’t know what it was or what it did. Hopefully after reading this article you now know. To increase your ATP levels it is important to supplement with creatine, as getting creatine from food alone can be difficult and expensive.