Here is a piece I've written which details the four ATP generating energy systems, how and when these energy pathways are used, linking this up with how they fit into human movement and physiology, and finally how to use this knowledge to train specific performance goals.
Cellular Respiration and ATP
Most of us are familiar with calories and that these are units of measuring the potential energy we get from proteins, carbohydrates, fats and alcohols. While these do end up giving us the energy and building blocks to live and grow, if they are used for energy they are first broken down into a chemical called adenosine triphosphate (ATP), which is an adenosine molecule with three phosphate groups attached. Whatever energy is needed and generated by our body to be used chemically, mechanically, thermally or electrically, this is achieved by an enzyme splitting one of the phosphate groups off of the ATP molecule, which yields energy. This then becomes ADP (adenosine diphosphate; ATP with just two phosphate groups remain), which can continue to be broken down to AMP (adenosine monophosphate, a single phosphate group), and so on. Whether from the broken down nutrients from food or the storage forms of nutrients, the forming and reforming of ATP is the goal of our energy systems.
In a lot of ways this may seem simple and in a lot of ways this may seem complicated. Putting it simply, our nutrients (proteins, fats, etc.) are broken down until they are either building blocks or ATP, and then the phosphate groups are stripped off one at a time for energy as needed. The important thing to know about all of this as an athlete, a trainer, coach, or other medical/exercise practitioner, is that there are four ways that our body turns large nutrients into ATP, with each of the four taking a different amount of time to refill our ATP potential energy stores, but each one also has a different capacity/endurance. One system restores ATP for use very quickly, but the energy that it makes can generally only yield 4-12 seconds or so of activity, while the opposite end of the spectrum can be quite slow to generate the energy we need but has an incredible endurance if we are not demanding highly intense amounts of energy use for long periods of time (aka this is the energy system that keeps us breathing and alive, whenever we are not actually in moments of intense exercise).
And what does this all end up meaning? If you want to train to be a sprinter...you need to emphasize training of the right energy system. If you want to be an endurance cyclist, it will require the development of a different energy system than a sprinter. Each different goal in sport and life when it comes to movement will require a different mode of training if you want to improve physical performance. As the fundamental unit of energy for the human body, anything we need to use energy for (as listed above as for mechanical, thermal, chemical, or electrical needs and actions) uses ATP, but I will be chiefly approaching the use of ATP for exercise and other movement. ATP is used in the contraction and release of our muscle fibers and so for proper training methods to optimize energy systems for any given action, so in this text it is muscular ATP needs that are being addressed most directly.
In this text I will describe each of the four energy systems, how they chemically work in our bodies to (re)generate ATP, how they function during bouts of exercise in different intensities and durations, as well as other topics such as how supplementation and stresses on the body can alter the ability of these systems to work at peak potential. Through this I will cover how to appropriately train for your particular sport or activity with a better chance of optimum performance and improvement.
Most people have heard of the terms aerobic and anaerobic before, often being taken to mean light cardio work and then high intensity exercise, respectively. The word aerobic means with oxygen, the word anaerobic means without oxygen. Two of the four energy systems are anaerobic and only last a short while because they can be done without oxygen (so they generate lots of waste without disposing of it at the same time), while two are aerobic and offer energy over much longer periods of time and with much higher efficiently (albeit at a slower rate). It must be noted that all four of your energy systems are always going at any given moment, but the degrees at which any given energy system is providing the majority of energy being expended varies depending on how intense your activity is (from sitting in a chair to sprinting a 100 meters, with a lot of room in between) and for how long it lasts.
The four energy systems are the creatine phosphate system, fast glycolytic system, slow glycolytic system, and beta oxidative system. This list goes from the fastest ATP generating system while having the lowest overall output (runs out the quickest), to the slowest but most enduring system. The first two are anaerobic and second two are our aerobic systems. During our periods of lower intensity movement and energy demands it is the last two systems that supply the majority of our energy needs because they are most efficient. While slower to process they generate by far the most energy. Below we see two tables, one which compares the energy output of each system, as well as about how long they last. Also note, oxidation is a process which can turn several types of nutrients into ATP, and they have been separated into two systems in one of the graphs below as they do have some differences in time and yield.
As stated before, each of the energy systems is always contributing some portion of the (re)generation of our ATP stores in our muscles and throughout the body, even after they have ‘fatigued’ and are not longer the main source of energy supply. When at rest or low intensity activity it is our aerobic systems that are most responsible. At high(er) intensity levels the anaerobic systems dominate, until they fatigue. If you are lifting heavy weights in a gym that are really challenging you, you are probably resorting to your creatine phosphate system, and as it quickly fatigues it mingles with the next fastest system until it phases out, and so on until the body can lower its intensity or rest to refill the anaerobic systems, or the body simply fails to continue to perform at such high intensities and it slows down until it can get enough oxygen for the aerobic systems to cover its energy expenditures.
In the most vigorous activities our body uses up its stores of ATP in each muscle, or other activity related cell, quickly and its first reserve system to refill it’s energy stores is the creatine phosphate system. This is a very simple one for one reaction where our now partially expended adenosine diphosphate takes a phosphate group from the molecule creatine phosphate (an enzyme is often used for these processes to separate a portion of a molecule off for some biological process, in this case creatine kinase, though we will not go into the detail to describe most of the other enzymes for the other energy systems, which are far more complex), resulting in one ATP and one creatine (the enzyme is always unchanged after a reaction, it is just a facilitating molecule). This process, as well as the next (fast glycolysis) happens in the cytoplasm (or sarcoplasm in muscles fibers) of each cell, which is the ‘watery’ open space of your cells. This can be analogous to maybe the middle of your living room floor.
Here we can see an ATP molecule, with the right sections being
adenosine, and the three phosphate groups being visible on the left.
The chemical equation for this would be: ADP + Creatine Phosphate ---> ATP + Creatine. With each reaction a single ATP is created, which is not a lot of energy but as only one reaction is required it is very quick, so at the first few moments of an intense exercise (only 4-12 seconds!) it provides a burst of energy. Towards the end of these few seconds fast glycolysis starts to merge with creatine phosphate as the chief ATP re-supplier.