Your body has 3 main systems, or "engines", that produce energy and power your various activities in the gym and in life. These are known as your metabolic pathways.
You can get a quick history lesson by referencing the CrossFit Journal’s original article on “Metabolic Conditioning” by clicking here. Greg Glassman (the creator of CrossFit) distills the complex science of how our bodies create adenosine triphosphate (ATP) in an easily palatable read that conveys almost everything we care to know about the subject. Believe it or not, we don’t need a Ph.D.-level understanding of bioenergetics for this information to be useful.
Today, we’ll discuss metabolic conditioning through a lens that will set some context for the next few weeks of programming. We’ll start by identifying the three pathways our bodies use to create ATP.
Short, fast, and powerful.
The phosphagen pathway is responsible for efforts where we exert up to 100% of our body’s potential. What’s the heaviest load you can squat for 1 rep? How fast can you complete a 100-m sprint? What’s the highest target you can touch at the apex of a max-effort vertical leap? Each of these involves the maximum application of force to accomplish the prescribed task.
Note that each of these efforts is completed in just a few seconds. The phosphagen pathway allows the body to fully express the top-end of its power output but quickly begins to fade after the 10-second mark. This is why the pace you start at for your 100-m sprint will begin to slow if you continue running beyond the 100-m mark. It’s why your 1-rep max squat is only 1 rep — after the 10 seconds it takes to complete the work, your body can no longer create that level of power. And in order to generate that power again, the phosphagen pathway requires a work-to-rest ratio between 1:12 and 1:20. In most instances, that’s a range of 2-4 minutes of rest depending on the task.
Moderate power, moderate duration.
The glycolytic pathway is responsible for fueling efforts that exert around 70% of our power output. These are efforts that are manageable but quickly grow uncomfortable as time progresses. If you’ve ever run 400-m as fast as possible, you have felt what we’re talking about. The same goes for an all-out 500-m row. The first 30 seconds are strong and fast — you feel confident and powerful. As the clock continues to roll past the 1-minute mark, your legs feel heavier as your heart rate increases. The pace you started at is still within reach, but you have to work hard to hang on to it.
One minute is where the glycolytic pathway peaks, and it continues to hang on until somewhere around the 2-minute mark. During that time, lactic acid is produced by the muscles as a byproduct of creating ATP anaerobically. The phosphagen and part of the glycolytic pathways are anaerobic — they don’t require oxygen to create ATP. However, once athletes reach their lactic threshold — the point at which lactic acid levels in the blood rise faster than can be controlled — they’re forced to rest and recover. That recovery time ranges from a 1:5 work-to-rest ratio for shorter efforts (~45 seconds) to a 1:2 work-to-rest ratio for longer ones (~2 minutes). Rest can range from 90 seconds to 4 minutes, and this is often why interval work is programmed into CrossFit — to allow for more use and development of the Glycolytic pathway.
Long and slow, but sustainable.
The third and final metabolic pathway is the oxidative pathway. It is responsible for fueling efforts that live around 40% of our maximum power output and continues for as long as the effort lasts. The oxidative pathway begins to take over beyond the 2-minute mark. It is the primary pathway for creating energy and it serves to support our longer-duration efforts. A 1,600-m run or a 2k row fits this bill, but really any effort spanning longer than 3 minutes utilizes the oxidative pathway.
The oxidative pathway is almost entirely aerobic, meaning, it does require oxygen to create ATP.
The Simple Version
The phosphagen pathway is used for very short efforts that are max-effort. It’s rapidly fueled by ATP already stored in the muscles, allowing it to produce tremendous force. However, that supply is quickly depleted and its mechanism for creating ATP cannot occur quickly enough to be able to repeat those same tremendous efforts.
The glycolytic pathway is used for moderate efforts at moderate intensities. It’s the pathway we use most in our workouts. For example, a set of 15 thrusters can take 30-40 seconds to complete unbroken, and a 500-m row can be finished in under 2 minutes. Both of these efforts are glycolytic, however, using this pathway generates lactic acid in the muscles — that burning feeling — that forces short periods of rest.
The oxidative pathway is used to fuel longer-duration efforts lower in intensity. Because oxygen is used to fuel these efforts, and because humans will continue to breathe during exercise, a steady supply of air to create ATP allows these efforts to be sustained for longer periods of time.
While the time domains each pathway falls under are pretty clear, it’s not cut and dry. Our bodies are technically always using all three systems, but one is always the main supplier of energy while the other two contribute to a smaller degree. Think of this like three dimmer switches for the lights on your ceiling. All three can be on, but one can be turned up high and the others low.
As we continue through the month we'll share more about these different pathways and how our body uses them to produce energy!