Energy Systems Used in Endurance Athletics

I’m not an exercise physiologist, and I also don’t feel the need to completely re-create the wheel, when it comes to discussing energy systems in the body. There are plenty of articles, blog posts and videos out there about the various energy systems our bodies use to fuel different levels of exercise intensity. That said, some of them are very confusing and can be hard to digest (pun intended) into usable chunks of information. My intention here is to share some resources with you to establish an understanding of what’s happening in the body during different types of activity (long, easy or moderate efforts vs. short, high-intensity bursts of effort). This foundation will help us build on future discussions about everyday fueling and pre- and post-workout fueling.

So, without further ado, let’s get to it.

If you are a visual learner like me, you might benefit from this great little video that reviews the 3 energy systems in the body

There’s another good (albeit more in-depth) explanation of the energy systems on runnersconnect.com.

If you don’t have time to watch the video or read the blog post above, here is a cursory overview of our body’s energy systems:

The body’s energy systems use carbohydrates (sugars), fats and proteins to make a molecule called ATP (adenosine triphosphate) commonly referred to as “the currency” of energy in the body. This “currency” moves around inside muscle cells and, upon breaking apart, results in muscle contraction. ATP fuels everything from the functions of our heart, lungs, and all organs in the body, to the muscular demands of the highest intensity activities (sprints, single-set power lifts, 10-minute dance routines and 50-mile endurance races). While all three of these systems run simultaneously in our bodies at all times, the type and duration of activity dictates which energy system is predominantly producing ATP at any given moment. Here are the three energy systems:

1. Anaerobic Alactic System (alias: Phosphagen System or ATP-PCR System) This energy system is called upon during VERY short bursts of intense movement—think box jumps, a 10-15 second surge in a race, or the initial few seconds of effort when the gun goes off. Like the anaerobic lactate system, this energy system is not oxygen-dependent. In fact, it uses ATP that has already been made and stored in the muscle cells—about 10 seconds worth of stored ATP. This system is called upon during VERY short bursts of VERY intense activity.
2. Anaerobic Lactate System (alias: Glycolysis or Glycolitic System) This system kicks in as your physical intensity level increases, eventually resulting in a deficit of circulating oxygen in your blood stream. Being an anaerobic system means that the body can make ATP from sugar molecules without the use of oxygen. Maximum effort activities fueled largely by this system can last between 10 seconds and 2-3 minutes. This energy system dominates during activities such as the final sprint finish in an endurance race, during a longer set of lifting heavy weights, and in sporting events that include a lot of starting and stopping (think soccer, basketball, etc.). You’ll know you’re tapping into the anaerobic lactic system when your muscles start to burn and feel weak–due to the muscle tissue becoming more acidic from hydrogen molecule buildup–and breathing becomes faster and shallower. Training your aerobic system at slightly higher intensity levels (such as during tempo workouts) helps the body to remain in the aerobic system longer before having to engage the anaerobic lactic system for energy.
3. Aerobic System (alias: Oxidative System) This system dominates during long-term, low to moderate intensity activity. As an aerobic system, lots of circulating oxygen in our blood stream is required to break down fats, carbs and protein molecules to make ATP.  Activities fueled by the aerobic system can last anywhere from minutes to hours.
   While initially preferring to “burn” carbs to make ATP, the longer the activity lasts, the more likely this system is to use fat and protein to make ATP.

Those of us with a little science geek in our core will appreciate this depiction of energy system interplay:

Image courtesy of http://www.aflcommunityclub.com.au/

If you really want to go deep into this energy systems stuff, check out the handbook from the International Triathalon Union: https://www.triathlon.org/uploads/courses/54802/4-Energy-Training-Module_1.pdf

In a follow-up post, we’ll talk about how to optimally fuel these different types of energy systems.