Energy Systems, Lactate and What Age-Group Athletes Need to Know

By Mark Turner | April 03, 2017, 4:59 p.m. (ET)

runner tying shoe 

ATP, ATP-PC, ADP, Krebs, glycolysis, phosphorylation, ATPase, lactic acid, lactate, pyruvate. All of these and more are important biological and scientific terms and descriptions that underlie endurance training protocols. However, they and the processes they describe can make the average age-group athlete's head spin. So what is actually necessary for endurance athletes to understand about how the human energy system works?

Very few of the athletes I coach want me to get so detailed in my discussions with them about their training that I start to sound like their high school biology teacher more than their coach. That being said, a basic understanding of the human energy system, or more accurately, the three energy pathways of the human body does help in explaining such simple ideas as: Why easy days should be easy or why low heart rate aerobic sessions are just as important as key higher intensity workouts.

The concepts that I want my athletes to grasp are simple. First, I want them to grasp the basic difference between aerobic and anaerobic work. Then within the anaerobic energy pathways I want them to distinguish between the glycolytic and phosphagenic systems. While the average age-group triathlete does not need to have a full grasp of the chemical formulas that accurately describe what is occurring, they do benefit greatly from a better understanding of the role of two key players: carbohydrates, or more simply sugar and lactic acid, or more correctly lactate. Athletes also benefit greatly from understanding some basic math in the following form: 9 is greater than 4. Why is this important? Because most age-group athletes are unaware that both 1 gram of carbohydrate and 1 gram of protein have 4 calories each while fat has 9 calories per gram. 

The point? Fat is far more energy dense than the standard calorie go to for most amateur athletes, which is sugar. To the degree that we can adapt the athlete's body to run more efficiently in their aerobic zone and burn both sugar and fat is the degree to which we are building a better endurance athlete. Short, easy aerobic training sessions rarely need to be fueled with extra sugar. Allowing the body time to oxidize fat for the primary fuel greatly enhances the body's ability to increase the level of performance in the aerobic zone. This explains why utilization of the aerobic energy system makes up the majority of the endurance athlete's training. As USA Triathlon Level III Certified Coach Coach Mark Sortino points out, “When you consider the total of aerobic-specific sessions and the warm ups, cooldowns and recovery intervals of higher intensity sessions, aerobic work comprises 80-85 percent of a training cycle.” Smart training protocols are designed to increase the athlete's velocity over time at the upper end of their aerobic zone. In other words, the goal of aerobic training is to make the endurance athlete both stronger and faster but still remain aerobic for most of their event by training their aerobic system to be “smarter.” The key word there is “most.” And that is why smart training will also include both forms of anaerobic work in order to build explosive strength, neural conditioning to heightened demand and the ability of the body to more quickly recover to an aerobic state. While all three of the energy pathways can be described in more detailed and technical terms, for most athletes a simplified description linked to the “why” of each workout's purpose is more than sufficient.

A simple way to regard the aerobic energy system is to look at three main components that make up what is known as the Krebs cycle: sugar, oxygen and pyruvate for most of the cycle or lactate above moderate efforts. Oxygen delivery to the system relies on the cardiovascular fitness of the athlete to deliver optimal amounts of oxygen. Sufficient sugar relies on the body's ingestion of sugar or ability to convert fat into sugar. And finally pyruvate, which the body produces in balance with energy demands at low to moderate levels of effort or lactate at higher effort outputs, which gets stored in the blood, as a by-product of glycolysis. At easy to moderate efforts of endurance training very little lactate is produced, and energy demand can be largely sustained with the available pyruvate. Efforts that go above moderate and begin moving toward the upper ends of the athlete's aerobic capacity will bring about the increased production of lactate. At this point an athlete is drawing close to tipping over into the anaerobic zone. In a critical high-intensity workout interval or a short race or part of a longer race (e.g. passing a bike on the bike course) where energy demand will be higher, this is not only acceptable but desirable. However, in a longer endurance session or race, “going anaerobic” too many times or for too long will burn your energy “matches” at a rate that is not sustainable.

In the anaerobic energy systems the energy exchanges take two very different forms. In the glycolytic anaerobic pathway the athlete is going “hard” for anywhere from 30 seconds to 2 minutes. During this exchange, the fuel sources are blood glucose and muscle and liver glycogen, unlike the aerobic system, which also leans heavily on adipose and intramuscular fat as fuel. By duration contrast to its partner anaerobic system, the phosphagenic system is utilized for very short high intensity bursts of up to 10 seconds or so. Think a must needed course correction on a swim or a burst of speed needed to avoid an obstacle. In this process Jason Karp points out, “No carbohydrate or fat is used in this process; the regeneration of ATP comes solely from stored CP.” [1] What this means for the endurance athlete is that these very short bursts of energy are available but limited. What the endurance athlete should be working toward is not increasing the length of their capacity to produce anaerobic energy but rather to decrease the amount of time needed to recover from anaerobic energy demands and return to an aerobic state. 

One other important point that needs to be addressed is what happens when the athlete does increase lactate production as a result of sustained or vigorous effort. Lactate production has been the cause for misguided alarm among endurance athletes for too long. As Owen Anderson points out in his extremely informative book “Running Science,” the body's ability to produce lactate or lactic acid is actually not a cause for concern at all. That endurance athletes still believe that “excess” buildup of lactate in the muscles is the cause of the “burn” in their muscles is merely the unfortunate result of an often repeated myth that has no real grounding in science. As Anderson puts it, “Lactic acid doesn't produce burning sensation, it doesn't induce soreness, and it is not a form of metabolic garbage that must be eliminated from muscle cells as quickly as possible.”[2] Workouts that push the athlete to lactate threshold and beyond, far from creating damage to the muscles from which they must be “healed”, are, more accurately speaking, training the body to more efficiently utilize lactate in a manner that over time increases the overall metabolic efficiency of the athlete. Training the body to utilize the lactate that is often present in abundance will result in improved performance on race day.

I love mental images, and the one that comes to mind for me whenever I think of the aerobic energy system is my 1970 VW Beetle. When I first purchased it back in 1983 a mechanic told me the only thing I needed to get a bug to start and run were oxygen, a spark and fuel. If the car wouldn't start and run then one of them was missing. Over time, I have come to understand that he was almost right. Because you also need timing. And that is where training your endurance athlete engine comes into play. Just like my VW, the basic ingredients are only as good as the work you put in to fine tune their efficiency. 

Team MPI Senior Coach Mark Turner is a USAT Level I Certified Coach, IRONMAN Certified Coach, USAT Cat 3 Rules Official and USMS Level II Coach. He can be reached at

The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.

[2]   Anderson, Owen, PhD. 2013. Running Science. Champaign, Ill.: Human Kinetics.