Design Better HIT Workouts With This Guide To Energy Systems

Designing workouts is about a lot more than picking your favorite exercises. Many factors go into putting together a series of workouts that have the power to change your body. In addition to exercise selection, you must account for the number of repetitions, rest periods, tempo (how fast the different phases of the rep are performed) as well as other factors that affect performance such as life stress and physical activity.

To do this, you need an in-depth understanding of the three energy systems that govern the body’s ability to produce energy. This article will provide an overview of the energy systems and help you understand how to use this knowledge to create effective workouts.

Aerobic Versus Anaerobic Exercise

Hopefully, you remember from elementary science class that the body uses the molecule ATP for energy. There is a very small amount of ATP already stored in the muscle, but the rest must be synthesized from other fuels in the body: phosphocreatine, glucose, fat, and protein. There are three distinct chemical processes for producing ATP, which are known as “energy systems.”

Two of the systems are anaerobic and do not require oxygen to be present (“anaerobic” means “without oxygen”), whereas the third is aerobic and does require oxygen (”aerobic” means “with oxygen”).”

You might wonder how exercise can occur without oxygen. After all, you are always breathing. When you perform a short, very intense bout of exercise such as a 50-meter sprint, you are moving at such a rapid rate that the body is not able to inhale and circulate oxygen to the working muscle fast enough. The body can produce energy without oxygen, but only for a short period of time.

The alternative is aerobic exercise, which occurs when oxygen is present. You’re working at a pace that is moderate enough that you are able to breath in oxygen and it has time to circulate to your working muscles. Aerobic exercise can go on indefinitely.

As a general rule, strength training and sprints are more anaerobic in nature; however, because these exercise modalities are intermittent and are repeated over and over, they tap into the aerobic system during rest periods. Distance running, cycling, and walking are all examples of exercise that favors the aerobic system. Additionally, when you are resting, sitting or standing at your desk, or walking across a room, you are predominantly using the aerobic system.

The Three Energy Systems

We are going to look at the three energy systems now, which can get confusing. Don’t give up on us here! We’ll try to make it as clear as possible.

#1: Phosphagen System (a.k.a. ATP-CP)

First, there is the short-term phosphagen system, which burns phosphocreatine that is stored in the muscle. The phosphagen system is also known as the ATP-CP system and it allows the body to produce a small amount of energy very quickly when oxygen is not present.

It fuels the highest power output, but only has limited fuel available from small stores of ATP and phosphocreatine in the muscle. This system provides energy for up to 15 seconds of intense activity such as lifting an air conditioner off the ground, going up a flight of stairs, running a 50-meter sprint, or doing a maximal effort deadlift.

#2: Anaerobic Glycolytic System

Once the ATP-CP system runs out of energy, the second energy system known as the anaerobic glycolytic system takes over. It is a 10-step system that provides enough ATP to fuel activity for 60 to 110 seconds once the ATP-CP system conks out. This system burns carbohydrates from glucose or glycogen to produce ATP and is most active during a 400-meter race, when doing a certain number of deadlifts or chin-ups for time, or doing a shuttle run.

During glycolysis, your cells produce something called pyruvate, which has one of two fates. If no oxygen is present (because you’re working at a very intense rate), pyruvate is converted to lactate. When lactate accumulates at a rate faster than the body is able to metabolize it, it leads to the buildup of hydrogen ions that cause a burning sensation in the muscle. The muscle loses strength capacity and exercise intensity decreases.

On the other hand, if you’re training more moderately and able to get enough oxygen to the muscles, pyruvate is turned into acetyl coenzyme A, which enters the Krebs cycle and is used to make more ATP. This is part of the oxidative aerobic system—also known as aerobic glycolysis, with “aerobic” referring to the fact that oxygen is present and “glycolysis” referring to the fact that carbohydrates in the form of glucose or glycogen act as a fuel source of this third energy system.

#3: Oxidative Aerobic System (a.k.a. aerobic glycolysis)

Many people get confused about the relationship between anaerobic glycolysis and the oxidative aerobic system and the easiest way to think about them is that the oxidative aerobic system is just a continuation of anaerobic glycolysis that occurs when you’re working at a pace that is slow enough that oxygen is present. Your power output is low enough that pyruvate (the product of glycolysis) is turned into acetyl coenzyme A (instead of lactate) and enters the Krebs cycle, resulting in ATP production.

The oxidative aerobic system is also capable of burning fat or even protein when oxygen is present. Triglycerides, which is the physiological term for body fat, are released and transformed into fatty acids. Fatty acids are broken into acetyl coenzyme A, which enters the Krebs cycle to produce ATP. The great thing about burning fat is that each fat molecule can produce a LOT of ATP (129 ATP per molecule), which can fuel low-intensity activity for a long period of time. The one drawback to burning fat is that the process is fairly slow and can only be tapped into when power output is low.

In contrast, glucose molecules (from carbohydrate) that go through the Krebs cycle yield far fewer (38 ATP per molecule), but the process is faster. This is why when people talk about the “fat burning zone” they are referring to working at a fairly easy pace with a low power output. This “zone” was erroneously believed to be useful for losing body fat, but in actuality, calorie expenditure is what matters for losing fat, and this is maximized when working at higher intensities even though it uses carbohydrate to a greater degree.

The oxidative aerobic system provides much of our energy when we are at rest, or when performing easy to moderate exercise intensities, such as walking or distance running. Another time that the oxidative aerobic system kicks in is during rest periods when weight lifting, sprinting, or performing any type of anaerobic exercise that is intermittent in nature.

Relationship of The Three Energy Systems

It’s important to know that multiple energy systems can be active at once. A simple way of understanding the progression of the three energy systems is to think of an all-out sprint, to a slower run, to a jog, to an eventual walk. Throughout that progression, all three systems will contribute, but the ATP-CP system will dominate for the first 10 seconds. Once that is exhausted, the intensity decreases slightly, and the anaerobic glycolytic system takes over for another 90 seconds until your pace slows significantly more and you drop off to a jog/walk.

Although duration and power output both play a role in determining which energy system dominates, power output is more important. Here’s an example that makes it easier to understand:

If you get up and walk across the room for 10 seconds, which energy system would you predominantly use?

The answer is the oxidative aerobic system because the power output is so low. Even though you are only walking for 10 seconds, the power is low enough that you could sustain if for hours and hours.

In contrast, if you sprinted all-out for 10 seconds, what system would you predominantly use?

The answer is the phosphagen system because it is the only system capable of producing ATP fast enough to allow you to produce the high degree of power necessary for a sprint.

Work-To-Recovery Ratios

Now that you have the three energy systems down, it’s time to talk about how to apply this knowledge when designing workouts. The key to putting workout protocols together is to use work-to-recovery ratios.

Whether you are training sprints, weights, or some other high-intensity variation, your work is intermittent in nature. Simply, this means that you are performing repeated vigorous efforts interspersed with recovery. The duration of your work efforts and your recovery time will dictate which energy systems are dominating as well as the unique adaptations you get from your workouts. Exercise scientists have devised work-to-recovery ratios to describe the duration of work and rest bouts to target each energy system that are evident in this chart:

Approximate % of Maximum Power	Primary Energy System Stressed	Typical Exercise Duration	Range of Work-To-Recovery Ratios
90-100	Phosphagen	5-10 sec	1:12 to 1:20
75-90	Glycolytic	15-30 sec	1:3 to 1:5
30-75	Glycolytic & Oxidative	1-3 min	1:2 to 1:4
20-35	Oxidative	>3 min	1:1 to 1:3

Let’s take an example, such as a 400-meter sprint. Say you can run a 400 in 70 seconds. This would utilize primarily the anaerobic glycolytic system (and a small amount from the phosphagen system at the beginning of the effort). If your protocol tells you to run a total of four 400s, how long should your recovery period be?

Looking at the chart, you can see that you want a work-to-recovery ratio ranging between 1:2 and 1:4. If you pick the 1:2 ratio, your rest intervals would be 140 sec or 2 min and 20 sec, whereas if you pick the 1:4 ratio, your rest would be 280 sec or 4 min and 40 sec. That’s a big difference. Which to pick?

The answer comes down to what your goal is. When running a 400 all-out, you use the anaerobic glycolytic system, which means there is a considerable amount of lactate accumulation in the primary muscles involved in the run. The body can metabolize lactate during rest when oxygen is present, using it for energy. However, this process is relatively slow.

If you run your second 400 too soon, not enough of the lactate will have been cleared from the muscles. Remember, that once lactate accumulation surpasses a certain threshold, it leads to the build-up of hydrogen ions in the muscle, causing acidity and fatigue. When this happens, the muscles start to lose power and performance decreases.

Determining the correct work-to -recovery ratio requires you to identify the goal of training. Consider the following diverse goals:

#1: Peak performance in which you shoot for a personal record on each 400

#2: Fat loss in which you want to produce as much of a metabolic disturbance as possible, which is best done by a significant build-up of lactate

#3: Sports conditioning in which you want to increase your repeated sprint performance and ability to tolerate lactate

If your goal is #1 peak performance, you want to allow complete rest so that as much lactate is cleared as possible before repeating each 400. You should shoot for the high end of the range (1:4), and you could even round up 4 min and 40 sec to 5 min rest periods to be on the safe side.

If your goal is #2, we know that lactate accumulation correlates with release of the catecholamine hormones and growth hormone, which have lipolytic, fat burning effects. Lactate accumulation is also linked with EPOC, which is the increase in metabolism you experience during the recovery period after an intense workout. These factors are known to improve fat loss.

Therefore, using shorter recovery intervals is indicated to produce as much lactate as possible. Of course, training above the lactate threshold is painful and not easily tolerated, so you have to use some finesse in determining the ideal ratio for the trainee. For an athlete experienced at suffering through tough workouts, the low end of the range (1:2 could be used), whereas for individuals with less training experience, you should start on the high end and work down as the trainee becomes accustomed to pushing through the pain.

For #3, you’re going to err on the low side of the range with shorter rest, however, you don’t want performance to drop off too much. For example, if an athlete is running four 400s and the goal pace is 70 seconds, if they run 70 sec, 77 sec, 82 sec, and 85 sec because you only gave them 2 min recovery, their power has radically decreased. Dropping from 70 to 77 sec from the first to second repeat is a 10 percent decrease, which is quite significant. This is not ideal

Instead, you’re going to get better results from building up the athlete’s conditioning by starting with training distances that allow them to produce and maintain a power output that correlates with that used in competition. You can progressively shorten the work-to-recovery ratio, and/or increase the number of repeats, as the athlete gets in better shape.

Final Words: One of the most common errors when designing workouts is the “more is better” mantra. Unfortunately, this leads to overly long workouts, too little rest, poor all-around performance, and ultimately, overtraining.

In reality, you should strive for the minimal dose of training necessary to get the maximal effect. Understanding work-to-recovery ratios can allow you to do this so that when your goal is to train for power and performance you are actually achieving those goals instead of having your workouts evolve into long, aerobic-centered slogs. Doing less and doing it right is a good thing!