Understanding the 3 Main Conditioning Energy Systems

Written by Dave Thomas

I am going down a tricky rabbit hole today that I have avoided going down for six years. Conditioning science.

It’s impossible to come out unscathed as opinions and data vary greatly, it’s very difficult to pinpoint energy systems in a mixed modal setting, and data that has been observed often gets very misrepresented. It’s very difficult to assign and prescribe energy system training, as each athlete will be slightly unique and different within a class. My ten minutes of hard work might look a lot different than your ten minutes of hard work. Accordingly, we may be in completely different systems in the exact same workout.

Additionally, while I love what 8 – 25 minutes of conditioning can do in a group setting, the reality is that the model does have holes. Very, very small holes, and for some people no holes at all. But there does exist a natural crack in the armor and the aggressive front that, “high intensity solves all!”

Higher dosed intensity is better than lower dosed intensity for efficiency when working within a finite amount of time, like a 60 minute class. These workouts will accomplish a ton and I anticipate they will make up the vast majority of my personal fitness regiment for as long as I am able. However, it is a straight up misrepresentation of “conditioning” (and fitness, in general) to say that we accomplish everything across all levels one might need in a 60 minute window.

So, rest assured I have no agenda to “sell” you anything here today, other than maybe getting smarter with your training. We’ll show you what energy systems are in play for conditioning workouts, where you are benefiting, where you are not, and what you can do on your own to address some areas that may be deficiencies.

6 Variables of Conditioning

There are six variables to our circuit training. All of these variables blend together in a given conditioning circuit to dictate the energy system we will be in, and what result we can expect to achieve from it.

  1. Duration – Time or distance of training.
  2. Intensity – Speed of training.
  3. Load – Resistance of training.
  4. Volume – Number of reps or amount of training.
  5. Duration of Recovery – The length of rest between work sets in training.
  6. Nature of Recovery – The type of rest between work sets in training. Is it active (100m jog) or is it passive (8’ on/2’ off density circuit).

What About Density?

You’ve heard us talk about density before if you’re a member, but density is not a true variable because it is your finished product of added variables above. It’s the answer to the equation, not part of it.

For example, let’s say over a 10 minute circuit, you are able to complete 7 rounds of 5 push press, 10 RKB swing and a 100m run. If you were to perform the same workout a month later and finished 8 rounds, you improved your overall training density. You did more in less time, meaning increased intensity also occurred, to create this higher yield. Increasing your training density improves strength endurance and work capacity specific to those time domains, and is a primary goal of performance-based circuit training.

The improvement could be the result of a lot of things. Lactate buffering, ventilation, improved biomechanical efficiency, strength, movement transition, a better understanding of your pace. Probably a combination of all of them. We have three main energy systems when it comes to fitness that all contribute to these differently, and today we are going to examine them in the realm of conditioning.

I want you to understand and remember two points.

  1. When conditioning in a group fitness setting, we are never exclusively in one system, but rather, a blend and sort of back and forth between all of the components involved. Make no mistake. This is effective.
  2. Time domains are a helpful guide, but they are not the law. A beginner going for 5 minutes all out will not be in the same energy system as a higher end athlete going 5 minutes all out. So, to say “90 seconds is glycolytic” like I will below is not totally accurate.

Here is a look at how they blend to create different energy system training, and how that nets out for your fitness.

Energy System 1: Phosphagen System (ATP-PCr)

Conditioning Variables:

  • Duration – Very Short (a few seconds)
  • Intensity – Max (highest level of output)
  • Load – Body weight or light and explosive
  • Volume – Low

Benefits: Power, strength
Physiological Effect: Heavy breathing, but standing.

Let’s first understand what causes muscles to contract. Any muscle contraction or force exerted by the body is produced by a molecule called adenosine triphosphate (ATP). All three energy systems produce it, but some are faster than others. The Phosphagen system is the fastest at ATP production, but it burns out just as quickly.

Think of this system like a drag race car. It lasts only a few seconds, anywhere up to ~15 seconds and is responsible for maximal power and speed. PS is short, fast and intense. It’s the energy system we rely upon when lifting a maximal load, jumping at max height or sprinting at top speed. Because it primarily uses store ATP and does not generate new ATP at a rate fast enough to keep up with peak output, this system only lasts a few seconds.

Most conditioning we do will not be in the Phosphagen system because it is too short. This is primarily the strength pathway in a typical first tier. We may tap into the Phopshagen a little on an EMOM, where we get about 50 seconds of rest measured against 10 seconds of power work, but conditioning by nature is not Phosphagen based unless it’s sprint, power, or front-end alactic work on a monstructural piece like an erg, AirDyne, or foot sprint.

We are anaerobic in this system, as in we are not using oxygen as energy. We are also not producing lactate as a byproduct of working in this energy system (like we are in the Glycolytic system), so when I say ‘alactic’ I mean no lactate. No “burn”. It is taxing, but it is relatively repeatable with adequate recovery.

The 6 Variables of Conditioning

New athletes typically have trouble accessing this system because they are not strong enough to produce requisite, repeatable power. This is why we don’t do a lot of short term, high burst, high percentage strength training with new athletes. They have not yet developed the strength capacity or the engine to work hard enough to turn on true phosphagen work.

Training to Isolate the Phosphagen System:

  • 100m Run, Row or AirDyne Sprints/ Rest 2:30 – Repeat 5 Rounds. Rest for 7 minutes. Repeat Block.
  • MB Slam EMOM – 5 Reps. Rest 50″. On the minute.
  • Battling Rope Workout 

All in all, Phosphagen conditioning does not comprise a large piece of the pie, but it’s certainly an included ingredient.

Energy System #2: Glycolytic System

Conditioning Variables:

  • Duration – Short to Moderate (20 – 90 seconds)
  • Intensity – Very High (near max output)
  • Load – Moderate
  • Volume – Low

Benefits: Performance, body composition, power, strength endurance (horsepower), lactate threshold improvement.
Physiological Effect: Flattened. Back on the floor.

After maximum power declines around 20 seconds or so and we burn up all of our stored ATP and PC, we exit the Phosphagen System and move into Glycolysis, which is the breakdown of glucose for fuel. We tap into our bodies glycogen reserves, which are a product of food carbohydrates. This further fuels intense activity for up to approximately 90 seconds or so, and ultimately results in a hydrogen build up in the blood, which leads to a decrease in power, and muscle fatigue. The exact duration is different for each athlete, but this is what separates the top of the leader board in the timed challenge. The intra-muscular ability to “compete” and be able to work at a very high level of output, just under threshold. 

Here’s a summary of glycolysis and acidity build up.

  1. Stored glycogen is converted to glucose, which is then broken down to make ATP, which we continue to then use as energy.
  2. This chemical reaction results in creation of pyruvate and hydrogen, which results in muscles becoming very acidic, quickly.
  3. Because sufficient oxygen is not present (this system is anaerobic), pyruvate binds with hydrogen to create lactate (this is not lactic acid), so that it can be removed from the muscle cell, protecting it from becoming too acidic and leading to muscle failure.
  4. Lactate can then be re-shuttled back to muscles or the liver to undergo chemical reactions that are able to use it for further energy production.
  5. Without lactate production, we would burn out from the hydrogen build-up a lot sooner.
  6. Once lactate accumulates faster than we can remove it, we reach Lactate Threshold, which is the failure point.

The 6 Variables of Conditioning

Power peaks and then drops tremendously in a short amount of time in the Glycolytic system. Truth be told, this is the least necessary isolated energy system for the Gen Pop member. It is extremely grueling, challenging and is reserved for those who willingly wish to push themselves to a very high level of physical fitness. That said, this builds tremendous work capacity and carryover to aerobic endurance, and all around high level fitness. An athlete cannot be completely fit without a highly trained glycolytic system.

Remember, we are never in one system, so while true glycolytic training is difficult and not sustainable, a workout that is roughly ten minutes long will have a lot of anaerobic/glycolytic components to them, and will comprise a fair amount of our conditioning. We don’t really ever train in maximum 60 – 90 seconds burst. To have a true anaerobic training session you’d need a lot more rest than we provide. However, the workouts where we drift in and out of aerobic/anaerobic are by far our most common, beneficial, and productive conditioning pieces that we perform.

Training to Isolate the Glycolytic System:

  • IWT – i.e., 8 Hang Power Cleans + 300m Run as fast as possible. Rest for 6-9 minutes.
  • 400m Run – Rest 6 minutes. Repeat 4 times.
  • 500m Row – Rest 6 minutes. Repeat 4 times.

Energy System #3: Aerobic System

  • Duration – Moderate to Long
  • Intensity – Moderate
  • Load – Light
  • Volume – High

Benefits: Engine building, lactate threshold improvement, oxidization of body fat, endurance, cardiovascular health
Physiological Effect: Various. At lower intensities, tired. But repeatable. At higher aerobic intensities, hands on knees. Winded.

Think of this like a Prius. Low power yet high duration that can go on much longer than it’s Glycolytic Mustang cousin. When we continue efforts past the glycolytic stage, power will drop and we will enter the Aerobic system. This is our slowest form of energy creation, but we have a huge supply of it because we use carbs, fats and sometimes protein to make ATP. We transition into the aerobic zone past a few minutes of sustained effort.

Why Do We Run Intra-Workout?

Active recovery that involves the working muscles helps buffer hydrogen from the bloodstream, which would otherwise prevent failure and defeat of the purpose of the circuit, which is to sustain. It is simply not possible to have a productive aerobic circuit without active recovery.

We avoid that acidic burn due to the presence of oxygen in this system. The trade off is this is much lower power than anaerobic glycolysis. This typically occurs after 90 – 180 seconds depending on intensity of output, and ATP is made in three different stages of the aerobic system (Aerobic or “Slow” Glycolysis, the Krebs Cycle, and the Electron Transport Chain). 

Let’s call a spade a spade here. There is functional fitness aerobic (mixed modal, varied intensity) and there is real aerobic (monostructural, same pace). Functional fitness aerobic is longer, less intense conditioning workouts that have non-power based movements usually at a bit higher reps. The 15 to 20 minute second tiers where ask you to settle in on say 75% rate of perceived exertion and focus on maintaining pace, not progressively increase intensity of effort. True aerobic training places endurance needs before power needs, so it makes little sense to have something like a 75% deadlift as part of an aerobic conditioning event. That’s why you won’t find that in conditioning Tier 2’s at our gym. Ever.

Traditional, “engine building” aerobic work is monostructural. Running, rowing, swimming. Something that is uninterrupted and has a slower RPM than mixed modal fitness.

We don’t do real aerobic training work in the traditional sense of the word. We just don’t. For a few reasons. We have 60 minutes to get you better, and proper aerobic work would take up half of that time and be boring as hell (would you consistently show up to 3-mile run day?) We can dress up mixed modal training all that we want and call it aerobic, and a lot of it is and is highly effective, for sure, but proper aerobic capacity work needs more time than what we can provide, and is more strictly structured on pacing.

The good news is that most people don’t need it for what they are really looking for (stronger, leaner, more in shape). I’ve seen mixed modal aerobic work effective at transferring to monostructural aerobic arenas like marathons, triathlons, and half marathons with our athletes, but it will not replace the real thing of longer, steady state aerobic work that really draws out and builds the base. There is no more largely debated topic in fitness than whether or not bouts of steady state aerobic work are necessary.

My view is this. If you wish to get into great all around shape and be time efficient as hell, it is not needed. If you wish to explore your true performance potential as a 1% high end advanced athlete, it may be needed in a smaller, complimentary role once you achieve a very high level of fitness in the gym. I see it as a plateau buster in certain scenarios for advanced folks who need to expand the size of their engine.

Beyond that, I am yet to be convinced it’s needed for the everyday athlete looking to get into great shape.  Either way, it should not make up the foundation of your program but would likely benefit anyone with some complimentary work outside the gym.

Training to Isolate the Aerobic System:

  • 3 Mile Run
  • 2,000m Row
  • A long swim
  • A weighted hike

One important note on aerobic training is that it will absolutely not interfere with strength gains. Coaches who claim “strength interference” are likely making one of the following programming mistakes.

  1. Adding power based movements at just below strength percentages to the majority of aerobic work, thus producing athletes with who constantly decrease power. Remember, aerobic training places endurance before power. 
  2. Strength training their athletes too often or misalign their strength days. 
  3. Are not educated enough or have enough front line experience to see that it doesn’t.
  4. Or, the athlete at hand is not balancing strength work with aerobic work. 

I have never, in a single instance, seen a strong athlete get weaker as a result of aerobic conditioning. Never.

Final Takeaway

The majority of conditioning we do is in the 15 minute range and we are never in one particular energy system. There are touches of each, drifting back and forth depending on the movement, distance, and load. The shorter the work bout, the more anaerobic components we will. The longer the work bout, the more aerobic components we have.

Withers et al demonstrated that 96% contribution of energy from the anaerobic pathways with 10-seconds of work; 75% contribution at 30-seconds; 50% contribution at 60-seconds and only 35% contribution at 90-seconds.

A good rule of thumb to determine if you are working glycolytic (anaerobic) or aerobic is the amount of recovery time that you need. If you need a long time to recover and repeat intensities, you were typically more anaerobic. If you can recover and repeat quickly, you were typically more aerobic. This is why IWTs need up to 6 – 9 minutes of recovery. They are the most pure form of glycolytic training since most stop at around 1:30. A 500m row is another example on the more intense end of the spectrum. That you would be done for the day. It is not repeatable.

Do not get terribly caught up in exact quantification, as general conditioning rarely works like that, and I fully stand by the efficacy is high to moderate intensity dosing making up the majority of a program.

It will improve:

  • Anaerobic Capacity
  • Aerobic Capacity (to a level)
  • Build Muscle
  • Burn Fat
  • Support Linear Progression

If you feel you have some weakness, always remember that n=1. Always. You are the laboratory. You’ve been given some generalizations in each energy system on how to best target them on your own, at open gym, or on the track. My guess is you already know where you are strongest and where you are weakest.

To understand energy systems is to jump into deep waters. There are aggressive arguments within the industry GALORE on what constitutes proper energy system training. I am observer in those debates, not a participant. I find what I’ve put together for you here today to be the most realistic breakdown of all of the information at hand when it comes to the application of general population group fitness.

I hope this helped.

Dave Thomas Performance360 Coach Trainer-Dave Thomas




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Related Reading:

What is the Difference Between Anaerobic and Aerobic Conditioning?

How to Make Sure Your Conditioning is Helping, Not Hurting You


Vandewalle H, Peres G, and Monod H (1987). Standard anaerobic exercise tests, Sports Medicine, 4: 268 – 289.

Withers RT, Sherman WM, Clark DG, Esselbach PC, Nolan SR, Mackay MH, and Brinkman M (1991). Muscle metabolism during 30, 60 and 90s of maximal cycling on an airbraked ergometer. European Journal of Applied Physiology, 63: 354 – 362

Dr. San Millan, Inigo. “What Is Lactate and Lactate Threshold.” TrainingPeaks. N.p., n.d. Web. 27 Jan. 2017.

Fitzgerald, James. “Energy Systems Training.” Opex Fitness. OPEX, 14 Jan. 2017. Web. 03 Feb. 2017.