19 min read

Energy System Development

AKA "Cardio" but Energy System Development is so much more than that. A lot more. Find out how much more...
Energy System Development


  • Train all 3 energy systems on a semi-regular basis.
  • Be mindful of how you integrate these forms of training with resistance training.
  • Place emphasis on the Anaerobic System over the Aerobic System only a few times a year, the rest of the year should probably focus more on the aerobic system (for recovery and heart health), if you lift (and you should). Or at least be mindful of what you can actually recover from.
  • Your Medium Energy System means anything lasting 10-120 seconds, with typically up to 4 times the rest, but more often a 1:2 or 1:3 ratio.
  • Your Long Energy System will last roughly up to 20 miles which is typically where people bonk in Marathons without adequate training, or about 90-120 minutes (depending on your level of training) before your carbohydrate stores are depleted and additional carbohydrates must be consumed.
  • Intervals done in the Long Energy System typically have an inverse work to rest ratio (think walk-run protocol) or even. Often the rest is considerably shorter than the work phase, often 1:1, 2:1, 3:1 or even up to 10:1 or higher.
  • If training more than one system within a given workout, train the systems from quickest to longest.
  • All 3 systems work interdependently, not exclusively, so take most recommendations with a grain of salt.

As far as I know, Energy System Development is a term coined by Mark Verstegen, Founder of EXOS (formally Athlete’s Performance).

Mark’s a smart dude, but also the term obviously describes the intent, much the same way Neuromuscular System Development encapsulates training for the nervous systems and muscular systems.

Most people only know about ‘Aerobics,’ or ‘Aerobic Training,’ or what I often hear people talk about in terms of ‘cardio,‘ and maybe a few refer to as ‘conditioning.’

Aerobic simply means, ‘with oxygen,’ and only compromises one of three energy systems.

Two are actually Anaerobic Pathways — which if you’re wondering, basically means, ‘without oxygen.’

  1. Alactic (relies exclusively on available ATP)
  2. Lactic (uses lactic acid as a fuel source)

We are capable of producing energy in an oxygen environment and in an environment exclusive of oxygen.

Oxygen is a catalyst for energy production, your aerobic energy system is actually far more efficient (~16x) at generating energy via the production of adenosine triphosphate (ATP) — the most basic component of energy that your body utilizes — but at the expense of speed and muscle power output.

You can’t sustain a high level of energy output for too long, before the aerobic system must kick in to cope with the energy demands. This is why you can't sprint all out for miles.

Not relying on oxygen leads to lower total energy production, so you can't go for as long or as far, but you get more speed and muscle power output.

1. The Quick System

AKA: ATP-CP Phosphagen System (Alactic Anaerobic Energy System)

This is your immediate or shortest duration energy system, lasting roughly no longer than 10-12 seconds.

You’ll find this energy system heavily involved in sports like javelin, shot-putting, American Football, Baseball and short sprints (<100 m).

It uses the ATP stored directly in your muscles, to produce energy quickly to your biggest muscle motor units. The obvious limitation is endurance and it’s questionable how trainable this system actually is.

A lot of neuromuscular training is actually already geared to this system.

Strength training, particularly explosive strength (AKA plyometric training AKA shock training) with less than ~5 reps is often completed in less than 10-12 seconds and features long rest intervals in excess of 3-5 minutes.

One could argue that improvements in sprint or explosive strength performance are related more greatly to the neuromuscular improvements than our ability to harness energy in such short time frames with this energy system.

That said, however small the energy system contribution may be, it's probably not nothing. It's simply hard to determine significant contributions from the energy system directly over such time frames.

A lot of the neuromuscular coordination training approaches to improving performances over 10-12 seconds of time, are also the same things you'd likely use to train the energy system anyway.

And improvements in this domain already happen in tenths or hundreds of seconds.

A tenth of a second is the difference between first and third at the most recent Brazil Olympics for the 100 m, or less than the difference between third and sixth for more perspective.

Going from a 4.6 second 40-yard dash to a 4.5 second 40-yard dash is a monumental achievement that some athletes spend years trying to achieve.

Is it the neuromuscular system improvements, or the energy system improvements that matter here?

Who cares? As long as you see improvements, it's hard to even care about what caused the improvements. Especially considering the exercises and approaches you'd use train likely train both the energy system in question and the neurological elements simultaneously.

Practically Speaking

You could make a strong argument that proper plyometric training and high-intensity lifting (≤5-6 reps) could substitute for this in the average person's routine. That might not train this energy system directly, but it would lead to similar improvements.

Trying to target this energy system on an ESD training day/session will be some kind of sprint based exercise with very long periods of rest.

Still typically an exercise associated with cardiovascular exercise that someone could do for longer durations:

  • Running
  • Jump Rope/Jumping
  • Cycling
  • Rowing
  • Skiing
  • Etc...

For example, a ≤12 second sprint followed by 72-144 seconds of rest. Effectively a :6 or 1:10 work to rest ratio. Maybe a little bit more than that or a little bit less, but to maintain quality of efforts, you want long rest periods for this sort of energy system work.

You may even see rest in this domain characterized by up to 20 times the rest relative to the work period. Enough time to allow a chemical process to restore ATP again within the muscle. Depending how how high the intensity is, or you want it to be.

For example, 40m, 60m, 80m and 100m sprints are mostly geared to this energy system, and are among my favourite protocols for maintaining cardiovascular fitness via this system.

Often I’ll just do a quick sprint, walk back slowly and repeat.

However, sprinting is also technical, so you need to show technical proficiency to work on them, otherwise you’re best off with a bike, rower erg or something with less of a learning curve.

I like these because they remind me of the days I used to compete more competitively in sprint sports. I've always preferred higher intensity, longer rest work.

And the nice thing about this type of training, is the trickle down effect I'll discuss at the end of this article.

You can do significantly less of this work overall but still get great health benefits, while better better preserving muscle tissue, so they are time efficient.

The downside of this type of training is that it’s hard to recover from, so you can’t do it often. Maybe 30 minutes (including work and rest) twice a week to any reasonable effect for those of us who are not currently elite athletes.

It’s questionable how much you can do in conjunction with resistance training (not very much as they are very similar) or how much resistance training you'd have to displace to do these often.

Yeah, you could modify the intensity, but then it would truly be taxing this energy system. Anything below about 90% effort or close to 100% is probably training the neuromuscular qualities of the action, more than the energy system qualities.

As I said, this is hard system to improve as it is already, so focusing on it might not be the best path for most folks.

It's certainly not a great option for fat loss. I know the internet continually hypes up HIIT and SIT as the ultimate fat loss tools but that's HIIT geared to the "medium system" that I'll discuss next.

I recommend using it sparingly a few times a year, in low doses (less than 20-30 minutes not including warming up), followed by resistance training (rather than on separate days), with aerobic training done on off days to aid in recovery.

It is however more relevant for athletes in power sports than the average Joe or Jane. And it's much harder to fit into the tighter schedules of working professionals.

2. The Medium System

AKA: Glycolytic System (Lactic Anaerobic Energy System)

The glycolytic system or your sugar system, still requires energy expenditure in an environment without oxygen like the ATP-CP system. But after that initial 10-12 seconds the body basically starts utilizing simple carbohydrates (glucose) as a fuel source, rather than pure ATP.

Sugar requires some breakdown and chemical changes to occur for this system to be effective and training this system leads to a better utilization of glucose and a greater duration of exercise at a higher intensity.

This system is predominantly used in Power Sports like Soccer, Basketball, Hockey, Rugby, and 200m, 400m, 800m track events.

The downside?

Lactic Acid build-up, which isn’t as bad as most people think – it's actually a fuel source for this system. Eventually it will disallow you from continuing at that intensity because your body produces more of it than can be utilized.

This is also often referred to as the Lactate Threshold or Anaerobic Threshold (whatever you want to call it). It's that moment in time when supply outstripes demand. I guess?

Your body is good at dealing with it for about 90-120 seconds, depending on training, before it has to start utilizing the aerobic energy system and your power output has to drop.

If you want to deplete muscle and liver glycogen, this is the energy system you want to target with your training. And this is the HIIT training that everyone is talking about, although there is little carry over to the ATP-CP system.

The nice thing about this system is that recovery requirements are lower than the ATP-CP system. With a little bit of rest, you can repeat your performances and tax it pretty heavily.

Practically Speaking

A lot of moderate intensity lifting (8+ reps) does help develop this system, and might be a viable alternative for some.

That said, this is where energy system and neuromuscular elements start to diverge. You will feel considerably more fatigue working on this system and truly developing it will require more than just moderate intensity lifting.

A good example of training protocols that develop this system would be 1:1 work like 60 seconds on, 60 seconds off; 1:2 work, like 30 seconds on, 6o seconds off; Or even 2:1 work, like 60 seconds on, 30 seconds off.

The Tabata protocol (20 seconds on, 10 seconds off) also falls under this umbrella because 10 seconds rest provides incomplete recovery and the work duration exceeds 10-12 seconds.

Rest deficits, whereby rest intervals exceed work intervals also mean you can’t hold out as long, which is why the basic Tabata approach is only 4 minutes in total length of time (8 rounds of 20 on, 10 off).

Anything where the work durations get up to 90-120 seconds (2 minutes) and rest is characterized by maybe half to double or triple that work interval probably falls under this umbrella.

Ultimately, if you want to improve fatigue resistance, you lower the rest interval, if you want to improve the quality of the contribution of the glycolytic system to training, you typically use longer rest intervals, especially as they relate to sports.

45 seconds on, three minutes off, as in most hockey shift scenarios, is still an example of Glycolytic Interval Training.

*Note more rest is needed when training the quick energy system, less for the medium, and even less for the long system.

These are the types of intervals you'd want to do if fat loss was your goal. I still wouldn't run in this regard because that's technical and opens the door for injury but almost anything else works wonders, especially the bike or rower.

One of my favourite protocols and according to Dr. Steven Boutcher at the University of South Wales, the most effective for fat loss (per time spent, and if using a bike for comparison), is an 8 seconds on, 12 seconds off protocol. 1:1.5 ratio.

Interval training in these zones, has been shown to make significant improvements in markers for aerobic capacity and cardiometabolic health.

Again the downside of these higher intensity training sessions is that the total duration of training and frequency of training them must also drop. You can’t just use high intensity training protocols all year round, despite how some people might approach it these days.

Like the Quick system, you can likely only handle about 20-30 minutes or less in any given training session, probably only twice a week, maybe 3 if you're not also resistance training (much) in special circumstances. 36-48+ hours of recovery between bouts is likely ideal for performance outcomes.

Be careful mixing it with resistance training protocols with this as it’s still relatively high intensity (most moderate set lifting is in the glycolytic zone).

I won't get into the tricky balance of volume vs intensity here but one of these days I'll write an article about that.

What’s interesting is that a great deal of athletic research indicates that this system can be trained rather sparingly and the benefits last for quite some time. It’s highly trainable and the effects linger.

Meaning the benefits of doing glycolytic training three times a week (I’d skip resistance training this week or use very low volume anyway) for one or two weeks can last upwards of 12-16 weeks.

For performance and basic health reasons, you can probably get away with a week or two of this type of training every 2-3 months. Perhaps the odd maintenance session here or there instead of a pure resistance training session can extend the need out even further for such an isolated burst.

Where possible to do safely, I recommend a few phases of each type of training each year. With a few sessions here or there, depending on how they are planned to maintain during the rest of the year.

Of course, this all depends on your objectives.

A Brand New Day
CC Photo Credit: Thomas Hawk

3. The Long System

AKA: Aerobic System

Aerobic energy system training also what most people seem to think of when they hear the term ‘cardio,‘ but as you can see above, it’s only one of three systems.

This system can extend our energy output dramatically from 90-120 seconds to upwards of 90 minutes and beyond depending on the intensity (proximity to that anaerobic threshold) and training.

You can probably walk for hours and hours right now, but most of us would bonk trying to run a marathon without training.

You can't run on oxygen alone. There are still carbohydrates and/or fatty acids involved in energy production, even aerobically. Aerobic training relies more heavily on fatty acid utilization than carbohydrate utilization.

Which doesn't, contrary to popular belief, make it "better" either. Just different. When you train at higher intensities, more carbohydrates are used as fuel proportionately speaking than at lower intensities. The key consideration is that more total energy is also utilized at higher intensities.

The source of the energy doesn't matter as much as the total energy output. Not that I recommend worrying about energy expenditure from exercise, well...ever... really...

That's a separate long overdue article.

The main advantage of aerobic training is the frequency it can be applied. Almost daily. It’s much easier to recover from that other forms of energy system training.

While the main disadvantage (outside of boredom) tends to be the time requirement. It can take longer to yield similar results, which is why HIIT has gotten so much play in the media the last 10 years.

It's predominantly used in endurance sports such as long, typically slow (relative to their power sport counterparts) distance cycling, triathlons, marathons, kayak racing, and swimming.

Obviously. I get it, to many people, this is boring. I’m not a fan of doing the same thing ad nauseam for long periods of time either.

Thankfully, walking, hiking and biking all fall into this category and are a little more tolerable (personally speaking).

It's lesser know that you can make any interval approach more aerobic in nature by extending the interval out to longer than the 90-120 seconds your glycolytic system would manage to dominate at higher intensities.

You have to drop the intensity with aerobic training when you do that but you can train the longest system quite well with running distances as short as ~800m, rowing distances as short as 750m (most races are 2k) and cycling distances as short as 1.2-2k. Provided you rinse, lather and repeat.

Anything that will make the work effort last about 2 minutes or more, can be stopped, let your heart rate return to 130ish maybe (or until you can breathe through your nose-only again) and then repeat for up to 60 minutes.

These are often my personal favourite ways to approach it. I find the minimal distances (roughly) where I know the durations will last long enough, I rest, and repeat for a total duration.

Keeps my engagement higher, but your mileage may vary.

Unlike the Anaerobic pathways, training your aerobic system yields little improvements in speed, power or strength and is associated with degradation of lean tissues.

However, you are mostly in a state of aerobic energy expenditure all day, every day, anyway. So muscle loss has a lot more to do with negative energy balance, than aerobic work per-se.

It's also easily combated with resistance or strength training and impact (to a degree), combined with adequate protein intake.

I’ll note that research into this muscle building/maintenance "interference effect" suggests that non-weight bearing aerobic activities (like biking/swimming) do not have a significant interference effect when kept to less than three times a week for no longer than about 20-30 minutes at a time.

Practically Speaking

If you have decent aerobic fitness than your waking resting heart rate will typically be around 60 BPM.

That's my rule of thumb.

If you’re not there on a regular basis, then you might want to mix more into your routine for general health purposes than the other two for now. I think it’s something worth focusing on maintaining.

Aerobic training is definitely associated more so long-slow continuous training though. More than anything else, but pretty much anything that keeps you at a moderate heart rate (about 130-150 BPM) for 20-30 minutes upwards of 6x a week will have a significant aerobic training effect.

Keeping in mind this doesn't have to be continuously done as I mentioned in the last section. The time you spend recovering from aerobic work at higher – but still aerobic – intensities, closer to, perhaps even slightly above that ~150 BPM, still falls within range.

Depending on your age or conditioning and how long your heart rate stays in range you can get a significant aerobic conditioning stimulus while taking it easy for most of that 20-30 minutes, if you like. I top all of my deliberate training out at 6x a week.

Even brisk walking is of benefit and is worth considering on days you’re not lifting or doing some other form of cardiovascular activity.

Honestly, I don’t recommend the average person stretch it out beyond 60 minutes. I usually keep the majority of aerobic work at ~20-60 minutes zone. Less if you can do it more frequently, longer if you can't.

If your goals are suited to endurance sports, obviously you’ll have to stretch it out longer than that to 90 minutes or even longer in some cases and train aerobically far more frequently (3-6x a week). I've worked with some cyclists whose long rides last several hours every weekend.

Even if you aren't an endurance athlete the aerobic system is still important and with the recent popularity of HIIT, often overlooked. Serving several key roles in:

  • Heart Health (Specifically left ventricular elasticity)
  • Improved recoverability
  • Parasympathetic nervous system stimulation (relaxation/endorphin release) to balance the more sympathetic nervous system stimulation that resistance training and higher intensity forms of exercise promote
  • Improved energy/mood (with less fatigue)
  • Maintaining fat burning metabolic pathways (for all the meatheads who don't read my stuff)

Higher intensity forms of exercise appear to thicken the left ventricle of the heart, though the jury is out as to how detrimental that heart wall thickening might be.

Most of these benefits are greatest when 20 minutes of aerobic activity is strung together. Does that mean you won't experience anything of them if you can only commit to less? Not exactly, it all depends on your current level of fitness. Anything is better than nothing, so if you only have 10 minutes for a bike ride, you'll get some of the benefits still, even if it's only maintaining that level of fitness.

If you want to build muscle without losing some aerobic capacity, try to keep your work to 20-30 minutes at a time with non-weight bearing activity, or brisk walking.

Walking, is predominantly aerobic, and yields numerous health benefits, not to mention provides many people a much needed break within the day.

I’m not saying aerobic training is bad or good really.

For fat-loss, you can use it more frequently and recover better from it, but it might not have the muscle preserving effects either. Thankfully resistance training can easily fill that void.

I think a mix of methods is best as each method offers slightly different benefits/downsides.

If you don't have access to a heart rate monitor, then I find breathing through my nose, is the best gauge of how aerobic my training is.

If you have to breathe through your mouth, there is a good chance it’s become more anaerobic (or is approaching it) so back off, trying to breathe through your nose for the 20-30 minutes can be a reasonable indication that the intensity is low enough to be mostly aerobic.

Trickle Down Effect

One might think that 20 minutes of 60 seconds on, 60 seconds off only trains that glycolytic system, and you'd be wrong.

The last interval of a glycolytic HIIT training session might have as much as 40% of the energy contribution come from the aerobic system.

In other words, the aerobic system is always there, always helping you last longer.

It's really important to understand that your energy systems work together, not in isolation.

That said, the effects definitely flow (or trickle) downstream better from higher intensity to lower intensity, than the reverse direction.

Exclusively training the aerobic system won’t really contribute much in the development of the other two "high-powered" energy systems.

Meaning long-distance, low to moderate intensity work doesn't make people faster or yield more explosive power in shorter bursts. You'd still need to do some of that training. Otherwise, you learn to tolerate longer durations of relatively "easy" workloads only.

On the flipside, training the anaerobic pathways still leads to significant improvement of the aerobic system, as shown most famously in the Tabata Study. Keep in mind that those involved in that study, still did about seventy minutes of aerobic work per week. Something to think about…

A big part of that trickle down effect is that none of these energy system really run independently of one another, but rather interdependently. It looks a little something like this:

Notice that the aerobic energy system turns on the moment the fastest system turns on (as does the lactic system). You can't really use the anaerobic systems without also using the aerobic pathway, which is always on. Kind of the default for non-strenuous situations.

You're likely participating in aerobic metabolism right now, even if you're laying on your couch reading this on your phone. The extent of its involvement is just lowest when you're sleeping or laying down and not moving. And it's highest when you're running, cycling, rowing, etc... at what's termed your "anaerobic threshold" or "lactic threshold" depending on who you talk to.

There is a dynamic threshold for everyone at any given time whereby metabolism becomes dominantly anaerobic. Basically when your speed/activity, exceeds your ability to utilize lactic acid as a fuel (without oxygen) source and lactic acid begins to accumulate.

To be clear...

Lactic acid isn't a bad thing. It's merely a fuel. Nor does it cause muscle soreness, that's been disproven (it's the hydrogen ions). It's that when your ability to utilize it as fuel is slower than production, you can't maintain your power output.

Thus, you have to stop sprinting (or whatever you're doing) because you simply can't tolerate that intensity any further. But the aerobic system this whole time in the background has also kicked it up a notch to help you recovery ATP.

After you stop and allow for some recovery, your aerobic system can produce a lot more ATP while you're walking around huffing and puffing than your anaerobic systems can.

Effectively your aerobic system is a major contributor to activities like High-Intensity Interval Training (HIIT) or Sprint Interval Training (SIT). Always is during every rest interval.

Something sports scientists call "Repeat Sprint Ability" drives your ability to recover quickly between efforts and consistently repeat high levels of performance on each sprint. It's a very good indication of conditioning for power/mixed sports like soccer or basketball.

It's also a very good indication of an effective aerobic system. So while knowing that high-intensity activity yields a much better overall result, it might not be enough to facilitate ideal recovery. Especially at higher levels of athletics.

Developing more aerobic capacity via direct aerobic work is still often necessary.

The trickle down effect in the opposite direction is substantially smaller because it's rare for your long-distance (slow-steady state) work or moderate intensity continuous training (MICT) to hit many periods of activity where it needs a little extra umph.

For instance, going uphill might require anaerobic pathways to turn on, but anything lasting more than ~90 seconds of pure anaerobic energy production would mean you'd have to stop.

Ultimately most of that work ends up being right at the cusp of pure anaerobic energy contribution. It's a very delicate balance if you're in a position where you can't or don't want to ever stop.

As a result the anaerobic pathways end up far less "challenged" during predominantly aerobic activities, even if you do encounter the odd hill or strong current.

This is a reason HIIT training is so heavily touted but that's not without some risk considerations. It trains more energy system qualities in less time by increasing the intensity.

Too much time spent in the mythological ‘fat-burn zone‘ or too much low-level aerobic training is a bigger challenge to energy requirements and might degrade muscle tissue to some extent. Too much of it would interfere with any muscle building a person might seek, so a concern for some, but not all.

It isn't so much the aerobic activity itself, so much as, the durations and distances covered. It shear energy balance. The more you burn, the more you'd have to consume and eating a lot of food for some (AKA "hard-gainers") can be challenging, especially when you add a lot of aerobic conditioning into the mix.

Burning more energy than you take in, without the protein intake and resistance training to support muscle retention, will degrade more muscle tissue, even in the absence of aerobic training.

Ultimately, spending some time training in that aerobic zone is usually a good idea, all the same. Higher intensity forms of exercise thicken the walls of the heart when done to excess, while aerobic training results in greater elasticity of the heart. That quality appears to be associated with better heart health, even though higher intensity forms of energy system training still yield a lot of the same benefits.

The other important consideration is that aerobic training improves recovery from strenuous exercise and helps you recover better between sets of higher intensity exercise.

Basically everything has it’s pros and cons, you should be aware of that, and then cater your ‘cardio’ to improve what’s important to you.

Just because it’s been shown that higher intensity interval training has been shown to be very effective at eliciting metabolic and aerobic improvements, doesn’t mean you should just completely forget about aerobic training.

A lot of people these days hammer the @#%* out of this system in particular, expecting to somehow improve. If that’s you, you might want to consider spending some time on #1 or probably more on #3.


It is important to remember that these are more like guidelines, than rules.

We know that all three energy systems work interdependently, so it’s not exactly cut and dry.

For example, working your quick or medium systems, means your aerobic system is in use during the recovery period.

Doing resistance training will provide some benefits to these systems, depending on the type too.

If you want to optimize your training, it’s important to order any energy system work within a training session or workout appropriately, so that the quick energy system is developed first, then the medium system, finishing with the long system.

This is of course assuming that you plan to train 2 or more energy systems within a given session.

Also you are better served utilizing some form or combination of interval training in your weight loss efforts, instead of typical aerobic exercise.