Lift Large And Prosper

Don't Just Coast Through Life, Grow Through It!

The Ultimate Guide – Bioenergetic Systems

The Ultimate Guide series of posts aims to do one thing – educate. I’ll be explaining the ins and outs of how various systems within our bodies work and how you should structure your training around them!

What they won’t be is an exhaustive list of exercises that you can do – sure I’ll give an example or two, but the aim of these posts is to educate to a sufficient level so you can figure out if one form of training is a good fit for your goals or not!

Lets get into it!

One of my favorite systems within the body is the Bioenergetic system! This system is responsible for fulfilling all the energy needs that our bodies have on a day to day basis and is highly complex in nature.

Its role spans everything from the day in, day out requirements of just being alive, to dealing with the huge energy requirements of highly strenuous exercise.

With this guide I aim to give you two things:

  • A clear understanding of where the energy our bodies need comes from, and how it is replenished within the body; and
  • Knowledge on how you should train to improve your general fitness and conditioning level, based on your needs and requirements.

So let’s get started!

What is the Bioenergetic System?

The bioenergetics system comprises of a series of metabolic process that convert energy from various formats to one that our muscles can use. This format, known more scientifically as a molecule, is called ATP and is something that we will cover in a bit more detail later.

In layman’s terms: this is the system that is responsible for taking all that food we eat – be that a wholesome dinner or a chocolate bar – and turning it into energy that will fuel movement!

It has two mechanisms of operation, each of which can be further broken down into their own subsequent processes!

These mechanisms are;

  • Anaerobic – meaning without oxygen; and
  • Aerobic –  meaning with oxygen!

Any process which results in ATP being produced falls into one of these two categories, and each will have their own pros and cons. Before going into that however, it’s kind of essential that we know what ATP is; so, what is it?


ATP, short for Adenosine Triphosphate (notice the highlighted tri), is a molecule that is responsible for fueling muscular contraction! Without ATP a muscle cannot contract and would be left unable to do its job. Going into the science behind how ATP is used to drive this contraction would make this post about 3,000 words longer, so you’re just going to have to trust me on this one as we focus our attention solely on its production!

As I’ve already somewhat alluded to, the most important thing for us to take note off at this point is the “tri” in its name; ATP is, in part, made up of three phosphate ions and when used to contract a muscle fiber, loses an ion. This results in it being converted into another molecule called ADP – Adenosine diphosphate.

This new molecule will have to be reconverted back to ATP if it is to be used again – something we will discuss as we go into the different energy system processes!

The most important elements to take from this section is that ATP is comprised of three phosphate ions and is required for muscle contraction – that’s it!

Our Energy Systems

So, now that we know what our goal is – making ATP – let’s move our conversation onto the processes that produce it! We’ve already discussed how these processes fall into one of two categories, either Aerobic or anaerobic, but what exactly are these systems and how do they work?

There are three main systems or process that our bodies can use to produce ATP. These are;

  • The Phosphagen System (Anaerobic);
  • The Glycolic System (Aerobic and Anaerobic);
  • and The Oxidative System (Aerobic).

Now, before we go into any more detail on these systems individually it’s important to note that at any one time ALL of these systems are working simultaneously. Never do we have just one system alone producing all the ATP needs of our bodies. So even though under certain circumstances one system will be the dominant producer of ATP, all systems will be contributing some percentages of our energy requirements.

Now, onto the systems themselves!

The Phosphagen System

Picture this. You’re walking down the road, minding your own business thinking about what to cook for dinner, when out pops a hungry lion not 30 meters ahead of you! You’ve two options; turn and run, or just wait for the inevitable death that comes with being eaten alive!

Hopefully you chose to turn and run, in which case you will be placed right into a situation where your Phosphagen system is the predominant source of energy. Ok, ok, it’s hardly realistic but the picture I’m trying to paint is one of high intensity, full speed and a short time frame. Anything that fits this description is fueled by your Phosphagen system!

So, how does it work?

We’ve already discussed how ATP is the fuel which drives muscle contraction, and how it looses one phosphate ion and becomes ADP. So, after a period of time, we have a bunch of ADP just sitting around which can’t be used for much– what do we do? Enter the Phosphagen system, which converts it back into ATP.

The conversion of ADP back into ATP looks a little something like this…


Ok, let’s take a pause for a second! I’ve added a couple more terms here which need to be touched on. Creatine, yet another molecule in the body, is one that indirectly aids in the re-conversion process. It is rarely stored on its own and usually is bonded with a phosphate ion to form creatine phosphate.

When called upon, it is willing to give up this phosphate ion it was paired with and donate it to ADP, converting it back into ATP. Simple, right?

It’s kind of like asking for your friend for some change at the store – you’re getting yourself a coke, short a few cents and you need a little top up. If your friend can oblige, it’s a pretty quick process and in just a few moments you are out of the shop enjoying a refreshing beverage! Easy!

This process is similar in that there isn’t really much work to be done. The creatine phosphate is stored right within the muscle, so when called upon it is right where it needs to be; there isn’t any “moving around” so to speak! So, when we need lots of immediate energy for fast, explosive movement, the phosphate system is one that can kick in right away!

There is a catch though – we don’t have huge levels of creatine phosphate within our muscles and so it quickly becomes depleted. This means that if this energy requirement goes on for longer periods of time, this system is no longer of benefit!

Glycolytic System

When we consume carbohydrates our bodies convert them into glucose, which is more commonly known as “blood sugar”. When this glucose gets stored in our muscles and tissues it takes another form called glycogen, which for our benefit can be regarded as the same thing! This glycogen is mostly stored within our muscles and liver where it is kept for future use!

The glycolytic system is responsible for the production of ATP through the breakdown of this glycogen, which can happen in one of two ways – either Fast Glycolysis or Slow Glycolysis.

Fast Glycolysis

Fast glycolysis, obviously the faster of the two glycolysis processes, is the conversion of glucose directly into ATP. It takes place when there is a lack of oxygen availability within the muscle cells.  This lack of oxygen however causes issues with the conversion process, resulting in an unwanted byproduct being created – lactic acid!

The drawback of lactic acid production is that it also leads to an increase in hydrogen ion concentration within the tissue, which not only causes fatigue, but also inhibits further glycolytic reactions from taking place and reduces the maximum possible contraction force within the muscle! (1,2)

Our bodies, being as advanced as they are, try to combat this buildup of lactic acid by converting it into its base, Lactate.  This conversion process is carried out by the blood and liver system and lactate thankfully doesn’t have any of the downsides lactic acid does!

One way to measure “fitness” is by monitoring a person’s ability to convert lactic acid to lactate. As you would suspect, the faster someone can carry out this process the longer they can go without fatiguing, and the faster they can recover after a heavy hitting session! This ability to deal with lactic acid is something that can be affected though exercise, something which we will discuss later in Lactate Threshold section of this guide.

Slow Glycolysis

Slow glycolysis is the second variation of the glycolytic system and is actually very similar to the fast variant – the only difference being that the reaction can fully take place now that sufficient oxygen is present, meaning we now no longer have to deal with the lactic acid production that causes issue during fast glycolysis.

This also results in another compound being produced – pyruvate, which is subsequently converted into another enzyme called Acetyl COA, which will be used in the oxidative system which we will discuss in its relevant chapter!

Lactate Threshold

As we discovered when discussing fast glycolysis, because the process in an anaerobic one we end up with an unwanted byproduct, lactic acid. This lactic acid is then transported from the muscles to the liver through our blood system where it is subsequently converted into lactate.

But how much lactic acid can we actually tolerate before it becomes an issue? Well, like a lot of things, that depends!

As our exercise intensity increases, there is a point at which the levels of blood lactate within our system sees a dramatic increase. This point, called the Lactate Threshold (LT) is usually around the 50-60% of maximum oxygen uptake for untrained subjects and 70-80% for trained subjects (3, 4).

Similarly, there is a secondary inflection point at which these lactate levels begin to rise even further, a point which is called the “Onset of Blood Lactate Accumulation” (OBLA).


Studies have shown that training near or above the LT or OBLA points pushes them both to the right of this graph – meaning that lactate accumulation is lower for a given intensity level (5, 6).

But what does this mean in layman terms? Well, very simply it means the more we train around and above the areas at which lactic acid starts to be produced, the better we become at dealing with it – allowing us to train at higher percentages of our maximum oxygen uptake without as much lactate buildup within our blood – essentially becoming “fitter”!

Oxidative System

Out of the three energy systems that our bodies call on, this is the slowest but also the most effective method of producing ATP. It is the primary source during rest or low intensity activities and utilizes mainly carbohydrates and fats as its fuel source, but can also call on proteins during longer periods of activity. (7, 8)

At rest, the ratio of carbohydrates to fat utilization falls around 30:70, with this ratio trending more towards carbohydrates as activity levels increase. If, however, these activity levels continue on for prolonged periods of time (think 3k runs and up), this reliance on carbohydrates more or less falls off completely and fats and protein take over the workload!

As I discussed in my post “My thoughts on Cardiovascular Exercise“, although this does mean that long term, steady state cardio “burns” fat, it doesn’t mean that it is the best method for actually losing it. This is because it signals to the body that we need more stores to fuel our activity, promoting further fat storage on our otherwise lean and defined frames!

But how does the oxidative system actually produce ATP? Enter: The Krebs Cycle.

Krebs Cycle

This is where things get quite complicated, and rather than trying to paint a picture in 1000 words or less, I’ll make no apologies for using images in this section!

With that said, in its simplest form the Krebs cycle can be described as a series of continuing oxidation (aerobic) reactions where various molecules are converted into ATP. Depending on the molecules being converted, they will enter this process at different stages.

Krebs* Image taken from Wikipedia

As you can see there is a lot going on here; far more than most will need to know. The important thing to take from it is that it takes time, is quite slow, and mainly uses carbohydrates and fat as fuel. So although it can provide huge levels of ATP, the slow speed means its great for long term, low intensity exercise, but we need the other forms of ATP production in times where this isn’t a viable option.

Fat and Protein Oxidation

As mentioned, the Krebs cycle can call on carbohydrates, fats and proteins as fuel sources for its reactions, so it’s worth spending a second explaining the steps that drive these processes! We’ve already covered carbohydrates during the glycolysis section (Glucose -> Glycogen -> Pyruvate -> Acetyl COA), so that just leaves us with fats and proteins!

For fats: triglycerides stored in fat cells are sent to the mitochondria within muscles. Here, they undergo Beta oxidation and are converted into a compound called “acetyl COA” and hydrogen. These, although not directly converted into ATP, can enter the Krebs cycle where they will result in the production of ATP.

For proteins: they are broken down into their amino acids, which are then converted into glucose, pyruvate or other Krebs cycle intermediates. The exact intermediate each amino acid will get converted into depends not only on the type in question, but also the state and situation currently present within the body!

Summary Of Energy System Roles

That was a lot of information, more then some of you will want to know. In any case, a handy table breaking down the roles and times each energy system is the predominant source of ATP would be hella handy. Thankfully, I can oblige!

Bioenergetics Table 1

* Note: These are not only estimations, which will vary from person to person, but also only list the predominant sources of ATP. All systems work simultaneously to supply some percentage of the immediately required ATP.

For healthy indivudals it can be assumed that short bouts of high intensity activities are fueled mainly by the phosphagen and fast glycolysis systems, with longer, lower intensity activities being supplied with ATP through slow glycolysis and the krebs cycle.

However this isn’t just due to the volumes of ATP that each can supply, but is also heavily driven by the speeds of said delivery also! Studies have shown that even at maximum oxygen update, power output is only at approximately 20-30% of a persons all out maximum (9), meaning that if we want to get closer to that elusive 100% power mark, we need other ways of providing ATP without relying on oxygen to drive the process.

Substrate Depletion, Repletion and Limiting Factors

All these molecules and substances that we’ve talked about – Phosphagens, glucose, lactate, amino acids – are called Bioenergenic Substrates.

Simply put, a bioenergetic substrate is anything that provides starting materials for bioenergetic reactions. In order to create ATP, varying levels of these molecules will be used and consumed, meaning they will need to be replenished if we are to rely on them again further down the road!


Studies have shown that there can be a 50-70% reduction of creatine phosphate muscle stores after just 30 seconds of high intensity activity. (10,11,12) Thankfully, our bodies are quite good at replenishing this system and it can be totally recovered after just 8 minutes of rest (13). Although this energy system is itself an anaerobic one, this replenishment occurs through aerobic processes – not a detail that is of huge important to us in this situation.


The total volumes of glycogen that we hold within our bodies is somewhere in the region of 300-400 grams, with around 100 of those being within the liver alone (7). As we use glycogen to produce ATP, this number runs down and can in fact become totally depleted, with the only way to replenish it being carbohydrate ingestion.

This resting volume of glycogen that we can store can be effected through exercise selection and type, which can also impact the storage location of said glycogen (14, 15, 16, 17). Undergoing regular high intensity, short duration anaerobic exercise will result in an increased capacity directly within the muscles, whereas longer aerobic bouts promotes increased liver stores!

Limiting Factors

Depending on which of the predominant energy systems are fueling the majority of our ATP requirements at any one time, we will be limited by the availability of one or more substrates. Below is a nice handy table which outlines which substrate will limit us for a given activity level and duration! Remember, the hydrogen buildup is a result of the lactic acid production caused by fast glycolysis.

Table 2

Oxygen Uptake

Oxygen update is a fancy term given to the bodies ability to intake and use oxygen. During low intensity activity, oxygen uptake slowly increases over a few minutes until it reaches a steady state level which is high enough to drive the ongoing workload. Before this steady state level is reached however, we still need to provide our bodies with the ATP required to carry out that action, and this is where anaerobic processes come in to take the slack. This slack is called the oxygen deficit.

After the activity ends, it usually takes us a few minutes to catch our breath – this is our bodies returning once again to the baseline level of oxygen uptake and it is called the oxygen debt or  excess post-exercise oxygen consumption, EPOC for short!

If, however, the steady state level of oxygen update required to sustain activity is higher then our max uptake, anaerobic processes have to take over as the primary suppliers of ATP.


* Image taken from

Training the Energy Systems

If you’ve made it this far having read every word – well done! There was a lot in it but the essence of these Ultimate Guides will be to teach you everything you need to know about how the systems work so you understand what you need to do to address your goals.

With that said, it’s time now to turn our attention to just that – training the energy systems!

Interval and Combination Training

When it comes to improving fitness levels, there are a number of camps that exist saying different things are best. In reality, there will probably be pros and cons to every form of conditioning that you do; but its a matter of choosing the one that will get you the best results, for the least effort, in the shortest amount of time.

Some studies out there suggest that athletes performing in anaerobic based sports could see some benefit in recovery by undergoing regular aerobic training, given that recovery is mainly an aerobic process. For decades footballers, weight lifters and other athletes performing in anaerobic sports have jogged as part of their recovery process – all of which stems from this mentality.

The downside of this though is that it also impacts their anaerobic capabilities (18), slows muscle growth (19) and also reduces speed (19, 20, 21). The concern this raises is if the increased recovery is worth the reduced performance within sport.

The other option that is being studied a lot in the last number of years is anaerobic training, and it’s been shown that it can have a net positive effect on performance, even for those people ONLY competing in aerobic based sports (22, 23, 24). This comes from not only an increased capacity to deal with blood lactate for a given exercise intensity, but also through improved levels of muscle glycogen stores.

This isn’t just something I’ve seen in literature either. As part of our athlete training, our conditioning is all anaerobic based regardless of their sporting energy system requirements and all have seen nothing but positive results from it. This means that although their sport is mainly an aerobic one, anaerobic conditioning has seen them improve performance!

Does this mean that anaerobic training is the only answer? Absolutely not. There will always be a time and a place for both anaerobic and aerobic based conditioning methods, but based of my personal experience, the wealth of coaching experience in my facility, and the literature, it would seem that aerobic based conditioning is the way to go to improve both your anaerobic and aerobic performance levels.

What does this anaerobic training look like? Being anaerobic, its going to high intensity, but short duration, and our energy system sessions follow this mentality. A good place to start would be 8 all out efforts of some or varying activities for 10 seconds on, 10 seconds off. Repeat this process for 4-8 times before moving to 20 seconds on, 20 seconds off. Once you can last 4-8 rounds of that, then its time to up to 30 on, 30 off.

With that said, it’s improtant to remember that these efforts have to be high intensity – that means your exercise selection should be one that requires a lot of change of direction or explosion, and that you should be following through with that each rep of each set.

Follow that regime, being honest with yourself and how hard your pushing, and you’ll be well on the way to improving fitness and both anaerobic and aerobic performance.


The bioenergetic system is made up of three main processes: The phosphagen system, the glycogen system and the oxidative system, each having their own pros and cons. These systems are designed to supply the body with ATP which is then used to drive muscle contraction.

Each system can either supply ATP fast but in low quantities, or in high quantities and slow, and depending on the activity in question one system will be the predominant supplier of ATP, although it is important to note that at any one time all systems are supplying some percentage of our bodies requirements.

When it comes to training  it has been shown that high intensity combination training can have a positive impact on all of the systems abilities to produce ATP, but also to deal with any negative impacts each might have. The same cannot be said for lower intensity, long duration exercise, which may help in recovery but has also been shown to reduce power output, strength and hypertrophy capabilities!


  1. The interaction of cations with calcium binding site of troponin. Fuchs, F. 1970.
  2. The influence of hydrogen ion concentration on calcium binding and release by skeletal muscle sarcoplasmic reticulum. Nakamura, Y. 1972.
  3. Anaerobic recovery in man. Cerretelli, P. 1975.
  4. Plasma lactate accumulation and distance running performance. Farrell, PA. 1979.
  5. Anaerobic threshold alterations caused by endurance training in middle aged men. Davis, JA. 1979.
  6. Endurance Training affects lactate clearance, not lactate production. Donovan, CM. 1983.
  7. Insufficient carbohydrate during training: Does it impair performance? Sherman, W.M. 1991.
  8. Effect of initial muscle glycogen levels on protein catabolism during exercise. Lemon, PW. 1980.
  9. Peak power versus power and Max oxygen uptake. Conley, MS., MH Stone, HS O’Bryant. 1993.
  10. Breakdown of high energy phosphate compounds and lactate accumulation during short submaximal exercise. Hirvonen, J. 1987.
  11. Lactate in human skeletal muscle after 10 and 30 seconds of supramaximal exercise. Jacobs, I. 1983.
  12. Muscle power and metabolism in maximal intermittent exercise. McCartney, N. 1986.
  13. Biochemical causes of fatigue. Hultman, E. 1986.
  14. Influence of sprint training on muscle metabolism during brief maximal exercise. Boobis, I. , C. Williams, S.N. Wooten. 1983.
  15. Biochemical adaptations of human skeletal muscle to heavy resistance training and immobilization. MacDougal, JD. 1977.
  16. Effect of training on enzyme activity and fibre composition of human muscle. Gollnick, PD. 1973.
  17. Enzyme activity and fibre composition in skeletal muscle of untrained and trained men. Gollnick, PD. 1973.
  18. Oxidative and lysomal capacity in skeletal muscle. Vhko, V. 1978.
  19. The effects of running, weightlifting and a combination of both on growth hormone release. Craig, B.W. 1991.
  20. The available glycogen in man and the connection between rate of oxygen intake and carbohydrate usage. Hadmann, R. 1957.
  21. Interference of strength development by simultaneously training for strength and endurance. Hickson, RC. 1980.
  22. Potential for strength and endurance training to amplify endurance performance. Hickson, RC. 1988.
  23. Strength training effects on aerobic power and short term endurance. Hickson, RC. 1980.
  24. Health and performance related adaptations to resistive training. Stone, MH. 1970.