Aerobic Development for Athletic Performance


Part 1: Cardiovascular Physiology & Testing

Popular opinions in strength and conditioning are too often formed around the opinions of certain experts or gurus.  The industry, and many of the young coaches within it, tends to latch on to certain ideas and methods based upon the number of Instagram followers a coach has or the number of pro athletes they train.  We begin to mold our programming and processes around a guru, rather than specific concepts.  In extreme cases, we let the opinions of a select few widely known gurus steer us away from the basic laws of physiology and the research that exists to support it.

Aerobic training falls under the category of “extreme cases.”  Nearly 20 years ago, Mike Boyle wrote about energy systems and how athletes who play explosive sports should avoid long, slow cardio in favor of high intensity sprint work.  He’s obviously not the first or only person to share this view, but he does hold quite a bit of clout in the strength and conditioning industry.  Since that time, the phrase “train slow, get slow,” has been uttered by strength coaches everywhere- the idea that unless you’re a marathoner, cyclist, or some other endurance athlete- your aerobic work should consist of high intensity sprints at or near the speed with which your competition is played.

It is a reasonably logical conclusion to draw, right?  If my sport requires several repetitive sprints, as fast as possible for 5-10 seconds, then that is how I should train, and if I train slow, I will become slower.  The problem with this simple story that’s being told, is that it disobeys some of the basic laws of physiology and goes against what we’ve found in research.

First let’s take a deep dive into the science behind cardiovascular fitness.

The most basic molecule we need to understand is called Adenosine Triphosphate, or ATP.  ATP is what gives us life.  It is an energy storage molecule that powers every cell in the human body.  When ATP is broken down, energy releases and stuff happens.  Stuff like a heartbeat.  Stuff like muscular contraction.  Important stuff.  When we utilize an ATP molecule, it gets resynthesized and recycled for future use, in large part due to what we feed our body.  Think of your energy as a big pool full of ATP molecules.  The fuller your pool is, the more energy sources at your disposal, the more work you can do.  As you drain those energy sources, you fatigue, and need to replenish.  Therefore, the quicker you can refill that pool, the more work you’ll be able to do at a given time.

How do we replenish our available pool of ATP?

Humans have three possible strategies to put ATP back together once it has been used:

1) The Creatine-Phosphate pathway

2) The Glycolytic pathway

3) The Oxidative pathway.

There are several long equations involved with the specific molecules of each of these processes, but for our purposes the important pieces to understand are that ATP gets broken down (a process known as hydrolysis) and free energy is released.  This free energy is utilized by the corresponding tissue, to do the work for which that tissue is responsible.  For example, a muscle cell uses ATP to create muscular contraction, nerve cells found in the brain utilizes ATP to send signals to the rest of the body to perform vital functions, etc.

Also released at the time of ATP hydrolysis is a hydrogen ion along with heat.  Heat plus that little hydrogen ion are hugely important here.  Heat and hydrogen represent two significant threats within the human body. Your body is constantly striving to keep your body temperature and hydrogen load (the amount of free hydrogen ions floating around at any given time) in check.  Put simply, if heat or hydrogen load gets far too high or far too low for too long—we die.  The three energy management strategies named above are responsible for not only putting ATP back together again, but they also must account for the heat and hydrogen they produce.

1) Creatine-Phosphate pathway- does not COST any ATP but does not help with reduction of hydrogen or heat.  This reaction occurs within the sarcoplasm of a muscle cell and relies upon the cells’ storage of creatine phosphate (CP).  CP exists only in small doses in human muscle tissue and as such, the supply is used up very quickly- within the first several seconds of activity.

2) Glycolytic pathway- Glycolysis occurs in the cytoplasm of the cell and begins with the phosphorylation of glucose. This system creates 4 ATP molecules at a time, but your body uses 2 ATP molecules to power the glycolytic process itself.  This system also removes two hydrogen ions out of circulation.  So, for every “turn” of the glycolytic system, we have a net gain of 2 ATP and a net loss of 2 hydrogen ions.

3) Oxidative pathway- Unlike the two processes above, the oxidative process occurs within the mitochondria of our cells.  You might remember from biology class that the mitochondria is known as the powerhouse of the cell.  This is because the oxidative process is by far the most robust mechanism we have both in terms of energy production and threat (heat and hydrogen) reduction.

When a cell goes through one cycle of oxidative respiration, it produces up to 30 ATP molecules, 6 molecules of carbon dioxide (CO2) and 6 molecules of water (H20).  Notice that these two byproducts, CO2 and H2O have something in common: O.  Oxygen is necessary for this process to function.  When there’s enough oxygen in the cell, hydrogen and oxygen bond to form water, and water is easily synthesized.  This process is not costly from an energy standpoint and is the least threatening/most productive scenario possible for energy production.   As we exercise, our body goes through this system over and over and over in all our working muscles.  As exercise intensity and duration increase, the rate of hydrogen ion production begins to exceed the amount of oxygen available within the mitochondria needed to produce water.  As this occurs, the body is forced to deal with this excess hydrogen load in a quicker, though less efficient manner: it creates lactate.  Two hydrogen ions can be removed from the system (reducing system threat) by binding to a molecule called pyruvate, forming a molecule called lactate.  Pyruvate is a product of glycolysis (see #2) and is only utilized when our exertion levels have reached a point where the amount of oxygen available cannot keep up with the rate of hydrogen production within the cell.  The human body craves efficiency, and so it will prioritize the oxidative process for as long as possible before moving to the less efficient pathways mentioned above. As we’ll see later in this article, blood lactate levels are a popular mechanism to measure exercise intensity, and they tell us which of the above energy systems are being most utilized during a given training session.

The fact that doing mechanical work produces these threatening byproducts of heat and hydrogen tells us an important fact: Stress has a cost.  Any time we place the body under more stress than it’s used to, the body is forced to adapt to this new stress and the cost associated with this stress, all the way down to the cellular level, is increased production of heat and hydrogen.  When we’re smart about our training stimulus, the body handles this stress almost instantly by increasing our oxidative capacity and heat and hydrogen are dissipated with no ill effect.  When stress levels exceed our current oxidative abilities, and we become overly reliant on anaerobic metabolism, we become worse and worse at mitigating these internal stressors.  Excessive levels of hydrogen and heat in the body lead to hormonal changes, central nervous system fatigue, and other negative adaptions.  When these negative adaptations occur in the body over the course of weeks, months, and years, we see progress stagnate, stress levels increase, and recovery between sessions suffer.

Energy systems and substrate utilization

In the above sections, we can roughly put the first two systems (Creatine-Phosphate and Glycolytic) of energy production into an “anaerobic” bucket, while the third (oxidative) system fits into an “aerobic” bucket.  This distinction helps us qualify our exercise efforts, but it is important to know that almost every effort we make starts out being aerobic in nature, and only starts to become anaerobic when the intensity and/or duration of effort requires this shift.

With a basic understanding of the cellular mechanics at play, we now must consider what drives these systems to function, and this question begins with fuel source.  The body has two primary fuel sources: Fatty acids (fat) and glucose.

Exercise intensity, when broken down into its most basic components, comes down to two things:

1) The amount of force an activity requires

2) Rate of Force Development (RFD)

When strength coaches think about intensity, we like to talk about methods and minutia:  Things like percentage of 1RM, bar speed, jump height, barbells vs. kettlebells, etc.  Thankfully, the body does not understand these very recent inventions.  The body has evolved over millions of years to adapt to the force it is asked to generate.   The body does not know or care if you are squatting with 500 pounds on your back or you’re performing a long jump.  It only knows how much force it needs to create and how quickly it must create it.  This is the definition of intensity, right down to the molecular level.

The exercise intensity level determines the speed with which your muscles require ATP replacement.  The speed required determines which fuel source your body will prioritize.  If we scroll back up to the previous section comparing glycolysis to oxidative metabolism, we learned that an oxidative process would occur within the mitochondria until the speed of hydrogen production exceeds your body’s ability to pair it up with oxygen to form water.  When the intensity level is low enough, mitochondrial oxidation can fuel ATP production using fat as the primary fuel source.  Fat is like diesel fuel for your car: it’s efficient and gives your body significant miles per gallon.  The draw back to utilizing fat for fuel is that it’s slow to break down compared to glucose.  As exercise goes from low to high intensity, and the need to replenish ATP accelerates, the body has no choice but to prioritize the quicker, albeit less economical fuel source: glucose.  This when the glycolytic system mentioned above begins to be the overwhelming driver of metabolism.  Glucose becomes the primary fuel source and fat stops being utilized.

Energy systems and motor unit recruitment

So how do we as humans determine which energy system we’re using, and which substrate we’re prioritizing as fuel?  Reading the previous paragraph, it’s certainly reasonable to come to the conclusion that spending more time in the “fat burning” aerobic zone could be advantageous from a performance standpoint, and also from a body composition standpoint when compared to the anaerobic/glycolytic zone.  So, how does one accomplish this?  While we can make an impact on fuel prioritization through nutrition and strength training, the biggest determining factor appears to be exercise intensity, or more specifically- muscle fiber recruitment. Human skeletal muscle has several different types of muscle fibers.  There are some fancy names for each type, but for sake of simplicity let’s separate muscle fiber types into three main categories:  Slow twitch, fast twitch, and intermediate.

1) Slow twitch muscle fibers are comparatively low motor unit recruitment, meaning they are not capable of producing tons of force, but they are built for endurance.  They are VERY slow to fatigue.  Remember the part of the cell we talked about earlier, the mitochondria?  These slow twitch fibers have tons of them.  Because of the sheer volume of mitochondria found in our slow twitch fibers, these fibers are oxidative in nature.  Fat can only metabolized in the mitochondria.  Therefore, when we are using our slow twitch fibers, we are prioritizing fat as our fuel source.  To utilize these fibers, we must adhere to their characteristics: low force production, long duration.

2) Fast twitch muscle fibers exist on the other end of the spectrum.  They produce high levels of force but fatigue quickly.  They rely upon those first two energy systems we talked about earlier- Phosphagen and glycolysis.  They are innervated by a greater number of motor units, leading to their increased force production but they quickly follow the glycolytic pathway: demand for oxygen exceeds oxygen availability pyruvate binds with hydrogen to form lactate.

3) Intermediate fibers or transitional fibers exhibit characteristics of both fast and slow fibers.  They live on the edges of either of these two classifications and it is believed that we can influence, through training, these intermediate fibers to perform either in a slow twitch or a fast twitch manner, depending on the needs of the situation.  For example, when we go from a fast walk to a jog, we might exceed our slow twitch fibers’ capabilities, however before we reach completely for a fast twitch response, we will dive into our available pool of intermediate fibers to get as much aerobic metabolism as we can before switching to a less efficient fuel source.  These fibers have more mitochondria and are more oxidative than the fast twitch group, but less than the slow twitch.  They can produce more force than slow twitch, but less than fast twitch.

To summarize the characteristics of each energy system, refer to this chart:

This understanding of aerobic/anaerobic metabolism as well as the fuel sources and muscle fibers involved in the process forms the basis of how we program our training.   When we spend a lot of time training our aerobic system, it becomes more robust.  When we spend a lot of time training our glycolytic system, our body adapts and becomes good at glycolysis.

We need to start thinking less in terms of the “thing” we’re doing that day and think much more about the specific adaptations we’re aiming for.   This is where planned, monitored programming is key.  We can’t talk about programming without first talking about testing.

Aerobic testing

Let’s quickly discuss a few key aerobic tests which help inform our decision making as it relates to training:

1) VO2max testing:

Long held as the gold standard of aerobic testing, a VO2max test determines the greatest amount of amount of oxygen a person can use at a given time.  The units of measurement here are important.  VO2max is measured in milliliters of oxygen, per kilogram of body weight, per minute (mL/kg/min).  To measure VO2max, the lucky subject is hooked up to a gas exchange analysis machine and begins to walk on the treadmill.  Every so often, the speed and incline of the treadmill increases, and the test administrator is getting readings on the amount of oxygen the individual is consuming.  The longer the test goes, the more intense it gets, the more mL/kg/min the athlete consumes.  Eventually the rate of oxygen consumption plateaus or the athlete is unable to continue and this point in time is deemed the VO2max.  From here, we can make programming decisions based upon VO2max, such as prescribing a 60-minute run at 70% of your VO2max.

There are several drawbacks to VO2max as a gold standard test.  First and foremost- it’s a dreadful test.  Being hooked up to a tube, having blood pressure and heart rate constantly monitored while you try to reach your maximum level of exertion is uncomfortable for all the wrong reasons.  It’s also very expensive as it requires specialty lab equipment to do.  Beyond these constraints, VO2max probably isn’t what we should be concerned about anyways:

-VO2max is least trainable component of aerobic capacity, meaning you can work on improving it by working near your maximal levels, but it’s only going to improve to a limited degree.

-VO2max is largely limited by your genetics.

-In order to train your VO2max, you must be working at an incredibly high intensity, somewhere around 90% of your max heart rate or above. This is only possible using high intensity intervals.  As you train at this high intensity, your heart muscle becomes bigger and stronger to eject more blood per stroke.  As your heart increases in size, it begins to run out of space within the pericardium.  When your heart cannot grow any larger in response to training, your ability to increase your VO2max hits a plateau.

-From a practicality standpoint, the desired adaptations of training at or near VO2max do not seem to outweigh the negative adaptations such as increased stress response, decreased recovery, etc.  If we think back to energy systems, you will be consuming oxygen the entire time you’re training.  However, you will stop utilizing your slow twitch muscle fibers, and therefore you’ll stop utilizing fat as your primary fuel source as soon as the intensity level surpasses aerobic levels.

*Make no mistake- I am not saying that we should not train at a high intensity! As you’ll see in part 2 of this article, high intensity training is an absolute must for peak athletic performance regardless of sport.  What I’m saying here is that we should not be training with the sole intent of increasing VO2max, because once an athlete is relatively trained, VO2max isn’t going to change all that much.  The cost of doing business here is quite high, and the payoff is limited.

2) Blood lactate testing

Testing for blood lactate levels during exercise is a much simpler, more practical way to assess aerobic fitness.  It’s the only aerobic test we perform at Paragon.  The test can be done on any piece of cardio equipment and ideally, we’d use the piece of equipment the individual is most comfortable using.  Cyclists can use a stationary bike.  Runners can use a treadmill.  Using the treadmill as an example, the subject starts at a very slow walk.  Every 3-minute stage includes: 1 blood sample to tested for blood lactate levels and an increase in speed and incline on the treadmill.  The technician administering the test is looking for two particular “turn points” in blood lactate levels, and once the second turn point has been assessed, the test can stop well short of maximal capacity.

If you recall back to our discussion on energy systems, we saw that your cells will utilize fat for fuel through aerobic metabolism as their primary source of energy when intensity levels are low.  As intensity levels increase, the glycolytic energy system becomes more dominant.  A product of the glycolytic system is lactate, which can be measured in the blood.  Blood lactate is measured in millimoles per Liter of blood (mmol/L).  At rest, humans usually have around .5 to 1.0 mmol/L of lactate in their blood.

In the first stages of a blood lactate test, the aerobic system is efficiently using fat for fuel and blood lactate levels stay close to resting but start to rise in response to increased intensity.  Remember the definition of intensity?  How much force are you creating and how quickly are you creating it?  By increasing the speed and incline of the treadmill, we’re forcing the body to create more force at a faster pace, thereby increasing overall intensity level. As blood lactate levels hit approximately 2.0 mmol/L, we’ve reached our first turn point.

Turn point #1: Aerobic Threshold: 2.0 mmol/L

This turn point (though it does vary some from individual to individual) represents the point in time when the body has begun to switch fuel sources from fat to glucose, and therefore has begun moving from aerobic to the glycolytic pathway.  When the technician sees this 2.0 mmol/L reading, he notes the subject’s heart rate and the test continues.  We have now determined the highest heart rate this subject can reach before his body begins to switch fuel sources.  This represents the top of the low intensity zone, also known as aerobic threshold (AeT).  We know from this point forward, there will be a mixed use of fuel sources between fat and glucose.

Turn point #2: Anaerobic Threshold: 4.0 mmol/L

As the test intensity continues to increase, blood lactate continues to rise until we reach our second turn point, known as anaerobic threshold (AnT) or lactate threshold (LT).  This turn point typically comes out to 4.0 mmol/L of blood lactate and represents the intensity level at which we’ve moved away from aerobic metabolism and are relying completely upon glucose for fuel.  Leading up to AnT, the relationship between intensity level and lactate production is relatively linear.  A 1% increase in intensity results in something close to a 1% increase in lactate production and corresponding lactate clearance. This means that blood lactate levels steadily rise from resting (between .5 and 1.0) to AnT (about 4.0).  Once we hit this second turn point, that relationship stops being linear.  The amount of lactate production quickly exceeds our ability to clear it because we’ve surpassed our oxidative capacity.

We’re also much more reliant our faster twitch muscle fibers at and above AnT.  Remember, it’s all about Force and RFD.  When we increase the amount of force required to complete the task and increase the speed with which that task must be completed, Type I muscle fibers begin to tap out as the faster twitch types step up.

At this point in the test, we can either continue to push the athlete as close to their maximum as possible if we want to get an accurate reading on their functional max heart rate, or we can stop. For the purposes of lactate measurement, we’ve identified what we need: The top of our low intensity zone as well as the bottom of our high intensity zone, with heart rate numbers at each turn point.  This gives us three distinct intensity zones: low, moderate, and high.

The obvious question is, what do we do with this data?  By assessing our heart rate correlated to the different energy systems, we now have an answer for the questions from above:

1) Your 5-mile run had you at an average heart rate of 150 beats per minute—what intensity zone does that place you in, meaning what energy system are you utilizing?

2) Your apple watch tells you that your 5-mile run at 150bpm burned 400 calories—what fuel source were those calories coming from?  Fat or glucose?

Based on your lactate testing, we can answer those questions with some certainty.  And more importantly, we can tailor our training program accordingly.

*A few miscellaneous notes on lactate testing:

-Aerobic fitness level is the biggest driver of lactate measurement scores.

-Test-day nutrition does make an impact on test results.  We ask our clients to fast for at least 4 hours prior to the test, and an overnight fast will deliver even more accurate results.

-Hydration is another important factor in lactate testing.  The more hydrated you are, the easier it is for your body to buffer hydrogen.  This means that if you come in well-hydrated, you’ll be able to reach a higher intensity level before relying upon glycolysis.

-Like many other things in strength and conditioning, the female population has not been studied well enough or long enough to be able to definitively say whether there are major differences between men and women.  The few studies1 that have been done indicate that gender differences alone do not account for differences in blood lactate response.

-As you’ll see in part 2, the results of this test inform our decision making when we’re laying out a plan for training.  We’ll ask the individual to train within specific heart rate ranges (called Zones), with the goal of developing the various energy systems we’ve already discussed.  This follows a foundational principle in biology: Specific Adaptation to Imposed Demand (SAID).  When we do a bunch of bicep curls (imposed demand), the body responds by creating a bigger, stronger bicep muscle (specific adaptation).  Your cardiovascular system responds the same way:  When we spend enough time using our aerobic system, the body adapts by making the aerobic system more effective.  For this reason, we’ll often re-test blood lactate profile every 1-3 months, depending on the individual.  Aerobic and Anaerobic threshold will respond to training and change over time, and so our specific exercise prescription will change accordingly.

Where do we go from here?

Following this deep dive into cardiovascular physiology and aerobic testing, it’s time to get down to the stuff that we strength coaches enjoy more than anything:  Training.

Part 2 of this article will give you practical applications for aerobic training that you’ll be able to immediately use with your athletes. Part 3 will put everything together and show you how to build a resilient athlete by concurrently programming for strength and aerobic adaptations.

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About the author 

Adam Reeder

Adam Reeder is the owner of Paragon Health & Fitness, a private training center located near Cleveland, Ohio. Adam has over 12 years of experience in the strength and conditioning industry and has helped athletes of all ages, ability levels and sports. More information, including personal training services can be found at or by contacting Adam directly at

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