A few weeks ago I posted the results from a recent week of testing at 626. Included in these results were a battery of strength indicators, looking at the broad categories of: squatting, single leg, upper body pushing, upper body pulling, bending and core.

Also included were a series of work capacity (conditioning) tests broken out into 4 categories: alactic, lactic power, lactic endurance and aerobic. These tests differ from each other primarily in duration and the power output required.

Examining the results across all of these tests provides insight into the relative strengths and weaknesses of our members. And while the idea behind testing strength is familiar to most, work capacity may not be, so I thought it’d be interesting to dig a bit deeper into those tests by discussing the energy systems involved.

It’s Not All Cardio?

Most people are familiar with the concepts of aerobic and anaerobic work and these ideas are related to the work capacity tests we perform at 626. Aerobic refers to work done with oxygen and anaerobic is work performed without oxygen. But what does that really mean?

At a more detailed level, the chemical energy necessary to produce muscular activity is a compound known as adenosine triphosphate (ATP). Now don’t worry, I’m not going to get super geeky with the chemistry here, I just need to provide you with a cursory knowledge to help you see the bigger picture.

Our bodies maintain stores of ATP and this is what we use to power our most intensive tasks. Think 1 rep max attempts or perhaps a 40 yard dash. The reason we can’t just keep knocking out reps at our 1RM though is that we are only able to store a limited amount of ATP – enough for roughly 10 seconds of activity.


So, what happens when we deplete our ATP stores? Our body doesn’t just shut down leaving us unable to activate our muscles. Nope. We create more and depending on the nature of the demand, we leverage different systems to accomplish this. These energy systems are called the phosphagen, glycolytic and oxidative systems.


The phosphagen system utilizes creatine phosphate (CP) which, similar to ATP is stored in the body in limited quantities. When we break down CP, a large amount of energy is released, but due to the limited quantity stored, it is only enough to power the body for 10 – 30 seconds. The process to create ATP from CP is relatively simple and thus this system can also be used to drive high power activity. Since there is an intermediate step to creating the ATP though, the power output available is not quite as high as when we’re leveraging stored ATP directly.  As a side note, supplementing with creatine has been proven to aid this energy system.


The gylcolytic system picks up where the phosphagen system leaves off. This system burns sugars (glycogen) to synthesize ATP. This is a more costly bodily function than creating ATP from CP, thus the power output available is even lower. The glycolytic pathway can leverage glycogen for fuel for several minutes, but will eventually deplete stored sugars and the body will need to look elsewhere for fuel.


This leads us to the oxidative system, which produces the sustainable energy necessary to power activities lasting longer than a few minutes. This is how our body keeps us moving during extended activities such as rowing a 5k or running a marathon. It is the most costly method on the body to produce ATP, so the resulting power output is the lowest – you can’t run a marathon at your 40 yard dash speed, right?

The stack up of these energy systems can be seen in the graph below.  As you move out in time on the graph you can see the different energy systems dominating power production and the total output power capacity falling.


Another way to think about all of this is to consider your car. To get it started, your car requires a very short burst of high energy from the battery – this parallels the phosphagen system. After ignition occurs from the spark, gasoline is used to power your car – two unique energy systems working together.

Now if we shift this analogy slightly and consider a hybrid vehicle, at low speeds the battery provides all the power to the car – which is usually higher torque (power) than when operating on the gasoline motor, but drains the batteries fairly quickly. This parallels the glycolytic system. At some point, the batteries drain and the gasoline engine has to take over for the long haul, which is equivalent to your aerobic system taking over.

Not a perfect analogy, but hopefully it helps provide a bit more understanding of the human body and how it powers activity.

Another important point is that these systems do not operate in isolation. We are using all three of them for every activity we perform, we just happen to lean on one more than another depending on the power requirement and duration.

You Can Feel Them Too!

One of the interesting things about the different systems is that you can often feel the difference depending on which was the dominant source of energy. Specifically, the glycolytic system can be very painful. This is why high turnover, shorter duration workouts like FRAN hurt so much and lead to this….


Returning to the concepts of aerobic vs. anaerobic, we can now look at how these relate to the various energy systems.  The phosphagen and glycolytic pathways don’t utilize oxygen when creating ATP and thus they are anaerobic. As the name implies, the oxidative pathway requires oxygen to create energy, thus making it aerobic in nature.

Now, I glossed over many of the biochemical details and perhaps oversimplified some points, but I think the discussion above is enough to get you thinking about your training at a higher level.

Energy System Training

For balanced fitness, we need to make sure that we’re training and testing each of these systems. The work capacity tests at 626 dive even deeper down the energy system rabbit hole by evaluating four areas: alactic, lactic power, lactic endurance and aerobic.  I’ll save these concepts for a future post though. The purpose here was really to lay some of the ground work and educate you a bit on energy production in the human body.

However, with just the knowledge provided here, you have enough to start taking a keener look at your training.  Consider your workouts over the past week.  What energy system dominated your training each day?  Are you getting a balanced mix?  Is the intensity of your training varying from week-to-week?  Start paying closer attention to all of this.  As an athlete, the more awareness you have here, the more you can get out of your training.

This is a huge topic, but the intertwined nature of the various energy systems necessitates a directed approach if you’re looking to improve on all fronts.  A collection of random workouts won’t cut it and very likely will be counterproductive.  Each energy system requires a unique approach to building volume, intensity and adequate recovery.  When combined appropriately, it’s truly a beautiful thing where improvements in one energy system compound and can be leveraged to improve the others.