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 Understanding the Energy Systems

Whether you’re paddling for fitness or training to race, it’s useful to have some basic knowledge about how your body provides the energy to make the muscles you use to move across the water contract.  It provides you with an understanding of what’s happening when your heart rate rises and you begin to get out of breath, why you can’t sustain a really hard pace indefinitely and why you feel like your muscles eventually fail if you try.  It also provides you with insight into the different types of fitness involved in paddling and what type of training you should be doing to improve them, and if you’re a racer, how to improve your performance.  So, let’s take a look at the human energy systems, how they work to make muscles contract, how they interact with each other, how they affect athletic performance, and how we can train them to improve our health and paddling.  

ATP - The Energy Currency

The muscles we use when we’re paddling (or for any other movement) require energy to make them contract.  The source of that energy is a compound produced in muscle fibers called Adenosine Tri-Phosphate (ATP).  It’s composed of an Adenosine molecule with three phosphate molecules attached to it through high energy bonds.  When one of the high energy phosphate bonds is broken and a phosphate is ripped off the ATP where contraction occurs in the muscle, it releases the energy the muscle uses to contract.  So, if we’re going to do anything that involves our muscles we need to provide those muscles with ATP.  If we’re going to contract those muscles forcefully and repeatedly, we need a constant supply of ATP in sufficient amounts.  Nothing happens without enough ATP being produced in our muscles in sufficient amounts to sustain contraction.  

ATP isn’t just something that exists in our body in an unlimited amount so that is readily available whenever we need it.  It is produced by the energy stored in carbohydrates, fats and proteins from food we each through energy pathways occurring in muscle fibers.  

When we start to exercise and our muscles are asked to contract more than at rest, our muscle cells begin to produce ATP in the amounts required to drive the contractions.  The harder we want to work and the more muscle contraction that is involved, the more ATP needs to be produced.  There are actually three different and separate pathways used to produce ATP, and these are what we are referring to when we’re talking about “the energy systems”.  

Let’s look at each of the three different pathways.

The ATP-CP or Creatine Phosphate System

This is the simplest of the three different pathways and provides the least amount of ATP.  It’s based on a phosphorylated form of creatine (a creatine molecule with a phosphate molecule attached to it) donating its phosphate to adenosine di-phosphate (ADP) to form ATP.  

You’ve probably heard of creatine because of creatine supplements.  Creatine is produced by the liver, kidneys and pancreas and is naturally found in your muscles as well as in red meat and fish you consume in your diet.  Some people take creatine supplements to augment this supply.  

There is actually a tiny amount of ATP stored in our muscles but it is just enough to last for a couple of seconds worth of contraction.  After that another source of ATP is required to keep muscles our contracting.  This is where the ATP-PC system comes in to play.  Stored creatine in your muscles can produce ATP through this system by donating its phosphate to ADP for another 6 to 10 seconds before creatine stores are depleted.  There are no limiting by-products produced in this reaction, it occurs in the absence of oxygen, and it is very rapid.  For these reasons it is the pain pathway for fueling muscle contractions in explosive events like throwing, jumping and sprints.  However, because it doesn’t last for long its value to endurance athletes is limited.  Furthermore, it doesn’t produce a lot of ATP, producing only one molecule of ATP for every molecule of creatine-phosphate.  

Creatine stored in muscles does replenish fairly rapidly with rest or low-level activity.  Within 3 to 5 minutes creatine stores can be nearly fully restored. The problem is, the pattern of being able to use this system for 10 seconds or so every 5 minutes is not a recipe for success for endurance athletes.  Endurance athletes need another source of ATP production.  Because this energy producing pathway occurs in the absence of oxygen and does not produce any limiting by-products like lactic acid it is often referred to as “anaerobic-alactic” metabolism.


Glycolysis is the second ATP producing pathway.  It is also anaerobic in nature, occurring in the absence of oxygen.  

This pathway is more complex, relying on several different reactions to produce ATP from glycogen which is really just glucose stored in the body.  This stored glucose comes from the carbohydrates we consume in our diets.  Glycolysis produces twice as much ATP than the ATP-PC system, producing 2 molecules of ATP for every molecule of glycogen.  

The problem with this pathway is that the by-product of ATP production is a substance called lactic acid.  Lactic acid is a limiting factor to athletic performance, accumulating in muscles as ATP is produced until it overwhelms the muscle and causes muscle failure.  For most people lactate builds pretty rapidly during glycolysis and after about a minute muscle contraction becomes more difficult.  I can remember when I was teaching kinesiology the textbook we used said that the limit for this system was about 3 minutes.  I’ll be honest, I’ve had the privilege of working with some of the top paddlers in the world - Olympic Champions with the most developed anaerobic abilities imaginable, and I have never encountered someone who can rely on this system exclusively for 3 minutes.  Most athletes will begin to experience serious issues with lactic acid becoming a limiting factor to this method of ATP production within 60 to 90 seconds.  Clearly, this pathway is of limited use to athletes in long endurance events.  

Because glycolysis occurs in the absence of oxygen and produces lactic acid it is often called the “anaerobic-lactic” pathway.  

Oxidative System or Aerobic Metabolism

By far the most useful pathway for production of ATP for endurance athletes is cellular respiration or aerobic metabolism.   This pathway differs from the anaerobic-lactic pathway in a number of fundamental ways.  

For starters, this pathway only occurs in the presence of oxygen, carried to the muscle fibers via oxygenated blood.  Oxygen is deposited at the cell membrane of the muscle fiber and carried by myoglobin within the muscle cell to the mitochondria where aerobic metabolism occurs.  

Aerobic metabolism produces ATP from glucose as well but the process is much more complex than what we see in the anaerobic-lactic system.  There are two forms of oxidation involved in this pathway, glucose oxidation and fat oxidation. Both of which produce the energy within the oxidative system. 

Through both glucose and fat oxidation, two molecules of ATP are produced for each molecule of glucose/fat with the difference being that fats are oxidized preferentially at rest and lower intensities than glucose which is used as intensity increases.   Unlike in anaerobic-lactic metabolism, in the presence of oxygen pyruvate is produced as a by-product instead of lactate.  The pyruvate then enters something called the Krebs cycle where two more ATP molecules are produced as well as compounds capable of storing high energy electrons.  These compounds then enter something called the electron transport chain where 32 more molecules of ATP are produced as well as water and some free radicals, neither or which limit muscle contraction.  

The net result is that for every glucose or fat molecule, 36 molecules of ATP are produced.  The aerobic system produces vast amounts of ATP compared to either anaerobic system, and because there are no limiting by-products produced it can occur indefinitely as long as oxygen is present.  In fact, since fats and even proteins can be used to fuel this system if needed it can last beyond depletion of glucose stores.  

Of course, nothing is perfect.  The aerobic system is slower to activate, taking up to 3 to 5 minutes to start producing ATP efficiently, and is only as effective as your oxygen delivery system is at getting oxygen to the working muscles. 

So, what does all this mean?  How do these three different energy systems interact when we paddle?  

The Interaction of the Energy Systems During Activity

These three energy-producing pathways or “energy systems” do not operate in isolation. As athletes competing in an endurance sport we rely on all three to some degree.  While all we really care about when we’re paddling is that ATP is continually being produced to allow our muscles to keep working, it’s worth understanding what’s going on behind the scenes as these three systems work together to meet our energy demands.  Such an understanding can help us pace races or workouts better and avoid making tactical pacing related mistakes.  So, here are some examples of how these energy pathways interact to keep our muscles working.

Working Aerobically and Pacing Yourself

As long as we’re warmed up and our aerobic system is activated it is producing ATP.  You’ll recall that it takes up to 3 to 5 minutes for this system to become fully activated and capable of producing ATP effectively.  This is why a good warm up that involves at least 3 to 5 minutes of reasonably hard (level 2 to 3) aerobic activity is essential shortly before a race.  You’ll want to make sure your aerobic system is activated before you pull your first race stroke so that you can use it to produce ATP immediately.  It’s far better to properly warm up your aerobic system before the race rather than during it.  

The reality is that, once activated, the aerobic system works all the time you are engaged in your activity.  Even if your effort level exceeds the limits of your cardiorespiratory system’s ability to deliver oxygen to the muscles you’re using, the aerobic system will still be working.  It just won’t be producing enough ATP by itself to maintain the desired rate and intensity of muscle contraction.  Something that can produce ATP without oxygen needs to back up the aerobic system to make up for the ATP shortfall, and here’s where anaerobic metabolism comes into play.   

When the demand for ATP to keep muscles contracting exceeds what the aerobic system alone can produce, the shortfall is made up with contributions from the anaerobic systems.  Assuming there is creatine phosphate available, the first 10 seconds or so of this shortfall can be made up without any of the negative side effects of lactate production by the anaerobic-alactic system.  However, if you are going to continue to work harder than your aerobic system alone allows beyond this point, you’ll have to top up your ATP production using the anaerobic-lactic system.  In this scenario the clock starts ticking as you can only use this pathway for a limited period of time before you’ll have to either slow down or stop.  

Lactic acid will begin to build fairly quickly as soon as you start using the anaerobic-lactic system to start producing ATP.  The more you’re using it (in other words the harder you are going) the faster it will build.  At some point between 60 and 90 seconds later you’ll need to consider slowing down so you produce less lactate and rely solely on aerobic metabolism again.  If you don’t slow down within the range of two to two and a half minutes you risk having to stop entirely due to muscle failure.  

Pacing is really just the ability to work at the threshold where the aerobic system is maxed out and cannot meet all your energy demands on its own.  Provided you stay just the tiniest bit under this threshold, called the anaerobic threshold or AT, you should be able to paddle indefinitely.  However, every time your work rate surpasses that threshold you’ll begin producing lactate more rapidly which has consequences.  Knowing how hard above threshold you can go and for how long is important when it comes to pacing in order to avoid muscle failure and being forced to stop or slow down excessively.  

 What Happens to the Lactic Acid you Produce?

Lactic acid produced during glycolysis is removed from the muscle via the blood stream and eventually deposited in the liver for removal.  Unfortunately, when you’re working anaerobically lactate is produced far more quickly than it is removed so your blood lactate levels rise dramatically.  However, once you back off in your effort and return to producing all the ATP you require using aerobic metabolism alone your blood lactate levels will stop rising and slowly start to drop, taking about an hour at rest to return from a high level to baseline.  

If you aren’t resting and are still actually paddling aerobically your heart rate will be elevated above resting levels, meaning more blood flow to the working muscles removing lactate and carrying it to the liver for removal more quickly than at rest.  So, when you’re racing or doing a workout any lactate you build that is below critical levels which force you to stop will be removed when you return to working aerobically.  If you’re in a long enough race you should be able to repeatedly do harder bursts fueled by anaerobic metabolism which produce lactate.  You just need to manage your lactate levels by ensuring you don’t go too hard, for too long or too often.  

When lactic acid gets to the liver it is removed from the bloodstream via a process called the Cori Cycle in which approximately 5/6 of blood lactate is converted to glycogen and stored for future use (the rest is eliminated from the body with other waste products).   Recall that glycogen, which is really just stored glucose, is used in both glycolysis and cellular respiration as fuel to produce ATP.  So, in the end, lactate you produce does provide some positive contribution towards future ATP production.  

How do the Energy Systems Interact in a Sprint or During a Start?

In any race we are doing, we can rely on our anaerobic-alactic system to produce the all ATP we need for the first 10 to 15 seconds. Remember, there is no lactate produced so there are no negative consequences to going as hard as we possibly can for this short period of time.  We can blast off the start, going as hard as we can go and for all intents and purposes those initial 10 to 15 seconds are free.  

After 10 to 15 seconds we begin to pay a price for going “as hard as we can” as the anaerobic-alactic system stops and the anaerobic-lactate system kicks in.  We’ll start to rapidly produce lactate and eventually reach a point where we’re forced to stop if we don’t begin to slow down.  

To avoid stopping we can back off a little in terms of effort to reduce the rate at which we’re producing lactate.  But as long as our work rate exceeds the rate at which our cardiorespiratory system can deliver oxygen to the muscles we’re using we’ll still be working anaerobically.

The Length of the Race or the Effort we’re doing Matters

Obviously, if we’re only doing short sprints lasting 10 to 15 seconds most of our ATP will be produced using the anaerobic-alactic pathway.  Lactic acid is really not much of a concern unless we are doing repeat efforts without sufficient time between them for creatine phosphate stores to recover.  For most athletes creatine phosphate stores should be completely replenished within five minutes, allowing us another “free” burst of 10 to 15 seconds with no lactate production.

If we’re doing a 200m race which could take anywhere from 45 seconds to well over a minute, we still get to use our anaerobic-alactic pathway but will switch to producing ATP via the anaerobic-lactic pathway after 10 to 15 seconds.  If we’ve done our training and have developed both our anaerobic ability and ability to tolerate lactic acid in our muscles, we should be able to complete this race without reaching critical values of blood lactate.  In other words, we should be able to complete this race going as hard as we can go.  We should be able to attack the effort as if it were an all-out sprint.  An ideally paced race would be one in which we’re forced to slow down due to lactate building up just after we cross the finish line.  However, if we are doing 200m repeats in training then pacing is a much greater concern.  If rest between pieces is not adequate for lactate levels to return to baseline we’ll accumulate higher and higher lactate levels from piece to piece.  We’ll need to slow down a little in each piece so that lactate is not produced as quickly, thus allowing us to complete all the repeats in the workout.  

If we’re doing races or workouts which are longer than 200m we’ll have to start thinking about pacing.  We’ll still be able to go as hard as we want for the first 10 to 15 seconds using the anaerobic-alactic pathway.  However, after that we’ll need to decide how to approach our race so that we don’t produce too much lactate before we finish.  Do we go really hard, working anaerobically and producing lactic acid at a maximal rate for another 30 seconds after those first 10 to 15 seconds and then settle into a slower pace where all the required ATP can be produced aerobically, before cranking it up again for an all-out, anaerobic finish?  Do we slow down just a bit so we are still working anaerobically but not at max (only just over threshold) for the first 2 km or so, establishing good position in a fast group, before easing into exclusively aerobic energy production?  Can this get us to the finish line without having to stop first?  How many anaerobic bursts can we do during the race?  How long can they be?  And how close together?  Experience and experimentation can go a long way towards answering these questions so that we can pace the perfect race for the distance.

Metabolic Testing

Olympic athletes in this day and age don’t leave much to trial and error.  To be sure there is always a little of that involved but, in most cases, the top athletes have access to metabolic testing that can provide them with information about the rate at which they produce lactate at various levels of effort.  

For example, by doing repeat efforts over their 1000m race distance at increasing intensities and testing blood lactate after each, Olympic canoe athletes can determine the pace and heart rate at which lactate production rapidly begins to spike.  This can help inform them about a number of things. 

Obviously, it can shed light on their pacing, helping them understand how hard they can go without running the risk of producing too much lactate.  But beyond that it can help them determine the heart rate at which they begin to produce large, and potentially crippling, amounts of lactate.  Knowing this heart rate value helps them to train at this level, both increasing it over time and developing a tolerance to lactate in the blood stream in the process.  

The rate at which lactate is produced at a given intensity or heart rate does not remain constant but rather changes over time as an adaptation to training. So, lactate testing is something that must be done at regular intervals throughout the training season to be truly useful.

Hopefully this sheds a little light into what is going on in your muscles when you’re paddling at various speeds, why you can’t go all out for an unlimited period of time and why there seems to be a speed and work-rate at which you can go all day.  Now that you know a little bit about the energy systems and how they interact when you’re racing, next time we’ll look at how you can train each energy system.  

Training the energy systems effectively is obviously a key in maximizing race performance, but it’s also a key in developing good cardiovascular health.  Remember, the energy systems we’ve discussed here depend entirely on the ability of your cardiorespiratory system to supply oxygen to the muscles we’re using.  When we train our energy systems, we’re training our oxygen delivery system as well – our heart, lungs and circulatory system.  We live in a society where cardiorespiratory diseases are still a leading cause of death.  It’s not hard to see how training our energy systems can lead to increased cardiorespiratory health.  

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