A Complete Overview of Energy Pools in Climbing

I had a request awhile back to thoroughly go over the various places energy can come from for a climb. I’ve referenced many of the pools several times, but never really diagrammed them out to the point you can see how they interplay.

Actually, this is a post I’ve wanted to write for awhile, but didn’t really know the right strategy to tackle it with. The problem was I wanted to write an article about the energy pools—to spin a narrative about each one and bring them together as a discrete whole—but that’s not a suitable approach here. This post calls for a more hierachical assembly, where each pool is discussed as a part of the system that contains it; in this format, I think we can dredge coherence out of it.

Let’s dig in.

The Three Energy Systems

Before we proceed further, it’s important to know that our body has three energy systems:

1. The Creatine Phosphate System, also known as the Anaerobic Alactic Acid System (because it doesn’t produce lactic acid). The principle end product of this system (aside from energy, of course) is ADP and inorganic phosphate.
2. The Glycolytic System, also known as the Anaerobic Lactic Acid System (because it does produce lactic acid). The principle end product of this system is pyruvate, which is then converted to lactate in order to absorb a couple excess hydrogen ions.
3. The Aerobic System, which doesn’t have an alternative name. The principle end product of this system is carbon dioxide and water—when people say that you lose fat by breathing it out, this is what they mean (that you release the carbons stored in a fat molecule as carbon dioxide). Also, the fact that burning fat releases water is how bears don’t die of dehydration during hibernation, which isn’t particularly relevant to this article but a fun thing to know!

The creatine phosphate system is powered only by creatine, the glycolytic system is powered only by carbohydrates, and the aerobic system is powered by both fat and carbohydrates, though fat is more energy-efficient per molecule (carbohydrates, on the other hand, are better energy producers per liter of oxygen—a frequent limiting factor in exercise).

For the rest of this article, however, I’m not going to separate out the different energy pools by the exact system they feed, but rather by the type of system they feed: anaerobic-only, anaerobic and aerobic, and aerobic-only.

Anaerobic-Only Energy Pools

There is only really one pool of energy available to us to use solely for anaerobic energy, but for the sake of being thorough I’ve included the minor amount of free ATP we always have stored in our muscles.

Free ATP in the Muscles

The very first source of energy used to power a climb is the free ATP sitting in your muscles. The pool of free ATP at all times is miniscule (approximately 8 millimoles per kilogram of muscle, or about 0.06 kcal worth of energy) and able to power less than a second of most exercise. In reality, this pool doesn’t exist to power exercise, but rather to ensure there is always energy available for the muscles while the other pools work to resupply the energy.

You cannot train this pool as it’s not really a pool so much as a temporary holding station for energy before it gets used. Free ATP is like the water stored in your cocktail straw when you’re not sipping a drink.

Creatine Phosphate

Creatine phosphate is the second source of energy used to power exercise, but is primarily relevant for high-intensity exercise. Our body can store significantly more creatine phosphate than free ATP (approximately 30 millimoles per kilogram of muscle), but it is still an extremely limited pool and only able to sustain maximal exercise for around 5-10 seconds (estimates vary, and will depend on the person, but usually center around 7 seconds).

While the total size of the creatine phosphate pool is small, it is also rapidly regenerated—though over the course of exercise, it will be depleted to lower and lower levels (and thus take longer and longer to recover from) unless you really take the time to rest and recover (13+ minutes between climbs of high-intensity). Thus, the “effective size” of the pool is larger than may otherwise be assumed since the pool is constantly refilling (fueled primarily by the aerobic system, which is in turn powered by both fat and carbohydrates).

You can increase your total pool of creatine phosphate in two ways:

1. Supplement with Creatine Monohydrate: Supplementing with creatine can increase your muscle creatine content by around 25%, a significant boost for such a small energy pool, bringing it up to almost 40 millimoles per kilogram of muscle.
2. Gain More Muscle: Since your total pool of creatine depends on your total muscle mass, you can effectively increase creatine by gaining muscle. Whether this provides an acute advantage separate from the additional strength of that muscle is debatable.

Anaerobic or Aerobic Energy Pools

The following energy pools can be used to power either anaerobic or aerobic exercise, depending on the current need. For climbing, it’s likely that both are significant as anaerobic glycolysis can quickly regenerate spent ATP (allowing for more power) and aerobic glycolysis can regenerate creatine phosphate faster when oxygen is limited (as is often the case locally in the muscles during climbing). All dual-purpose pools are carbohydrate-based.

Muscle Glycogen

Compared to creatine phosphate, our body can store significantly more muscle glycogen, but it’s still a limited pool. It’s estimated that the human body normally stores around 20 grams of glycogen per kilogram of muscle with a physiological maximum of around 36 grams per kilogram. Unfortunately, muscle glycogen is only available to the muscles it is stored in—so while our total pool of muscle glycogen may be large, the pool relevant to climbing may be much smaller. You can’t use the large amount of glycogen stored in your legs, for example, to get through a fingery crux. Still, the upper body can store a lot of glycogen, and the forearms can each store an estimated 15 grams (based on typical forearm muscle weight), which will go a long way.

For most breeds of climbing, total glycogen depletion is unlikely to be a reason for failure (i.e., you’re not going to “bonk” or “hit the wall”); it’s more likely that fatigue will set in from a combination of factors, including overall energy depletion (from creatine, muscle glycogen, and liver glycogen combined), accumulation of metabolites (such as inorganic phosphate and ADP), and neural fatigue.

You can affect your total muscle glycogen content in a few ways:

1. Directly through Exercise: The muscles adapt to exercise by increasing the local storage of muscle glycogen—whichever muscles you train will store more muscle glycogen for subsequent sessions. (This is assuming you consume carbohydrates; if do not eat carbs, you will not store muscle glycogen).
2. Indirectly through Exercise: Just as with creatine phosphate, you can effectively increase total muscle glycogen by increasing total muscle.
3. Through Diet: You will store more or fewer carbohydrates depending on the carbohydrate content of your diet. On one extreme—a ketogenic diet—you will store virtually zero muscle glycogen (because you aren’t providing the raw material necessary). On the other extreme, some athletes will carb load (eat significantly more carbs than normal) before important events in order to increase muscle glycogen content above normal levels. Most diets will fall between these extremes, and if you eat a relatively normal amount of carbohydrates (40-65%) then you are likely to have relatively normal amounts of muscle glycogen storage.

Liver Glycogen

We also store glycogen in the liver. Unlike muscle glycogen, which can only be used by the cells containing the glycogen, liver glycogen is released into the bloodstream for use by any cell in the body (including the muscles).

The average human stores 80-100 grams of liver glycogen, and the main purpose for liver glycogen is to keep blood glucose from dipping below a healthy threshold. There is more than enough liver glycogen to bolster blood sugar between regularly timed meals, but during exercise liver glycogen can be rapidly depleted, forcing the body to begin converting skeletal muscle into glucose (also in the liver; more on this later).

You cannot really affect liver glycogen storage, aside (again) from consuming a low-carbohydrate diet. If you eat carbohydrates, you’ll store those carbohydrates first in the liver, then the muscles, then finally in the fat if there is absolutely no other place for it to go. If you don’t eat carbohydrates, you won’t have any available to store in the liver. Regardless, you can’t increase liver glycogen storage above your own personal physiological max, somewhere in the range of 80-100 grams.

Blood Glucose

Blood glucose isn’t really a storage pool, but it is a pool of energy we can draw from when there is excess energy available—which is basically right after we’ve consumed carbohydrates. These carbohydrates will help power exercise (and will increase total power potential), but will not reduce the amount of muscle glycogen used (though liver glycogen will be spared).

There’s only one way to directly increase blood glucose aside from directly injecting it—eat a carbohydrate-containing meal. If you eat such a meal before exercise, slower carbohydrates are your best bet so they’ll be released at a steady pace during exercise instead of flooding your system too soon. Once you’re in the middle of exercise, though, you’re better off with fast carbohydrates such as those found in a sports drink so the energy is made available faster.

Aerobic-Only Pools

The final pools—mostly fat-based—are only ever used for aerobic energy. For climbers, they are excellent at regenerating spent creatine, but probably too slow to provide more than a trickle of energy to be directly used on a climb.

Muscle Triglycerides

Intramuscular fat is known to accumulate in two distinct populations: the obese, and athletes (specifically endurance athletes). But while intramuscular fat in the obese causes insulin resistance (and thus begins a cascade of problems leading towards diabetes), athletes remain insulin sensitive, making it a useful energy source with no downside.

Intramuscular fat becomes an important energy source primarily during moderate-intensity exercise (its use peaks at roughly 65% of your VO2Max). The advantage of intramuscular fat is that it does not require delivery from the adipose tissue to the active muscle. Unlike carbohydrates, fat is not water-soluble and requires special transporter proteins to be moved in the blood. Our body has a limited supply of those transporters and at some point the chain will bottleneck—intramuscular fat helps avert that bottleneck.

For endurance athletes, this is of tremendous importance as their activity is long and sustained—they must have a constant supply of energy lest they fatigue. This is exactly why endurance athletes develop intramuscular fat stores in the first place! For climbers, it’s questionable whether intramuscular fat would be helpful or even whether it would develop at all from normal climbing training. Most climbs are of too short a duration to be considered endurance exercise, and it’s unlikely the body would adapt to climbing by storing extra intramuscular fat since no bottleneck will be felt (and therefore no stimulus will promote such an adaptation).

That being said, you could affect intramuscular fat in two possible ways:

1. The Unhealthy Way: You could eat more than the amount of food you need for weight maintenance over a long period of time and eventually will start storing the fat directly in the muscle. This isn’t a real option, but I feel I’d be remiss if I didn’t mention it because it is still a direct way you can affect your intramuscular fat levels.
2. The Healthier Way: You could engage in significant amounts of endurance activity, though the intramuscular fat will only be stored in the muscles you’re fatiguing. For climbers, this would mean during extremely long bouts (30+ minutes, but probably 1+ hours) of near continuous climbing.

Again, though, it’s unlikely intramuscular fat will actually improve your climbing as the bottleneck in fat availability endurance athletes experience isn’t relevant to climbing; climbers use predominantly anaerobic systems, first creatine phosphate and second anaerobic glycolysis, to “power” a climb and then use fat mainly to recover those systems during rests, not to directly power the muscles.

Adipose Tissue & Plasma Free Fatty Acids

The other fat-based pool available to us is the fats we derive from the adipose tissue (our main storage location for fat). These fats are released as free fatty acids (as opposed to the triglycerides found in the muscles) and transported via proteins in the blood to the active muscles.

For endurance athletes who have intramuscular fat stores, free fatty acids are used significantly to replenish those stores after exercise has finished (as well as provide some energy during exercise). For those without intramuscular fat stores—including, probably, most climbers—free fatty acids will need to be transported during exercise to the working muscles in order to provide the necessary energy.

There’s currently only one study that has been done on climbers examining the roles of various energy systems (and the pools that feed them) in performance, and it determined that free fatty acids contribute around 40% of the total energy for any given climb. The study authors note, however, that most of this fat-derived energy is likely to be used to replenish spent creatine back into creatine phosphate as fat, by itself, cannot resynthesize ATP fast enough to power all but the easiest moves while climbing.

Even the leanest individuals have more than enough fat stored to power extended periods of exercise (you could run hundreds of miles on the fat stored in your body), but in this case the total size of the pool is less important than the available stream of energy it can release. Not only is your ability to use fat limited by the availability of transporter proteins in the blood, but also by the availability of oxygen.

Fat is thus an important energy source for climbing, but in many ways only an auxilliary source. The raw, biochemical fact is that the oxidation of fat for energy can only provide a fraction of the energy necessary to power a high-intensity sport like climbing—though between difficult sections or during rests, fat can go a long way towards replenishing your spent creatine.

Since increasing the total pool of fat (by gaining more adipose tissue) doesn’t do anything to solve the bottleneck problem, there is essentially no way to increase the output of this energy source. Similarly, while plasma free fatty acids increase after eating a meal, all that changes is the initial source of those fatty acids—the total available energy will not increase.

Muscle Protein

The final energy pool tapped during climbing (or any form of exercise) is the energy contained within the proteins of our muscles. This is the absolute last source of energy our body wishes to rely upon, and only becomes important when blood glucose begins to dip due to liver glycogen depletion. At this point, the body will break muscle proteins down into free amino acids and transport those amino acids to the liver for conversion into new glucose molecules (a process known as gluconeogenesis).

Since the depletion of liver glycogen is a necessary first step, muscle protein only begins to play a significant role after an hour or two of exercise (depending on the intensity). Furthermore, you can greatly stave off the use of muscle protein for energy by consuming carbohydrates before and during exercise, thereby helping prevent blood glucose levels from dipping in the first place.

The pool of available energy stored in the muscles is as limitless as the pool of available energy stored in our fat; in both cases, there is zero risk of depleting these pools as you will fail much sooner due to other reasons. Therefore, while you can of course increase the total pool size by gaining more muscle mass, there will be no direct benefit energy-wise to doing so.

I list this energy pool as “aerobic-only” because by the time you rely upon it, it’ll be used almost solely to feed the necessary glucose to the brain. By this point, the muscles will have become resistant to glucose (in order to save it for the brain) and thus the glucose made from protein is highly unlikely to power anaerobic exercise—actually, it’s unlikely to really power exercise at all.

A Quick Recap

Our body has numerous pools of available energy, some of which can be increased or modified, others which cannot. Knowing which pools have the most importance in climbing—and what you can do to improve them—can go a long way towards understanding how your performance can be improved or limited by your diet.

If you didn’t read through all 3,000 of the above words, here’s a much faster summary of the various pools:

• Anaerobic-Only Energy Pools
  • Free ATP in the Muscles
    • Extremely limited (less than a second’s worth).
  • Creatine Phosphate
    • Extremely limited, but rapidly regenerated.
    • Can be increased by supplementing with creatine monohydrate.
• Anaerobic or Aerobic Energy Pools
  • Muscle Glycogen
    • Limited (only an estimated 30 grams between the forearms).
    • Can be increased by training and consuming a higher-carbohydrate diet; Can be eliminated by eating a very low-carb diet.
  • Liver Glycogen
    • 80-100 grams total.
    • Released to prevent blood glucose from dipping below healthy levels.
    • Cannot be increased; can be eliminated by eating a very low-carb diet.
  • Blood Glucose
    • Only provides energy to muscles when spare energy is available; otherwise, the glucose is reserved for the brain during exercise.
    • Will not prevent muscle glycogen from being used, but may spare liver glycogen; increases total pool of anaerobic energy.
    • Can be increased by consuming carbohydrates in the hours prior to (slow carbs preferred) and during exercise (fast carbs preferred).
• Aerobic-Only Energy Pools
  • Muscle Triglycerides
    • Local storage form of fat that helps circumvent the transportation bottleneck experience by endurance athletes.
    • Can be increased by performing long bouts (30+ minutes, at least, with no rest) of endurance climbing—but of questionable benefit to climbers since they are unlikely to experience the bottleneck in fat transportation as an endurance athlete would.
  • Adipose Tissue and Plasma Free Fatty Acids
    • Adipose tissue is essentially unlimited, but can only release a trickle of energy compared to other sources due to constraints (transportation protein bottlenecks and lack of oxygen).
    • Can be increased by consuming dietary fats, but the same limitations will persist so virtually nothing will change other than the initial source of the fat.
  • Muscle Protein
    • Broken down into amino acids that are then converted in the liver to new glucose (gluconeogenesis).
    • Technically could be used to power anaerobic activity, but by the time muscle protein is a significant contributor, the majority of new glucose created will be used for the brain, not exercise—thus, not really a source of anaerobic energy.
    • Essentially unlimited; you will fail far sooner due to other reasons than due to a lack of skeletal muscle to convert to glucose.

Now, it’s an entirely different discussion as to how you can modify the use of the various energy systems in climbing, but suffice to say for now that your body will use the systems you train it to use provided that it can.

Fat can used extensively to replenish spent creatine phosphate, and you can train your body in such a way to increase the speed at which fat can do this job (helping you recover faster during and between climbs), but you can never train your body to use fat as its primary fuel source because fat doesn’t have what it takes to do the job—it’s too slow of a fuel, ideal for powering lower-intensity, longer bouts of exercise like cycling or running. Creatine can power brief spurts of extremely high-intensity exercise, but will always need to be replenished, and fat (or carbohydrates burned aerobically) remains the ideal candidate for the job. Carbohydrates offer a mix of the two, but will always be limited and unable to be directly replenished during a climb (though you can bolster it, at least).

Essentially, you can train each system to work better at the functions at which they already excel, but you cannot train a system to take the place of another system. But, that’s really a discussion for another time when we examine the roles of different types of muscle fibers as well.

In the meantime, I hope this view into our body’s energy pools was helpful, and give me your thoughts in the comments!