Energy Systems in Trail and Fell Running

Having an understanding of your so-called ‘energy systems’ - or in other words, the different methods by which the body generates energy - is really useful if you want to improve your fell and trail running abilities. This is because these systems dictate precisely how you’ll want to train and fuel for different types of events.

In this article we’ll explain the different energy systems and the implications these have for training and racing…

Overview of Energy Systems

In order for muscles to contract and propel you forwards, a source of energy is needed. The body’s energy source is a substance called ‘ATP’ (which stands for ‘Adenosine Triphosphate’). You can think of ATP like a battery – charged and ready to release energy when needed. To release energy, ATP is broken down to another substance called ‘ADP’ or ‘Adenosine Diphosphate’.

ATP is in very limited supply within the body, and therefore systems are required to continually resynthesize ATP so that it doesn’t run out (similar to recharging a battery). 

Three key systems are used to produce ATP during exercise:

1.  The phosphocreatine system.

2.  The anaerobic glycolytic system.

3.  The aerobic energy system.

These systems operate at different speeds and with differing capacities.

At one end of the spectrum, the phosphocreatine system produces ATP very rapidly, but this can only be sustained for a few seconds.

At the other end of the spectrum, the aerobic system produces ATP the slowest, but with a virtually unlimited capacity to sustain ATP production indefinitely.

Sitting between these two systems is the anaerobic glycolytic system, which for brevity, we’ll call the ‘glycolytic system’. This system produces ATP at a moderately high rate, but can only be sustained for 30-seconds to 2-minutes.

Contrary to what many people think, these systems are usually all acting to a certain degree, irrespective of exercise intensity, as illustrated in the figure below, adapted from Gastin (2001):

So, for example, when you do a hard ‘anaerobic’ effort, such as 1-minute all-out, there will still be a substantial contribution from the aerobic energy system.

Similarly, even during longer efforts, such as a 30-minute race or time trial (which is traditionally used to determine threshold pace or heart rate, as described here), there will be a notable contribution from the glycolytic system.

One of the main purposes of training is to develop or manipulate these energy systems such that you’re able to produce energy in a way that’s optimised to your chosen event type (e.g. short fell/trail races, vs long ultras). So, as a self-coached athlete, an understanding of these energy systems and how they might relate to your own discipline(s) is important.

Over the remainder of this article, we’ll take a look at each of these systems in a little more detail. We’ll try to stick to the key essentials so as not to be overwhelming!

The Phosphocreatine System

As mentioned in the previous section, the phosphocreatine system is the fastest system for producing energy. Phosphocreatine is a substance stored in the muscles. When ATP needs to be resynthesized very rapidly, phosphocreatine is broken down to release energy that can be used to reform ATP molecules.

There’s only a very limited supply of phosphocreatine within the muscles, and thus this system can only operate at a high rate for 2-3 seconds. This system is the main system used during explosive movements such as Olympic lifting. It’s very quick to respond to changes in exercise intensity, so also plugs the gap in energy generation before the slower glycolytic and aerobic systems have chance to catch up.

A classic example where the phosphocreatine system dominates is at the very start of a race, where the first few strides will be predominantly fuelled by phosphocreatine breakdown. It’s worth reiterating though that the other systems will still contribute to some extent, and no form of exercise is ever fuelled by one single system alone.

Once depleted, phosphocreatine is then resynthesized via the aerobic system. It typically takes around 30 seconds to replenish 50% of the depleted phosphocreatine stores (Sahlin et al., 2014). However, this recovery rate depends on the strength of your aerobic system. 

The phosphocreatine system is sometimes also referred to as the ‘anaerobic alactic system’, as it does not require oxygen (it’s ‘anaerobic’) and it does not produce lactate (it’s ‘alactic’).

The Glycolytic System

The glycolytic system is the next fastest system to respond to an increase in exercise intensity, taking around 3-6 seconds to kick in (Sahlin et al., 2014). It involves the anaerobic (i.e. without oxygen) break-down of carbohydrates to produce ATP in a process called ‘glycolysis’. These carbohydrates can either be in the form of ‘glycogen’, stored within the muscles and liver, or glucose within the bloodstream. You may hear this system referred to as the ‘anaerobic lactic’ system, due to the production of lactate.  

The figure above shows a summary of the chemical process involved in glycolysis. In addition to ATP, several by-products are formed by the glycolytic system. You don’t need to necessarily remember the names of these substances (for the sake of brevity, we’ll just refer to these by-products as ‘pyruvate’, but recognise that some other things are produced too). The key thing to understand is that there are two potential fates for the pyruvate, as illustrated in the figure below:

First, the pyruvate can be converted into a substance called lactate, and some positively charged ions (H+ and NAD+). This process is highlighted in red in the figure above. The NAD+ ions are useful, because they can be re-used in further metabolic processes, as illustrated by the dotted arrow.

The hydrogen ions (H+) are potentially a little problematic, however. If they were allowed to accumulate, they would turn the muscles and blood increasingly acidic. It was previously thought that the accumulation of hydrogen ions, and associated acidity is what led to the ‘burning’ sensation you feel when running very hard. However, the subject of what precisely causes fatigue is actually a bit more nuanced than this. There’s evidence that it’s not the acidity per se that causes the burning sensation and general feelings of tiredness, but actually a combination of the metabolites: lactate, hydrogen ions, and ATP, which signal that the conditions within the muscles are getting close to becoming damaging and indicate that exercise intensity needs to be reduced (Pollak et al., 2014).

In any event, the key take-home point is that when hydrogen ions accumulate, it becomes harder to continue exercising, and fatigue occurs quickly.

The second potential fate for pyruvate is that it can enter the aerobic energy system to produce more energy, plus some relatively innocuous water and CO2 (this process is shown in orange in the figure above.

This is the ‘preferable’ fate for these by-products, because (i) it produces more energy and (ii) it doesn’t produce hydrogen ions. How much of the pyruvate is processed by the aerobic system and how much is turned into lactate depends on the availability of oxygen at the working muscle, and the capacity of the muscle for aerobic respiration (Gray et al., 2014). We’ll discuss some of these factors further below.

The Aerobic System

Having looked at the phosphocreatine system and the glycolytic system, the final energy system we need to cover is the aerobic system. This system uses oxygen to convert fuel into ATP. The by-products of this process are carbon dioxide and water, which as mentioned previously, are both (relatively*) benign substances that don’t impair exercise performance. It’s for this reason that the aerobic system is the body’s preferred energy system for exercise of any extended period of time.

The main fuel sources for the aerobic system are carbohydrates and fats. Protein can also be broken down aerobically to produce energy, but this makes a very minor contribution under nearly all circumstances, so we won’t consider it here.

In the case of carbohydrates, these are initially broken down to pyruvate via glycolysis as described in the previous section. Thus, the glycolytic system and the aerobic system are linked, as illustrated in the figure above, and aerobic oxidation of carbohydrates cannot occur without the initial anaerobic stage. In contrast, fats (in the form of fatty acids) are processed by the aerobic system directly (although, fats that are stored as triglycerides in muscles and fat tissues around the body must be broken down by enzymes to fatty acids prior to entering the aerobic system).

The relative proportions of fat and carbohydrates that are used to produce energy via the aerobic system depends on exercise intensity, as illustrated in the figure below, adapted from Brooks & Mercier (1994). The use of fats is slower, and therefore dominates at lower intensities, whereas carbohydrates can produce energy more quickly, and therefore these dominate at higher intensities.

The relationship between exercise intensity and fuel utilisation also depends on fitness level and is something that can be trained. The body typically stores sufficient carbohydrates to fuel all-out exercise lasting up to around 1.5 hours, whereas even very lean athletes have enough fat to fuel exercise lasting for several days. It’s therefore often beneficial to train the body to become more efficient at using fats for fuel, so as to conserve the relatively limited carbohydrate stores. This is particularly true if competing in events that last 1.5-hours or more.

It can also be useful to train the body to become better at using fats so as to reduce the rate of glycolysis, and thereby produce less lactate and H+ ions (Bassett & Howley, 2000). Even when the capacity of the aerobic system can match the production of pyruvate, a proportion of the pyruvate will always be converted to lactate and H+. This is why training in Zone 3 can be quite stressful on the body, even though lactate and H+ levels remain stable.

The capacity of the aerobic system to produce energy depends on the rate at which oxygen can be supplied to the working muscles, and the rate at which those muscles can process the oxygen. This is known as ‘aerobic capacity’ or VO2max. We have a detailed article on what VO2max is, and how it can be trained here.

Implications for Racing

Having understood the different energy systems, and how their use depends upon exercise intensity, let’s look at some of the practical implications this has for training and racing.

All race types rely mostly on the aerobic systems

If you take one thing from this article, it’s hopefully that almost all trail and fell running types rely heavily on the aerobic systems. This means it’s important to develop your VO2max and general aerobic fitness. Some good sessions for achieving this are described in this article on VO2max.

Short and/or hilly races rely more on the glycolytic system

Although almost all trail and fell races are predominantly aerobic, the shorter the race, the more you will also call upon the glycolytic system to provide energy. This is particularly true for very short races (e.g. those lasting 30-minutes or less).

Races with steep, sharp hills that you want to climb at speed, and which force you to repeatedly produce relatively short hard efforts also tap into the glycolytic system to a greater extent.

For these types of races, you’ll want to focus on developing your capacity to produce energy through glycolysis, and to efficiently clear away the fatiguing metabolites that are produced alongside it.

Short intervals lasting 30-60 seconds, with long recovery (at least 5-6 times the work interval) are a good way to improve your capacity for glycolysis.

You can also work on your ability to clear and buffer the fatiguing byproducts by performing 30-60 second efforts with shorter recoveries (e.g. 1.5-2x the length of the work interval).

Nutrition can also help with these types of races. In particular, beta alanine and sodium bicarbonate are both supplements that have been proven to help neutralise the acidity caused by the H+ ions, and may enhance performance in short and/or hilly races.

Mid-length and longer races may challenge glycogen stores

Even at middle and lower intensities, there will be a non-negligible proportion of energy derived from glycolysis, which over time can challenge the body’s internal glycogen stores.

It’s therefore very important to make sure you fuel well with carbohydrates before and during events like these. Generally speaking, you’ll want to take on between 30-60g of carbohydrates per hour, with higher amounts (50-60g) for races in the region of 2-5 hours and lower amounts (30-40g) for events that are longer or shorter than this.

It is possible to take on carbohydrates at a slightly higher rate (60-90g) and in some cases this can further enhance performance. However, this requires a careful mix of ‘multiple transportable carbohydrates’ and sufficient fluid intake to avoid gastrointestinal issues, which in practice can be hard to achieve. So we generally recommend 60g/hour as an upper limit for carbohydrate intake during a race, except in special cases where detailed nutritional testing has been performed beforehand.

With these types of events, there’s also a benefit to specifically focussing on training your ability to use fats for fuel, which can prolong endurance as well as improve the pace you can sustain for an extended period. One good way to achieve this is through long runs performed at a Zone 2 intensity.

Key Take-Homes

We understand that the energy systems are quite complex, and there are lots of confusing terms like ATP, phosphate and H+ ions to get your head around! Don’t worry if you don’t understand everything in complete detail. The main points to understand are:

• There are three key systems used to generate energy and thus power muscle contraction. From fastest to slowest, these are: the phosphocreatine, the glycolytic and the aerobic systems.

• The phosphocreatine system dominates for maximal efforts lasting up to ~6 seconds. The glycolytic system dominates over maximal efforts up to around 1-2 minutes. The aerobic system provides the bulk of energy for anything longer than a few minutes.

• The glycolytic system operates quickly because it does not require oxygen. However, it produces acidic conditions within the muscles and blood, which mean that hard efforts can only be sustained for a limited length of time.

• The aerobic system does not produce any undesirable by-products and can sustain exercise far longer than any of the other systems. However, it requires a supply of oxygen, and the maximum rate at which energy can be generated by the aerobic system is therefore limited by the body’s ability to supply and process oxygen (‘VO2max’).

• The aerobic system involves the break-down of fats and carbohydrates. Fat use increases at lower exercise intensities and as aerobic fitness improves.

*Actually carbon dioxide can impair exercise performance if the levels become too high. This can occur if you have a respiratory limitation, and cannot expire carbon dioxide at the same rate at which it’s produced. However, accumulation of fatiguing metabolites from anaerobic processes is a much bigger cause of fatigue.

References

Bassett, D. R., & Howley, E. T. (2000). Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine and science in sports and exercise, 32(1), 70-84.

Brooks, G. A., & Mercier, J. (1994). Balance of carbohydrate and lipid utilization during exercise: the" crossover" concept. Journal of applied physiology, 76(6), 2253-2261.

Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports medicine, 31(10), 725-741.

Gray, L. R., Tompkins, S. C., & Taylor, E. B. (2014). Regulation of pyruvate metabolism and human disease. Cellular and molecular life sciences, 71(14), 2577-2604.

Pollak, K. A., Swenson, J. D., Vanhaitsma, T. A., Hughen, R. W., Jo, D., Light, K. C., ... & Light, A. R. (2014). Exogenously applied muscle metabolites synergistically evoke sensations of muscle fatigue and pain in human subjects. Experimental physiology, 99(2), 368-380.

Sahlin, K. (2014). Muscle energetics during explosive activities and potential effects of nutrition and training. Sports medicine, 44(2), 167- 173.