The human engine

When is an athlete considered fit? When he or she is fast, has stamina and pedals at high wattage numbers? Well, being fit is certainly a question of individual aspirations and sporting goals, and thus a matter of definition. But it can also be measured scientifically: by means of performance. Read on to find out what factors performance consists of, what is necessary to be able to perform and how performance is increased.

No performance without training

If you want to improve your performance, you should do it in a structured way, i.e., no sporting around just for fun, but planned training that sensibly alternates workload and recovery over several weeks, months or even years, with the goal of optimising the body’s systems. Such structured training improves the cornerstones of athletic performance, which are: nutrition, economy of movement (how efficiently an athlete moves) and physiology. The latter describes certain processes in the body and plays an important role in the formation of performance. Let’s therefore take a closer look at physiology.

Performance and energy metabolism

Performance as the most important parameter

In triathlon, long-distance running and cycling, the goal is to get from A (the start) to B (the finish) as quickly as possible. To do so, the athlete must be able to maintain as high a performance as possible over the given distance. Training should therefore focus on enabling him or her to do just that.

But what is performance anyway? Cyclists may have already encountered it in the form of wattages. After all, power meters cast this rather abstract parameter into numbers on the display that have meanwhile become quite reasonable. It is more difficult to measure performance in running or swimming. In contrast to cycling where the athlete moves in a very predictable manner, this is not the case with the other two disciplines. On the bike, the athlete sits on the saddle at a certain angle to the crank, with foot and shoe fixed on the pedal and moves the crank permanently over 360 degrees with a crank arm of predefined length at a corresponding rotational speed. In running and swimming, the execution of the movement can vary significantly, which in turn influences the execution efficiency. Is it nevertheless possible to determine performance? What level of performance is a good one, which is one to be improved and, in case of the latter, how is it done? To answer these questions, we first need to understand what performance consists of.

To define performance, there is a physical formula that some may still remember from school lessons:
Performance is the quotient of work done or energy per unit of time, or P = W/t.

Or applied to sport:
The more energy the athlete can convert, the is higher his or her performance level.

This means that an athlete’s performance is the result of his or her energy metabolism over a defined period of time. This energy comes from different sources that can be specifically controlled and adjusted to influence performance.

The sources of energy turnover

Carbohydrates – fast but “finite”

Cyclists, long-distance runners, and triathletes have two main sources of energy at their disposal during physical training: Carbohydrates and fats. They differ fundamentally in their main properties. Carbohydrates are quickly available for energy supply, but unfortunately the human body only has small storage capacities. Fats, by contrast, are available almost unlimitedly, however, they can only be converted into energy slowly and with great effort.

Carbohydrates are stored in the muscles as glycogen; athletes can take them in exogenously, for example in the form of bars, gels, or sports drinks. They provide the required energy quickly, but the body requires exponentially more carbohydrates with increasing training intensity. This would not be a problem if the body’s own stores (approx. 400-600 g, depending on body constitution) as well as the absorption capacity or resorption rate per hour (1-1.2 g per kg body weight) weren’t limited. Due to these limitations athletes cannot even do shorter endurance races on carbohydrates as their only energy source, because:

  • the body’s stores are not large enough to provide the energy needed.
  • we cannot consume as many carbohydrates as we needed to consume to cover energy needs.
  • an excessive carbohydrate intake would be necessary, which would cause gastrointestinal issues.
  • energy would have to be taken from one source only. In reality however there are always several systems involved.
“Infinite” fat reserves and their limitations

The body’s own fat stores, in contrast to carbohydrate stores, are huge, almost “infinite”. An athlete with a body weight of 70 kg and 15 % body fat, e.g., has 10.5 kg body fat mass, each contains about 9,000 kilocalories (1 g fat = about 9 kcal) worth of energy. This athlete therefore has 94,500 kcal on board. Even in top form, in which he should not drop below 7 % body fat, there would still be 50,000 kcal available for energy supply only. If it was possible to draw energy solely from fats, this amount would suffice to run about 10 to 15 marathons without additional energy supplies required.

So much for the theory. In real life, unfortunately, things are not so simple, because to metabolise the body’s own fats, two elementary things must be taken into account:

On the one hand, energy metabolism first needs to be trained in order for the athlete to fully take advantage of this ”infinite” source.

On the other hand, the effort the body needs to make to use fats as a source of energy is so huge that other limiting factors (e.g., mineral deficiency, reduced motor control of the working muscles, neuronal fatigue) will appear before the body fat reserves are even close to being exhausted.

Fats are a very important source of energy and especially during longer, low- to moderate-intensity exercise they should be the main source. However, as they are not as quickly and easily available as carbohydrates, energy supply remains a mixed calculation up to a certain intensity.

Energy supply

People are hybrids

While the automobile industry has only managed to build sensible hybrid models in the past few decades, humans naturally dispose of a hybrid engine. What’s more: a car is usually limited to two systems (petrol and electricity), people actually dispose of three ways to convert energy into propulsion:

  • the alactic system,
  • the lactic system and
  • the aerobic system

These systems differ significantly in their performance and capacity, but they all provide ATP and can even resynthesise it. ATP, or adenosine triphosphate, is necessary for a muscle to contract. Without muscle contractions, there is no locomotion. Let’s therefore take a closer look at these “engines”.

The energy-producing systems
The alactic system

The alactic system, based on the conversion of energy-rich phosphates, is by far the most powerful system and works extremely quickly. For one single jump or during the first few seconds of a sprint, the alactic system is the decisive one. However, high-energy phosphates are extremely limited. After just a few seconds of maximum intensity, they drop so significantly that the load must be reduced. For this reason, the peak in a sprint performance cannot be maintained for longer than a maximum of five seconds.

Since the alactic system is highly efficient but also very limited, humans use another support system during high physical stress: the lactic system. It is less efficient but lasts much longer. The lactic system converts glycogen from the body’s stores into lactate without oxygen (anaerobic), which is then used for the resynthesis of ATP and thus contributes to muscle contraction.

The lactic system

In sports science the lactic system is defined as maximal lactate formation rate (VLamax). The lactic anaerobic system is responsible for high-intensity loads, such as attacks, sprints, or some fast rides over cobblestone passages, as it provides energy quickly via glycogen and without oxygen. However, this system does not completely break down the sugar molecules, comparatively less ATP is produced, but lactate may accumulate. This energy source is exhausted after a few minutes at high intensities, depending on how much lactate accumulation and the associated acidosis an athlete can tolerate. The lactic system could be seen as some kind of “opponent” of endurance performance, as it

  • leads to acidosis at high intensities
  • uses limited carbohydrates as energy sources
  • significantly obstructs the activity of fat metabolism.
The aerobic system

The most important energy-producing system in endurance sports is also the body’s least efficient: the aerobic system. It converts glycogen and fatty acids into energy by using oxygen. The parameter that describes the individual capacity of the aerobic system is the maximum oxygen uptake (VO2max).
Compared to the alactic and lactic systems, the performance achieved by using the aerobic system is comparatively low; however, the load duration is often higher and can be maintained for hours or even days, as is the case, e.g., in endurance sports.

The limiting factor of the aerobic system are the substrates, i.e., fats, carbohydrates, and proteins. Since fats, as described above, are almost “infinite” and protein is of hardly any importance in energy production during exercise, the glycogen balance is decisive. If glycogen stores are depleted, the aerobic system can’t work properly anymore and no longer provides sufficient energy for muscle contraction.

Humans don’t have a hybrid switch

When it comes to basic energy-providing systems, humans are better off than cars. But cars have one decisive advantage: there is a switch to deliberately change between systems. Athletes cannot do this; in fact, all three systems are always permanently involved in locomotion:

The alactic system also helps to resynthesise ATP during long, low-intensity endurance exercise. However, it does so only to such a small extent that the ATP concentration in the muscles hardly changes. The lactic system works harder, and the production of lactate always increases when a certain proportion of fast twitch muscle fibres (type II fibres) are active. They cannot be completely switched off, even when we are running slowly. Depending on the training condition, we use more or less of them and therefore always produce lactate and use a corresponding amount of carbohydrates.

The aerobic system is continuously active in cycling, long-distance running or triathlon and provides the largest proportion of energy during such endurance performances. Via this system, the maximum oxygen uptake (VO2max), an athlete’s endurance performance can be assessed; the higher it is, the better the performance. However, it is only considered a gross criterion, as it just depicts the upper limit of performance.

Accordingly, the energy-supplying systems cannot be switched back and forth or on and off at will; which one is used to what extent depends on two factors:

  • the training condition and the individual physiological profile of the athlete
  • the intensity at which the athlete moves in training or competition.

Since the highest possible intensity, i.e., best performance, is always the goal when it comes to the second bullet point, performance needs to be worked on to unlock further potential, i.e., energy-producing systems need to be optimised. This can be achieved through training. This doesn’t provide athletes with a switch to change between energy systems, but they will get some tools to use optimally for themselves.

Increasing performance and pushing limits

Performance at the anaerobic threshold

Fitness and capability of an endurance athlete are best defined by the parameter of performance. A very specific one is essential: performance at the anaerobic threshold.

The anaerobic threshold is the state of the energy-producing systems in which the lactate production of the lactic system and the lactate metabolism of the aerobic system are in exact balance. The so-called metabolic steady state of lactate build-up and breakdown is thus in equilibrium. The higher the performance at which the athlete reaches this steady state, the better his or her performance.

There is one peculiarity, though: The aerobic system – defined by VO2max – increases linearly with increasing load intensity. Up to performance at the anaerobic threshold the athlete roughly needs to process about 12 ml of oxygen per minute per watt on the pedal. Unfortunately, the lactic system, which produces lactate, does not increase linearly but exponentially. Therefore, at a certain performance level both systems will intersect – that’s the anaerobic threshold.

The systems therefore do not only differ in terms of capacity and capability, but also in their progression relative to the exercise intensity. These progressions can be trained.

Training the systems

To become more efficient means to shift the performance at the anaerobic threshold upwards. There are two ways to do so:

  • Work on the aerobic, linear rising system, i.e.: improve it.
  • Reduce the lactic, exponentially rising system and make it rise at higher performance levels only.

It is desirable to make use of both options. For an endurance athlete with a very “consistent” load, such as a long-distance triathlete or a marathon cyclist, for example, the best possible optimisation is an aerobic system that is as large as possible and an anaerobic, lactic system that is as small as possible. This way, the systems intersect quite late, i.e., at high performance, which results in a high anaerobic threshold.

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