The performance equation: Introducing the key determinants of endurance performance
An article written by Sophie Herzog, Øyvind Sandbakk, Trond Nystad and Rune Talsnes
Endurance performance is influenced by a variety of physiological, biomechanical, psychological, and even sociological factors and therefore cannot be explained with a reductionist approach. In this article, we focus on the physiological factors that interact as key performance determinants for endurance performance. Our aim is to provide simple and easy-to-understand explanations for the current concept of key determinants of endurance performance.
Human curiosity has always driven us to strive for better performance, whether it is in athletics, academics, or the arts. Understanding the key factors driving development in athletic performance can be seen alike to mastering a musical instrument or cooking a gourmet meal. Just as a musician needs to understand rhythm, melody, and harmony, athletes need to understand the determinants that influence their performance[i],[ii].
In this article, we are discussing the role of the key performance-determining factors for endurance sports: 1) Maximal oxygen uptake, VO2max, 2) The fractional utilization of VO2max, 3) Gross efficiency/work economy, 4) Anaerobic metabolism/capacity and 5) Fatigue resistance/durability/resilience (see Figure 1).
Figure 1: The schematic of the key determinants of endurance performance should illustrate that various combinations of physiological factors can lead to the same performance. While the three “big ones” have been highlighted for decades, durability has been introduced as an emerging dimension, influencing the core dimensions to varying degrees. While the durability's exact impact and mechanisms are not yet fully understood, it plays a crucial role in overall performance. Adapted from [i],[ii].
VO2max is the highest rate at which the body can take up and utilize oxygen (per time unit) during severe exercise. When people talk about how powerful a car[1] is, they often refer to the letter “V” as well, followed by a number which refers to the number of cylinders the engine has for fuel intake. This is the engine capacity of a car. With more cylinders, the car can take in and utilize more fuel to convert to energy and eventually achieve greater speed. Similarly, athletes with higher VO2max also have a greater capacity to take up and utilize oxygen, which allows them to to utilize more fuel (mostly carbohydrates) to produce aerobic energy and translate it into better performance (power or speed). The good thing with oxygen is that it is an inexhaustible resource from the air around us. However, we need nutrition (especially carbohydrate and fat) to produce metabolic and mechanical work (i.e., performance). When the body wants to use carbohydrates and fats, oxygen helps process the waste products from “breaking down” (i.e., oxidize) carbohydrates and fats. Here, oxygen binds to C- and H-ions from carbohydrate and fat, turning them into H2O and CO2, which we then breathe out. So not only do we have traces of our food intake in our breath, this binding of waste products also makes sure that the cellular environment is kept normal, muscles can function well, and work can be sustained. To summarize, the amount of oxygen intake and utilization will determine the amount of energy release. The upper roof of this sustainable aerobic energetic capacity is the VO2max.
Fractional utilization of VO2max is the ability to work at a high percentage of VO2max during competition. To stick with the car analogy, one could say that fractional utilization is comparable to a car’s ability to maintain a high speed on cruise control for an extended period without straining the engine (i.e., internally damaging for example by overheating or running out of fuel). This factor is influenced both by the ability to sustain work at a high rate of VO2max (often indicated by the lactate threshold[2]) and how quickly the body can increase oxygen uptake to meet demands (oxygen uptake kinetics). Together, VO2max and its fractional utilization determine the total aerobic metabolism (i.e., the energy-generating process relating, involving or requiring oxygen) during competitions, also called performance VO2.
Anaerobic metabolism/capacity is similar to a car’s turbo boost, which provides a sudden increase in speed for quick acceleration, for example when overtaking another car or making a quick start. In humans, however, anaerobic capacity is not simply activated by hitting the “turbo boost”-button but can be explained by the ability of the muscles to generate and sustain high power. This power comes from breaking down phosphate molecules and carbohydrates in the muscles. Imagine this as high-energy batteries that release a quick burst of power when needed. However, these batteries have limited capacity and can’t be recharged immediately. Without oxygen available (hence it’s called anaerobic capacity) to take care of “waste-products” from this rapid energy release, the anerobic energy release comes to an end and muscle fatigue builds up quickly. Once the “batteries” are depleted, the muscles need a period of rest or lower intensity activity with sufficient oxygen available to recharge batteries and clear out the waste products to normalize muscle function. This aptitude and then also the ability to recover quickly and restore anaerobic resources (e.g., after a sprint or surge), can be decisive in certain endurance sports and sub-disciplines. Similar to electric cars who can recharge their batteries when breaking or driving downhill, athletes who can recover and restore their anaerobic capacity well, are typically excelling in the final stages of an event, in tactical races or in highly intermittent sports. The anaerobic component is less important in longer, steady races but plays a bigger role in sports with variable terrain, like cross-country skiing or cycling, compared to sports with more constant demands, like running a flat marathon.
Gross efficiency in the given exercise mode determines how efficiently an athlete converts metabolic energy to power and speed, while work economy is the metabolic energy, i.e., the energy it costs, to work at a given speed or power. These are basically trying to express the same but are used in different contexts. The latter is easy to explain with the car analogy – the less fuel needed to maintain speed, the better the fuel efficiency, i.e., the more efficient the car. Gross efficiency is simply the percentage of metabolic energy that is transferred to external power. If efficiency is 20%, it means that 20% of the metabolic energy goes to power, while the remaining 80% goes to heat production or mechanical or technical losses.
The ability to sustain the abovementioned factors at a high level – as close as possible to the initial level – throughout the entire competition is considered crucial to success. In general, it can be said that the more work performed by an athlete, the more the above-mentioned performance-determining factors decline. Very high-level endurance athletes, however, show a remarkable resistance to fatigue. Just like a reliable car can handle hours of driving without reduced capacity, athletes with high durability can sustain their performance throughout a long competition with minimal detrimental effect to the abovementioned factors. This resilience or durability, is associated with specific adaptations that delay muscular deterioration and fatigue, and enables maintenance of race pace, especially during long-duration competitions. Here, fuel utilization and intake play an important role and the ability to sustain good technical execution may be another important factor. However, limiting factors would differ across sports due to differences in sport-specific demands such as duration of competition, type of locomotion, etc.
In order to put the above-mentioned factors together to the best possible performance, one has to consider as well how the body uses carbohydrates versus fats (substrate use) and integrate it into metabolic energy calculations. In this way, the abovementioned factors can be combined to predict how well metabolic energy can be produced and converted into external power and speed. Elite level athletes across all endurance sports exhibit high, albeit varied, combinations of these factors. For instance, an athlete with slightly lower VO2max might compensate with excellent fractional utilization and/or efficiency. This interplay is akin to balancing flavors in a dish – a touch more salt can balance out a bit of extra sweetness. So, whether you’re an athlete or a chef it is important to understand the interaction of your key ingredients to optimize your results.
In our performance programs, we delve deeper into the key determinants of endurance performance, providing relevant and applicable education to athletes and coaches. For the reader interested in more technical explorations, we have included references to relevant scientific publications.
[1] We are neither mechanics nor car specialists, so please forgive us if the comparisons are not entirely accurate. Nevertheless, for the rest of the article we will draw comparisons with cars and engines, as they serve quite well as analogue examples to make the key performance-determining factors a little easier to understand.
[2] We will cover the topics of lactate and lactate thresholds in future blog posts. In the meantime, for the definition of lactate threshold we would like to refer to an external blog post from TrainingPeaks (https://www.trainingpeaks.com/blog/what-is-lactate-and-lactate-threshold/)
[i] Joyner, M.J. & Coyle, E.F. (2008). Endurance exercise performance: the physiology of champions. Journal of Physiology, 586, 35-44.
[ii] Sandbakk Ø. Physiological Determinants of Endurance Performance. Endurance Training - Science and Practice (2nd edition). 2023
