MPEs vs bursts

Apr 24, 2024 | ACADEMY

introduction

During movements on the pitch, players’ muscles use metabolic energy to produce mechanical work. For this reason, energy expenditure and mechanical work can be considered two sides of the same coin. Collecting players’ tracking data during training sessions and matches allows one to estimate both metabolic and mechanical power. The so-obtained energy expenditure and mechanical work are a more accurate estimate of the total workload. Compared to the total distance (one of the most widespread parameters used for this purpose), energy expenditure and mechanical work take into account the additional amount of effort due to the accelerations/decelerations, a not negligible aspect in team sports. In addition, the benefits of both these approaches are far more insightful once the interest shifts to high-intensity phases. If this is the case, it is important to define what is meant by high intensity. In everyday “field jargon”, the term high-intensity is frequently (mis)used: it’s commonly referred to high-speed phases (i.e. distance covered above 7 m⋅s⁻¹), but it may also apply to something else, like accelerations or decelerations, impacts, or any other measure related to the players’ activity. What are the high-intensity nuances provided by the metabolic and mechanical approaches?

the meaning of “high demands”

Often, when dealing with parameters calculated from tracking data, it’s a common practice to consider high demands as belonging to the same big family: the external load KPIs. However, the principles of the above-mentioned approaches (i.e. metabolic and mechanical) go in opposite directions, trying to estimate the energy demands, on one side, and the muscular load (more similar to internal workload), on the other. Then, where’s the boundary between external and internal load? It is quite difficult to answer this question because we should also consider the limits of the direct measures of the internal load (one of the most popular being the heart rate) to understand in detail how complicated a precise measure of workload is in team sports. Since this is not the purpose of this article, we can only conclude that any variable has its limits; knowing them can help us to use these variables as effectively as possible. This is the case with metabolic power events and bursts which, as the most demanding actions from a metabolic and mechanical point of view, respectively, may be confused with each other. What’s the real meaning of “high demand” when referring to the performance from an energetic or mechanical perspective?

the anaerobic spells

When dealing with energy demands in team sports, the erratic fluctuations between high-intensity running actions and recovery periods must be carefully considered. Indeed, it’s important to look at them separately because:

  • the most demanding phases are carried out thanks to the intervention of the anaerobic stores (mainly the phosphocreatine breakdown) and are usually characterised by a metabolic power greater than the actual oxygen consumption (see Figure 1, red areas);
  • the anaerobic phases must be followed by recovery periods in which the VO₂ is greater than the metabolic power and, as such, a fraction of the O₂ debt is paid at the expense of the oxidative processes (see Figure 1, green areas); this is the only way the player can maintain a certain average intensity throughout the match.

Figure 1 – Metabolic power (red curve) and VO₂ (blue curve) during an intermittent exercise. During the work phases, the energy supply is a combination of aerobic energy (blue areas) and anaerobic energy (red areas). On the contrary, during the recovery phases, the surplus energy given by the oxidative processes (green areas) allows to resynthesise, partially or totally, the anaerobic stores used during the preceding work phases.

It’s important to note that the combinations of velocity and acceleration that lead to metabolic power greater than VO₂max (and, even more so, greater than the actual VO₂) are often represented by activities that are far from the player’s maximal possibilities. Just consider that, during a maximal sprint of an elite football player, the highest metabolic power values attained (∼ 100 W⋅kg⁻¹) can be 4-5 times higher than her/his maximal oxygen consumption (∼ 20 W⋅kg⁻¹). As a consequence, there are many phases in which moderate speeds and accelerations carried out simultaneously are enough to require an anaerobic supply.

The energetic approach, despite the known limits of the metabolic power estimation, provides a great opportunity to describe the actions that require a large amount of energy to be performed (MPEs, Metabolic Power Events). A deeper analysis of the player’s capacities to reiterate these activities yields a lot of interesting information about the ability of the player to repeat high-cost actions with a certain tempo. Now we know quite well that an elite player can repeat these events, on average, every 30 seconds for the entire duration of the match and that the recovery time tends to increase from the beginning to the end of the game. Interestingly, if we refer instead to the energy-demanding phases (that are more often under the magnifying glass), they remain surprisingly almost unchanged during the game. In simple words, this means that the player can perform a high-intensity activity of the same characteristics for the overall duration of the match; however, each of these actions can be repeated less frequently as the game goes on. For this reason, we can conclude with the following question: can we understand more about players’ behaviour from high-intensity or low-intensity phases?

bursts: the smart accelerations

Accelerations are one of the most widespread metrics used to quantify the mechanical load (just for the sake of simplicity in this article, we’ll neglect the decelerations). Unfortunately, the expert fitness coach has realised for a long time that describing high neuro-muscular actions using only the number of accelerations is not right. We’ve already discussed it here. The reasons thereof are briefly summarised below:

  1. a single acceleration threshold may not be enough to isolate these kinds of efforts because, when referring to the entire speed spectrum, whatever the absolute threshold selected, it could be (i) a small fraction of the maximum possible at low speeds or (ii) the maximal acceleration achievable for the speed value in question or (iii) an acceleration that cannot be performed from this speed onwards;
  2. how the speed and the corresponding acceleration are filtered can greatly affect the results in our reports; indeed, there isn’t a broad consensus for a common filtering technique, partly because the notion of “acceleration” is poorly defined. Is it the acceleration peak at each step during the running activity? Is it the average acceleration of the pushing phase of each step? Is it the average acceleration within the entire gait cycle? Obviously enough, this makes the use of the acceleration concept somewhat foggy.

A solution must be found if the goal is to detect the most challenging acceleration values. In our opinion, the first step of the process is to define the Acceleration-Speed Profile (ASP) as obtained (i) through the traditional way from a maximal sprint or (ii) more simply via the “in-situ” method (see the link for more details). The ASP allows one to find the best accelerations achievable in the entire speed spectrum, from zero to the player’s maximal speed. Hence, the ‘acceleration threshold issue’ is solved. Indeed, without going into the depths of the mechanical approach (see the articles in the references for more information), this allows us to identify the acceleration values that bring the player near her/his ASP. We called these ‘smart accelerations’ bursts.

Is there any link between MPEs and bursts? We will discuss this topic in the second part of this article.

Further scientific content on mechanical analysis is available within the gpexe academy for all users (access from the web app footer).

Additional references:

P.E. di Prampero, “Mechanical and Metabolic Power in Accelerated Running-PART I: the 100-m dash” – PubMed
C. Osgnach, “Mechanical and metabolic power in accelerated running-Part II: team sports” – PubMed
C. Osgnach, “How easy is to stumble over acceleration and deceleration?
C. Osgnach, “The limits of acceleration 

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