Metabolic power: is it all so simple? (part 2)
The study our group published back in 2010 reported the results obtained with the adoption of Di Prampero’s et al method (2005). On that occasion data were collected from 56 matches of the Italian Serie A (First Division) and 399 subjects were involved in the research. At that time, due to the great number of players that took part in the study, it wasn’t feasible to outline the aerobic characteristics of every single athlete. Thus, as far as the choice of power categories was concerned, it was decided to set 20 W/kg as a reasonable reference value for the “average” top-level soccer player and consider it as an important parameter to express the concept of anaerobic energy.
Anaerobic energy plays a key role in terms of individual performance analysis since it establishes a correlation between the energy demand of the exercise and the personal characteristics of every single athlete. This is an analysis that not only takes into account mechanical variables (i.e. a certain speed profile and the number and type of accelerations and decelerations) but contributes also to relate them to the energy effects they tend to induce. The evaluation of each subject’s personal characteristics may be very helpful to investigate the role of aerobic and anaerobic mechanisms, a key factor which is extremely important to fully understand the effects of the training drill and their relevance to every single player (a topic we’re going to deal with in the following article).
Why are aerobic characteristics so crucial?
Let’s take for instance two subjects with very different aerobic characteristics performing the same exercise in a friendly game:
- subject A reaches a maximal aerobic power of 16 W/kg – VO2max ≈ 46 ml O2/(kg min) *
- subject B reaches a maximal aerobic power of 24 W/kg – VO2max ≈ 69 ml O2/(kg min) * (* above rest)
Data highlighted in green did not take into account the personal aerobic characteristics of each athlete whereas those in red are the expression of a detailed analysis which was heavily focused on the aerobic profile of the subjects. The example clearly highlights the major weakness of our 2010 study. On that occasion, indeed, we chose to set 20 W/kg as the value of maximal aerobic power and applied it to each player. Such an approach, though, allowed us to fix an average reference value which could be applied to a considerable number of athletes but was not detailed enough to give appropriate feedback on the performance of every single athlete.
Two sides of the same coin
Data in green reported in the table provide information about the effort put by athletes during a session and are not connected with the individual fitness level of each subject. The most significant ones are the following:
- total energy: the value that assesses the total energy expenditure induced with the training. The time variable might cause an increase of this value and, within the same time frame, differences among players performing the same exercise are linked to each athlete’s playing style or the kind of exercise proposed. It’s often associated with equivalent distance that is the distance that a player would have covered at constant speed using all the energy consumed during the exercise performance.
- EDI (Equivalent Distance Index): the difference expressed as a percentage between equivalent distance and the actual distance covered; the higher this percentage value, the greater the energy expenditure due to speed variations. Evidence showed that the energy cost measured in accelerated or decelerated running is higher than that developed during running at constant speed. Thus, certain exercises (i.e. a set of repetitions covered on mid-long distances and performed at constant speed) are not going to affect the index whereas other sessions may significantly stress the performance and lead to an increase of the value.
- average power: a parameter taken into account and regarded by many as the most interesting feature of the energy approach in the performance analysis of the soccer player. A value that somehow summarizes data concerning the performance of each athlete, but, at the same time, risks not to take into account fundamental information that is of paramount importance in accurately evaluating the kind of exercise drill proposed. As a matter of fact, a high average metabolic power value may be detected both in exercises performed at constant and high speed, as well as in high-intensity exercises carried out in reduced spaces and thus characterized by mid-low speed and frequent acceleration/deceleration phases.
Data in red, on the contrary, are closely connected to the evaluation of the aerobic fitness of each player. The example illustrated in the table serves to highlight the differences between subject A and subject B in terms of aerobic performance.
For the sake of simplicity, we deliberately chose to ignore oxygen consumption in accelerated and decelerated phases and decided to tackle the issue separately in a subsequent article.
Parameters closely intertwined with each athlete’s aerobic fitness are reported below:
- anaerobic energy: it assesses the energy expenditure that the subject needs to face in order to enhance his/her aerobic potential, i.e. the energy created by the anaerobic metabolism and exploited by the player.
AI (anaerobic index): the ratio between the energy expenditure above a certain metabolic power threshold and the overall energy expenditure. Evidence showed that the higher the ratio, the more stressing the performance of the player. As a matter of fact, the higher the energy threshold of anaerobic origin, the higher the oxygen debt contraction.
- total duration of events: a value that seems to increase with low-intensity events which create a small oxygen debt and decreases with high-intensity events which result in consequent high oxygen debt.
- total number of high-intensity events(> VO2max): a value which does not take into account the occasions in which the player exceeds an arbitrary threshold selected out of study reasons, i.e. when the athlete beats the speed of 25 km/h, exceeds a 2 m/s2 acceleration, or his/her cardiac frequency is higher than 180 bpm. On the contrary, it does express the frequency with which the subject has exceeded the maximal aerobic power. As illustrated in the table above, dissimilarities between the subjects are to be detected due to a substantial difference in terms of aerobic characteristics.
- average recovery time: the time interval measured between high-intensity events. A very important parameter, since it allows the coach to evaluate the time an athlete needs to pay off the oxygen debt owed to the performance. The lower the interval, the greater the number of high-intensity events.
- average recovery power: a value which expresses the way the athlete deals with recovery phases after high-intensity events. The higher the parameter, the higher the intensity the subject develops when recovering.
The approach we developed in 2010 was fairly appreciated because of its reliability in the evaluation of acceleration phases: as stated before, assessing metabolic power is very helpful to explain the changeability of energy cost in soccer, a sport characterized by the constant alternation between acceleration and deceleration phases.
As we’re going to see in details in following articles, the analysis can be furthermore improved by establishing a correlation between the individual performance of each subject and his/her peculiar characteristics.
See more at “Metabolic Power Performance Analysis”.