Speed zones vs acceleration events vs high power events

Player tracking technology has given the coach the opportunity to quantify players’ activity during training sessions and official matches. At the end of physical work it’s quite easy to get in a few minutes the average values of the main parameters:

• Total time
• Total distance
• Total energy expenditure (or total equivalent distance)
• Average speed
• Average metabolic power

Average values, although interesting, can give only “general” information on what the athlete did on the pitch and do not help the coach to analyze the players’ effort. For this reason (and also to get some “coach-friendly data”) there is an increasing interest in describing high intensity efforts.

High-intensity activities can be identified and summarized as follows:

1. Choose a parameter (speed, acceleration, deceleration, metabolic power, …).
2. Set a threshold.
3. Calculate the work done above the threshold.
4. Describe each action above the threshold.

The aim of the paragraphs that follow is to analyze advantages and limits in the use of different parameters to arrive at a complete picture of the best criteria to describe high-intensity efforts.

Speed zones

During these five minutes of an official match, the athlete runs 684 meters at an average speed of 8.21 km⋅h-1 . The time course of the speed is reported in figure 1 where we set a speed threshold of 19 km⋅h-1. We can easily detect the time windows in which the speed exceeded this threshold and summarize the total activity above it.

Figure 1: time course of the speed during 5 minutes of an official match.

Table 1: overall time, distance and energy above the 19 km⋅h-1 speed threshold.

Table 1 is simply the sum of all actions (event) in which the speed exceeded 19 km⋅h^-1. For example, in the event #1 (see figure 1) the athlete ran for 14 meters at a speed above the threshold with a corresponding energy expenditure of 64 J⋅kg^-1 (figure 2-left and table 2).

Figure 2-left: high speed event #1 corresponds to the area highlighted in dark red (in which the speed exceeded 19 km⋅h-1).
Figure 2-right: the acceleration phase of event #1 corresponds to the area highlighted in green.

This representation is legitimate, however, it leaves the following basic questions unanswered:

• How does the athlete achieve a high speed?
• Is it correct to define “high intensity” only the activity performed above a speed threshold?
• Which is the rule to set the speed threshold?

The answer to the first two questions is contained in the following paragraph (“acceleration”) whereas the response to the last question… is rather difficult! Indeed, it’s hard to find convincing reasons helping us to make a decision because apparently there isn’t a specific speed above which something happens, and also, it makes little sense to use the same threshold for athletes with different abilities in expressing high-speed.

Acceleration

It’s well known that accelerated running (essential to increase the athlete’s kinetic energy) is a very expensive activity both from a metabolic and a muscular point of view. Many team sports, such as soccer, are carried out by repetitive changes of speed and, even if the athlete may not reach high speeds, the impact of the resulting accelerations/decelerations can’t be neglected. As previously mentioned, the energy cost during acceleration can increase considerably as compared to constant speed running; therefore, the effect of considering “high intensity” only high-speed running is simplistic. Inspection of figure 2-right and table 2 makes it easy to understand how things are going: the acceleration phase has a double energy expenditure as compared to the phase of high speed running (101 vs. 64 J⋅kg-1) even if the athlete covered a lesser distance (11 vs. 14 m).

Table 2: details of event #1; red high speed phase; green acceleration.

However, even when trying to detect “high intensity” via acceleration, the scenario is similar to the previous one: how can we choose a meaningful acceleration threshold?

Figure 3: time course of the acceleration during 5 minutes of an official match; three acceleration events are detected setting a threshold to 2.5 m⋅s-2.

In figure 3, we set an acceleration threshold to 2.5 m⋅s-2 and only 3 of the 7 high-speed events were detected. Indeed, sometimes a high-speed can be reached thanks to a high acceleration (or rather, an acceleration higher than the selected threshold) and sometimes thanks to more moderate and longer acceleration… but everything would change simply by choosing a lower threshold! Of course, once again, it’s not easy to set an acceleration threshold with a specific meaning and above all, there is no point to use the same threshold for players with different muscle power characteristics.

A final consideration on this topic: whereas we generally use relative units when dealing with endurance activities (% of maximal heart rate, % of maximal oxygen consumption, …), it seems funny to base our analyses of high intensity speeds or accelerations, on the fixed threshold. Indeed, consider as an example the individual force-velocity-power profile of a maximal sprint (figure 4) [3]; it turns out that:

1. It’s an individual profile based on the maximal power abilities of the athlete.
2. The acceleration is strictly linked to the speed; in this example, an acceleration of 2.5 m⋅s-2 represent a maximal effort if the corresponding speed is approximately 5 m⋅s-1 but it’s only 60% of the maximum if the corresponding speed is 2 m⋅s-1.

Figure 4: individual F-V-P profile during a maximal sprint [3].

Metabolic power

Figure 5: time course of metabolic power (red line) and oxygen consumption (blu line). The horizontal grey line, corresponding to 18 W⋅kg-1, represents the VO2max of the athlete.

Recently a new approach has emerged that incorporate both speed and acceleration to determine metabolic power during accelerated running [1]. Metabolic power, which is the product of the instantaneous velocity and the corresponding energy cost, is a measure of the overall amount of energy required, per unit of time, to reconstitute the ATP utilized for work performance. It can be easily calculated at any given moment and any speed changes have a 2-fold effect: on one hand, because of the direct role of speed in setting metabolic power, and on the other, because the resulting acceleration increases (or decreases) the energy cost in proportion to the amplitude of acceleration (or deceleration).

1. The time course of the metabolic power is reported in figure 5 (red line) in which we can also plot the time course of estimated oxygen consumption (blu line) [2] and the metabolic power corresponding to the maximal oxygen uptake of the athlete (grey line). This representation has many advantages:
2. The threshold that identifies the high intensity is not a “random number” but it is the maximal aerobic power of the athlete and it can be set once we’ve measured it; therefore, every time that the metabolic power exceeds the VO2max of the athlete anaerobic energy is definitely required.
3. For each contribution of the anaerobic sources (not only when the metabolic power is higher than VO2max but also when it exceeds the actual oxygen consumption) we can detect “expensive energy event” which represents a critical phase because an oxygen debt is contracted and, at some point, it must be repaid.
   The so defined power events can be described in detail through both a general (table 3) or an analytical summary (figure 6).

Table 3: general summary of high metabolic power events.

Figure 6: analytical summary of high power events described by duration (x-axis) and distance (y-axis).

This representation is very handy for the coach because it allows him to understand exactly how the athlete obtains his average metabolic power during the match or the training session. Indeed, high metabolic power can be the result of several events, longer event duration, a higher metabolic power of the events, shorter recovery duration and higher metabolic power during the recovery: all these variables can concur to increase the total energy expenditure.

Figure 7: a high power event occurring during the high speed phase corresponding to event #1 of figure 5.

From table 2, if we merge the high speed phase and the acceleration phase, we get an overall event of 25 m with an energy expenditure of 165 J⋅kg^-1. We would like to stress that, when this same high intensity event is analyzed on the basis of the metabolic power, instead as separately from speed and acceleration (see figure 7), the obtained result is essentially equal (24 m with an energy expenditure of 153 J⋅kg ^-1). This strongly supports the use of metabolic power as a comprehensive high intensity index.

There are other evaluations concerning the energy balance of the selected period (aerobic, anaerobic alactic and anaerobic lactic energy) as well as the instantaneous availability of ATP-CP storages that could be monitored by the live application… but this is another story!

Conclusions

Whilst further improvements are needed to describe high intensity in team sports, metabolic power is one of the most convincing parameters that coaches can use to summarize the effort of their players. Speed and acceleration, although able to provide some information, have several weaknesses: the standard to set a critical threshold is not clear, furthermore, these thresholds fail to comprehensively isolate the high intensity activities. The metabolic power approach appears the best solution for managing workloads and, since it is based on energy expenditure estimates, it seems to be also suitable for assessing the balance between work and recovery.

However, besides the metabolic effort, the players have to “bear” also another type of “burden”: the mechanical load of the active muscle mass. This topic deserves a special analysis since it’s as important as the metabolic approach. Unfortunately, we have to postpone this argument to a future episode!

References

[1] di Prampero PE, Fusi S, Sepulcri L, Morin JB, Belli A, Antonutto G. Sprint running: a new energetic approach. J Exp Biol 2005; 208: 2809-2816

[2] di Prampero PE, Botter A, Osgnach C. The energy cost of sprint running and the role of metabolic power in setting top performances. Eur J Appl Physiol 2015; 115: 451-469

[3] Samozino P, Rabita G, Dorel S, Slawinski J, Peyrot N, Saez de Villarreal E, Morin JB. A simple method for measuring power, force, velocity properties, and mechanical effectiveness in sprint running. Scand J Sci Med Sports 2015; [article first published online: 21 May 2015; DOI: 10.1111/sms.12490]

Author: Cristian Osgnach