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  • Daniel Kamenetzky

IS THE GPS AN APPROPRIATE TOOL FOR DECISION MAKING IN SOCCER?

Updated: Jul 12, 2020

The soccer player performance told by a GPS


These devices integrate geolocation technologies (GPS), accelerometers, and often gyroscope and cardiotachometer. These tools, locate players in the field and measure distances traveled, speeds, changes in longitudinal and angular acceleration, and with some models even heart rate. From the integration of the obtained data, new variables are derived to “estimate”, among other things risk of injury, workloads, metabolic expenditure, fatigue, physical capacity, and physiological and mechanical effects of the load.


Although GPS is useful and the measurements are valid and accurate, they are being used to infer physiological and biomechanical data without scientific rigor. Here I present an analysis of data obtained by GPS to show the limitations of the information being used. The team's raw data of this work was provided by the Master in Sports Big Data that I carried out at the Catholic University of Murcia, Spain.

 

What DIRECT measurements do GPS devices, like those used by highly competitive soccer teams, do?

Spatial location: indicates where the player is in the field.

Speed: the time that the player takes to travel a certain distance.

Acceleration (and deceleration): indicates how fast the player can increase or decrease his speed. Basically how much the speed changes in a period of time:

Acceleration = Speed / Time

Another way to express acceleration is through Newton's Second Law of Motion. Relates the mass of a body and the force required to accelerate or decelerate it by a certain amount:

Force = Mass x Acceleration


In this case, the device only derives acceleration. The body mass must be added by the coach (many times it is obtained by survey). The force is obtained from solving the equation. This is one of the most used variables. What you want to obtain is the load that acts on the player throughout the game/training. Since the load is made up of muscular contractions, interactions with the environment (stopping and pushing on the pitch) and interactions with other players (collisions), it is impossible to quantify the magnitude, direction, and point of application of the forces acting on the player at every moment. Furthermore, there is a proportionality between the physiological and mechanical effects in each of the mentioned load components. Each of them generates a different type of "fatigue" in the player.


It is important to keep in mind that most of the variables that the GPS software offers are indices derived (or indirect) from the direct measurements and from the external data that is added by the coach (we do not know what protocols, what tools and with how much expertise each professional measures and generates the added data).

 

MATCH GPS DATA ANALYSIS FROM A PROFESSIONAL EUROPEAN SOCCER TEAM (DAY 4)

The “comments” in each point are the result of comparing three previous games (days 1,2 and 3) with the last one (day 4). They are the conclusion of the numerical analysis only and do not include the video match review.


* Total Distance Traveled

Comments: There is an increase in the total distance traveled by most of the players compared to the previous three games (days 1 to 3).It could be due to increased physical capacity, increased demand for the type of competition on day 4, and/or a change in tactic proposed by the coach.


* Sprints and Peak Speed


Comments: day 4 shows a rise in the number of high-intensity actions compared to the previous days. We could interpret this increment, as in the previous case, as an elevation in physical capacity, a greater effort due to the demand for day 4, and/or a change in tactic proposed by the technical team.

It is interesting to observe in the graph "Proportion of sprint distance from the total distance", that the sprinting actions represent for most players less than 10% of the total distance traveled. It is consistent with the data from the scientific literature, where it is indicated that "high speed sprinting" represents a small percentage of the total running volume in a European first division match (however, it is the physical capacity most trained in soccer!)

* Accelerations and Decelerations


Comments: In the case of the accelerations and decelerations analysis, we also observe significant increments in day 4 compared to the previous three, however, only in some positions such as central, midfielder, and winger. This could indicate greater rival's aggressiveness in attack since it would seem that the defensive systems have been put in a situation of greater pressure than in the previous days. Another option could be a more active first defensive line participation in the attacking game, which could happen if the Head Coach has developed a more aggressive offensive strategy for this day. It is not clear whether this rise in the number of accelerations and decelerations has had an increased fatigue effect.


* High metabolic loads (HML)


Comments: it is usually considered that the "increase in metabolic load" is associated with the rise in the distances traveled at high intensity. If the "high speed" has been achieved without significant increases in lactic acid, compared to the previous days, we would be in the presence of a new level of physiological capacity of each player and the team in general. Otherwise, the game has demanded a greater effort. This would be an important indicator for the coaching staff in order to decide new and more aggressive tactical models. It will be possible thanks to the team's capacity of maintaining higher levels of speed, changes in acceleration and physical contact for longer periods of time. However, the physiological endurance and power depend on the intensity, duration, and frequency of the stimuli. Therefore, without evaluating “directly” the metabolic effect (with measurements of lactic acid or maximum oxygen consumption), we cannot know if the athletes have gained physiological capacity or not. Nor is he/she is fatigued to the point of needing to be replaced.


* Energy expenditure


Comments: "metabolic expenditure" (inferred by the athlete's weight, heart rate, and running speed) and the "fatigue index" have increased despite the fact that the "fatigue/energy expenditure ratio" for day 4 indicates that in most cases it has been maintained or even reduced compared to previous days. This is an area to be evaluated in the laboratory and in training to establish the real load/fatigue ratio and its physiological outcome in order to design training criteria and game tactics. It is therefore impossible, following these indicators, to establish whether the athlete is fatigued and therefore requires replacement or if they are increasing the risk of an injury.

Conclusion The data provided by the GPS indicate that in "day 4" there has been an increase in metabolic load associated with the rise in the total distance traveled, and in the high-intensity actions compared to the previous 3 days. It is clear that the possible interpretations for those changes are various and only the appropriate one can be selected by integrating the GPS data with other performance variables (nutritional, physiological, mechanical, tactical, etc.). Therefore, the data provided by the GPS alone can't tell if: 1. The increase in metabolic expenditure is linked to the tactical proposal of our technical staff or to the tactical proposal of the opposing team. 2. The rise in metabolic expenditure is linked to the fatigue accumulated during the previous days and added to the increased last match load. 3. The recovery and/or pre-match training models have been adapted to the requirements of the latest competition. 4. If the current performance is consistent with an increase in the team's physical capacity (which would allow new tactical options), or a higher metabolic effort caused by the rival team's game proposal. 5. Players have reached a degree of metabolic fatigue such that they must be replaced during the game/training session. 6. The player is in limits of physiological and/or mechanical fatigue with an increased risk of injury.

 

In order to obtain conclusive information from the GPS tool, it is necessary to integrate its data with:

1. Physiological evaluations and thus establish baseline parameters for longitudinal comparison.

2. The physiological recovery evaluation, obtained after each workload (training and matches) through lactate and urea levels, and carbohydrate reserves (pre and post effort)

3. The effect of fatigue on cognitive abilities, through the analysis of the player's decision-making

4. The analysis of running and sprinting techniques to assess the metabolic load associated with mechanical efficiency (and not just running speed).

5. Evaluations of aerobic capacity and lactate removal capacity in order to establish metabolic loads during matches and associated with physiological recovery processes.


Interpretation of the athlete's performance and fatigue is only possible with a multidisciplinary battery of evaluations and integrated data analysis. The design of the sports training (to help the athlete!), and the competition tactics, will increase in efficiency when implementing integrated work models and using each tool according to its specific function. Today's attempts to interpreting complex biological changes with inadequate information are limiting the progress of players, the team, and the coaching staff.

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