# The Interaction of Fitness and Fatigue on Performance in Football
## 6 Discussion  
The primary aim of this study was to investigate the interaction between fitness and fatigue by developing a new performance score and analyzing its impact on match physical and tactical performance. The main findings revealed that the performance score indicator effectively discriminated lower physical output during matches and identified potential relationships with team tactical performance. These results underscore the value of invisible monitoring methods in tracking player status throughout the weekly microcycle and highlight the importance of integrating such approaches in elite sports environments to optimize match performance. **Feasibility and usefulness of invisible monitoring in the context of Elite Team Sport**  
This study proposed to use the fitness-fatigue model to objectively quantify performance and its relative effect on match day physical and technical performances. The use of statistical models to explain the relationship between training and performance has existed for a long time [4, 27, 28]. Interestingly, in the context of team sport, the interest of such an approach has been limited due to the so-called multifactorial nature of performance reducing its application to mainly endurance sports (e.g., running, swimming, cycling) where physical performance is central. However, with the use of new technology and advanced statistical analysis, the (re)integration of fitness-fatigue modelling in the context of team sports is now within reach. In the context of this research, such seamless integration within an elite performance environment has been possible with the incorporation of advanced statistical methods allowing continuous measure of multiple constructs (fitness and fatigue). This type of approach should be encouraged as it clearly showed some added value to optimize weekly training periodization in the context of elite football. **The effect on individual physical performance**  
One of the main results of this study showed a clear difference between players with different player status, suggesting that an adequate fitness-fatigue balance has a clear positive effect on individual physical performance. Indeed, the classification of players in the three conditions (optimal, intermediate and poor) changed from match to match. As highlighted in Figure 2, it is difficult during the season to have the entire team always in the optimal state. One interesting finding of this research highlights that when the players were categorized into the poor performance condition, they exhibited the lowest physical performance output, and this was even clearer in the second half, where moderate effects were found. This suggests that the expected transient fatigue state in the second half was more important for this specific category. Research is relatively scarce on this topic, making comparisons with other studies difficult. Indeed, recent literature has questioned the effect of previous physical status on physical performance, requiring performance staff to choose monitoring practices that ensure an optimal player status for weekly scheduling [29]. To the best of our knowledge, this is the first study showing that the balance between fitness and fatigue pre-match influences running performance. From a physiological standpoint, our results are logical; an optimal player status should result in a better physical performance. However, this has never been explicitly shown through original research, which is required in order to demonstrate the construct validity of the present approach. Such findings have clear practical implications. First, it highlights the necessity to implement appropriate monitoring strategies throughout the microcycle to observe potential abnormalities, intervene and ensure an optimal player status before a match. Indeed, the high inter-individual variability in performance scores reinforces such a need.