In soccer practice, it is well known that a single attack can make the difference between victory and defeat. It is quite common for one team to seemingly dominate the other, failing to take the lead and leaving the other team waiting for a decisive counterattack. To prevent this from happening, most teams station several players behind the ball to safeguard a possible counterattack in the event of a sudden ball loss. This tactical approach is referred to as rest defense. While it is widely acknowledged in soccer practice, there is only scarce research on it. To close this gap, this study aims to provide a definition of rest defense that enables a quantification of success, and to identify variables that distinguish between unsuccessful and successful tactical behavior of rest defense.

While soccer performance can be analyzed in the sub-areas of physical, technical, psychological, and tactical performance (Hohmann, 1983; Weineck, 2007), this study will assess the tactical behavior of soccer players in the outlined match situation.

In particular, the tactical behavior of the players differs depending on the playing phase of the game and their respective goals. Accordingly, the match situations in a game can be divided into four distinct playing phases (offensive play, defensive play, defensive transition, offensive transition) which occur in sequence depending on the course of the game (Hewitt et al., 2016; Navarro, 2018). In offensive play, a team controls the ball with the goal of attacking the opponent’s goal to eventually score. In defensive play, a team organizes itself to prevent the opposing team in ball possession from scoring and, in the best case, to regain the ball. The transition phases describe the switch between offensive and defensive playing phases after losing the ball (defensive transition) or gaining the ball (offensive transition). Consequently, there are differences in the behavior of players depending on the playing phase of the game, which should be taken into account when analyzing tactical performance.

Compared to the other playing phases, offensive play has received the most attention from practitioners and researchers. In detail, studies have predominantly investigated offensive on-the-ball actions such as shots (Lucey et al., 2015; Rathke, 2017; Anzer and Bauer, 2021) or passes (Szczepański and McHale, 2016; Power et al., 2017; Chawla et al., 2017). However, tracking data is particularly valuable for analyzing defensive play, as players' behavior off the ball is recorded and can thus be analyzed. Furthermore, defensive play has been revealed to be at least as important for a team's success as offensive play (Georgievski et al., 2019; Lepschy et al., 2021). Consequently, the number of studies analyzing defensive play has increased (Forcher et al., 2022a). For instance, defensive pressure has been assessed to analyze defensive play (Bojinov and Bornn, 2016; Merckx et al., 2021; Forcher et al., 2022b).

The aforementioned transition playing phases have been shown to be highly important for success in soccer as well. Several studies have indicated that short transitions after a ball gain/ball loss have the highest probability of scoring/conceding a goal (Tenga et al., 2010; Lago-Ballesteros, Lago-Peñas and Rey, 2012; Gonzalez-Rodenas et al., 2015). One reason for these effects is the increased vulnerability of an unbalanced defense after losing the ball (Tenga et al., 2010; Gonzalez-Rodenas et al., 2015). In contrast, research on the defensive transition playing phase is still scarce and will therefore be the focus of this study.

Defensive transition describes the behavior of players after losing the ball to restore the defensive organization and thus reach the defensive phase of the game (Bauer and Anzer, 2021). The main goal of this playing phase is to prevent an opponent’s counterattack by securing dangerous pitch areas and, if possible, directly regain the ball by increasing the defensive pressure in ball proximity (DFB-Akademie, 2022). While the defensive behavior of high pressure in close ball proximity after a ball loss is known as counter-pressing (Navarro, 2018; Bauer and Anzer, 2021), the behavior of players behind the ball protecting dangerous pitch areas (despite being the team in ball possession) is often referred to as rest defense (DFB-Akademie, 2022).

As stated earlier, there has been little research on defensive transition in soccer. Nevertheless, two studies examined defensive transitions in general (Vogelbein, Nopp and Hökelmann, 2014; Casal et al., 2016), and one study investigated the specific group tactical behavior of counter-pressing (Bauer and Anzer, 2021). Vogelbein et al. (2014) analyzed the time it took for a defending team to recover a ball (so-called defensive reaction time) in 306 German Bundesliga matches. They found that top teams recovered the ball faster in defensive transition compared to other teams. Similarly, Casal et al. (2016) showed that the duration of the defensive transition is a valuable key performance indicator for defensive transition. In detail, they predicted the success of the defensive transitions of eight matches of FIFA World Cup 2010 using a notational approach (Casal et al., 2016). Concluding, while these studies provide some initial approaches to analyze defensive transitions, their methodological approach does not allow for a detailed analysis of the tactical behavior of defending players (e.g. specific pressing behavior of defending players).

In contrast, approaching defensive transition in more detail, Bauer and Anzer (2021) automatically identified counter-pressing situations and identified crucial variables for the effectiveness of defensive transitions by analyzing over 4000 matches of the German Bundesliga. For instance, their results revealed that having four or more players behind the ball is important to the success of a defensive transition. This study is a great example of the use of player tracking data to get insights into the tactical behavior of players during the defensive transition phase and on the specific group-level counter-pressing in areas close to the ball.

Overall, the detailed knowledge of player behavior in defensive transition is still limited. Especially, none of the abovementioned studies analyzed the group tactical behavior of the players that are not in ball proximity with the task to protect their own goal by controlling dangerous areas and deny or delay counterattacks. As mentioned above, this tactical principle can be referred to as rest defense in practice. However, there is no scientific and generally accepted definition of rest defense in soccer.

Overall, the tactical behavior of soccer players can be analyzed at different levels according to the number of players involved, ranging from individual (1 player), over group (minimum of 2 players), to team level (11 players) analyses. While several studies have indicated that group level tactical behavior seems to be more insightful compared to team level analyses (Goes, 2023), few is known about the tactical behavior of subgroups of players in the defending playing phases (Forcher et al., 2022a).

Accordingly, the aims of this study are (i) to define the tactical behavior referred to as rest defense and (ii) to identify crucial variables (tactical measures) that are important for the success of the defensive transition phase regarding rest defense.

To achieve this goal, we will use a mixed-methods approach via interviews and an observational study. Using expert interviews, we aim to establish a clear definition of the term rest defense and the variables that characterize it. In a second step, we will use this information to analyze data of professional soccer matches to identify the determinants of successful rest defense using state-of-the-art analytics (tracking data and machine learning).

The aim of this study was twofold. First, we conducted expert interviews with professional soccer coaches to define the tactical behavior rest defense to establish a working definition that enables further performance analysis. Second, based on the gained knowledge of the interviews, we developed a data analysis to identify critical variables that are important to the success of rest defense.

In employing a mixed-methods research design that integrates both qualitative and quantitative approaches, this study aims to elucidate the conditions that underlie tactical behavior in soccer. Initial qualitative insights were garnered through interviews with subject matter experts, and these data were subsequently employed to inform a nuanced quantitative analysis. The analytical framework was designed to evaluate the success metrics associated to rest defense in the playing phase of defensive transition.

This detailed information gained by the qualitative expert interviews was applied to the quantitative analysis. To illustrate its quality, we provide some general information. The approach to detect rest defense situations identified players with the same tactical positions as those named by the experts as being involved in rest defense, as indicated by a high degree of agreement (93% defensive line & central midfielder). Accordingly, this approach using the area of rest defense seems to be valid for identifying rest defense situations and rest defending players.

The machine learning approach to predict the success of defensive transitions using information about rest defending players showed satisfactory predictive performance. Our final model (AdaBoost [excluding distance variables]) outperformed the previous approach to model the success of defensive transition by Casal et al. (2016), which showed an accuracy of 0.58 (our model: 0.97). In comparison to Bauer and Anzer’s prediction of defensive transition success, our model showed a comparable prediction performance (Precision: 0.72 [our model: 0.73], f1-Score: 0.67 [our model: 0.64]), however, poorer performance in distinguishing between the classes (AUC: 0.87 [our model: 0.60]). Their analysis included information of all players involved in defensive transition, also the behavior of defending players in ball proximity. Recent studies on defensive behavior in soccer showed that especially the behavior of players in close ball proximity is important for defensive success (Forcher et al., 2022b). In contrast, our study used predominantly information about the rest defending players who are not in ball proximity in the moment of a ball loss. Therefore, this study especially indicates the importance of players in rest defense (not neccessarily in ball proximity) for the defensive success in defensive transition.

After demonstrating the quality of our approach, the prediction model is discussed by evaluating each variable according to its value for the prediction. The most important variable in the prediction of rest defense situations during defensive transition was the duration of opposing counterattacks. The analysis of SHAP values (see Figure 2) suggests that a decrease in the duration of counterattacks increases the probability of successful rest defense. This finding is supported by the results of Casal et al. (2016) and Vogelbein et al. (2014), both of which found that the time to recover the ball in defensive transition is an important indicator of successful performance. Furthermore, Bauer and Anzer (Bauer and Anzer, 2021) indicated that the chance of conceding a goal is greatly increased if the ball is not regained within five seconds. Overall, this supports the conclusion that regaining the ball quickly increases the success of the defensive transition by giving the counterattacking team less opportunity to build their attack and deny their actions early.

Moreover, the prediction model yielded further insights that support the conclusion that duration is a critical success factor for defensive transition. In detail, the number of actions and the number of passes of the opposing counterattack were also highly important for the prediction (2^nd and 3^rd most important variables, see Figure 2). The identified distributions of both variables are comparable to other investigations, which indicates a higher generality of our findings. Specifically, we showed that on average an opposing counterattack that started in the defensive third had between 3-4 passes and 5-6 actions. Comparably, several other studies (of World Cup 2002 & Champion’s League 2014/15) found that successful counterattacks (ending in a scoring opportunity or a goal) required about 4-8 actions (especially when starting from their own defensive third) (Fleig and Hughes, 2004; Hughes and Lovell, 2019). While the variable number of actions showed a similar distribution as the duration of the counterattack (fewer opposing actions increase the probability of success of rest defense), the number of passes showed an opposite trend. This opposing trend could be due to high correlation between the variables pass number, number of actions, and the duration of the counterattacks which could have influenced the results of the prediction model. To sum up, the time it takes a defending team to recover the ball after a ball loss is a crucial success factor in defensive transition.

Besides, the height of the rest defense was the 4^th most important variable for the prediction of rest defense success. With it, the distribution of SHAP values in Figure 2 suggests that a deeper rest defense (closer to the own goal line) is beneficial for the success of rest defense. A possible reason for this is the smaller space behind the defense, which decreases the chance for an opposing counterattack to play in behind this deep rest defense. Following this idea, the deep spaces could be better secured by the rest defending players. This is in line with the stated primary goals of rest defense by experts, to safe deep spaces (see Figure 1: Goal of rest defense). In this context, Castellano and Casamichana (2015) showed that better teams in Spanish La Liga positioned their defensive-line closer to their own goal. However, they did not differentiate their analysis according to the playing phase and therefore, comparison to the current results is hardly possible. Moreover, this is an interesting finding when comparing the stated ideas to the group tactic of counter-pressing after a ball loss (in defensive transition). If a team aims to counter-press deep in the opposing attacking third after a ball loss one could argue that it is helpful that the last line is also high up the pitch to decrease the space for the counterattacking team to play and increase the pressure on the opponents. However, our result suggests kind of an opposing trend where it seems to be helpful for the success of defensive transition that the rest defending players are closer to their own goal. This might be a perfect example of risk and reward in a particular game phase in soccer, where opposite tactical behaviors can have both benefit and risk depending on the goal to be achieved (e.g. defensive line moved high up the pitch to enhance counter-pressing, which is assumed to decrease the performance of rest defense). Summing up, the effects of a deeper rest defense enhancing the success in defensive transition should be analyzed in the combination of counter-pressing in ball proximity.

Furthermore, the variable numerical superiority was the 5^th most important variable in our prediction with a higher defensive numerical superiority suggesting an increase in the success of rest defense. This finding is supported by the results of Bauer and Anzer (2021) who indicated that it is beneficial for the success in defensive transition when four or more players are behind the ball after a ball loss. This seems intuitive, as more defenders can more easily control possible counterattacking players and dangerous spaces (e.g. deep spaces). Similarly, in the analysis of the playing phase of defensive play, it has been repeatedly shown that numerical superiority is crucial for defensive success (Gréhaigne et al., 2002; Forcher et al., 2023a). On average, we found a defensive majority of + 1.7 (± 1) defenders in the rest defense area, which is comparable to the findings of Vilar et al. (2013) who identified +1 player superiorities in the center-back sub-areas of the defense. While these areas can be interpreted as the rest defense area this study also focused on the playing phase of defensive play. With the present study design, no conclusions can be drawn to the tactical positioning of rest defending players (e.g. flat or sandwich, see expert interview: Tactics of rest defense). However, the variables number of defenders and attackers are less important for the prediction as they are partly already mapped in the superiority measure.

The 6^th most important variable for defensive success in rest defense was the ratio of space control of attacking players with respect to the defenders in the area of rest defense to the defending team’s goal line (see Figure 1). In detail, the SHAP values indicate that a low space control of the attacking team enhances defensive success in rest defense. In this regard, numerous studies have investigated measures of space control to analyze tactical match performance in soccer (Filetti et al., 2017; Rein et al., 2017; Fernández et al., 2018; Low et al., 2021). For instance, Rein et al. (2017) found that passes causing a high space control gain in the attacking third are connected to goal scoring. Interpreted conversely, a small space control in front of the own goal is beneficial for defensive success which supports the current findings. By denying space control of the attackers, the defenders control dangerous counterattackers and deep spaces. Therefore, the metric space control appears to optimally quantify the idea of controlling deep spaces and areas around counterattackers. Both principles were stated to be highly important goals for experts in rest defense (see expert interview: Goals of rest defense). However, the results of outliers with extremely high space control of attackers seem to be counterintuitive, suggesting a positive effect on rest defense success (see SHAP values in Figure 2). A possible explanation for this pattern could be counterattackers standing behind the rest defenders (in offside), which would increase the space control tremendously, but decrease the chance of a successful counterattack as they cannot legally intervene in the game situation.

The spatial formation of the rest defense was partially important for the prediction of success in rest defense (7^th - 10^th most important variables for prediction, see Figure 2). The trend of SHAP values indicates that lower values of spread, surface area, length, and width of rest defending players (higher compactness) are advantageous for success of rest defense. In this context, there have been several studies analyzing the spatial organization (e.g. using surface area or spread) of the whole defending team (Moura et al., 2012; Bartlett et al., 2012; Castellano et al., 2013; Clemente et al., 2013; Welch et al., 2021). While it remains questionable whether higher compactness of the whole defending team is beneficial for defensive success, it was shown that a higher compactness in areas close to the ball could be an important success factor in defense (Forcher et al., 2023b). However, those studies analyzed the playing phase of defensive play and did not focus on subgroups of players that are not in direct ball proximity (such as rest defense) which makes a direct conclusion to the current results difficult.

Finally, defensive pressure showed only a weak predictive performance for rest defense success in comparison to the other metrics in the present study. The identified mean pressure of about 7 [%] is smaller compared to other findings on defensive pressure (11-20 [%] in the playing phase of defensive play (Forcher et al., 2022b), or 8-30 [%] on pass receivers irrespective of the playing phase (Herold et al., 2022)). In this context, it was shown that the defensive pressure decreases in areas further away from the ball (Forcher et al., 2022b). This is especially the case in the current analysis of the tactical behavior of players in the rest defense who are specifically not in ball proximity. In contrast to our findings, defensive pressure was previously shown to be a good indicator for defensive success (Forcher et al., 2022b). This could be due to the other variables (such as space control) that might represent the metric of defensive pressure with other measurement approaches (e.g. high defensive pressure possibly results from large space control). However, higher pressure on the attackers (to deny their actions) in the rest defense area (see Figure 1) seems to increase the probability of a successful transition.

This study showed how to combine qualitative and quantitative research in soccer, and how to use expert knowledge to enhance an up-to-date analysis of tactical behavior. With it, we presented practically important knowledge about how to behave in rest defense. Concluding, rest defense is defined as behavior of the deepest defenders during ball possession with the goal to prevent an opposing counterattack after a ball loss during defensive transition. Our results suggest that rest defending players should control deep spaces and dangerous counterattackers to successfully prevent a fast opposing counterattack. This could allow the defensive team to regain possession as quickly as possible in defensive transition to stop an opponent’s counterattack in the early stages, which was shown to be most important for success in defensive transition.