New Coach, New Risks? Injury Trends in Elite Football After Coaching Changes During an Eight-Season Period

Abstract

Although the effect of coach turnover is often discussed within the football community, there is a very limited body of knowledge on this topic. This study aimed to explore which coaching turnovers are associated with changes in injury incidence in an elite professional football club. A longitudinal study observed injury occurrence across the eight-season period, with the team experiencing 16 coaching changes, averaging 2 per season. All injuries were continuously monitored regularly recorded and saved in the club’s database. They were analyzed over three-time frames: for each season, 2 weeks after the coach was sacked, and also for 4 weeks after the coaching change. A paired sample t-test was used separately for the overall injuries and specifically for muscle injuries in the observed time frames. Overall injuries had an increase of 27.7% and 35.4% in the two and four weeks after coaching turnovers, respectively, while for muscle injuries, these rates were lower and amounted to 5.5% and 8.1%. There were no statistically significant changes, but a medium effect size was reported when comparing overall injuries in 4 weeks and the season in general. Muscle injuries appear to be less negatively affected by coaching changes. Club coaching and medical staff should be especially careful in controlling the training load and recovery techniques in the transition periods while also maintaining the club’s preventive methodology.

1. Introduction

Football (also referred to as soccer) is among the most popular and widely played sports globally. It is characterized by a complex and intense physical style of play that includes technical and tactical elements performed in teamwork and confrontation with the opponent [1]. It is also associated with a significant risk of injury at all levels of competition due to highly intensive and repetitive changes in directions, accelerations, decelerations, and other football-specific movements [2,3]. In professional football, injuries represent one of the biggest issues as they directly affect a player’s health and the team’s success and finances [4]. Mainly for these reasons, football injuries have been extensively investigated in recent times, and clubs and governing bodies are investing significant amounts of money in injury prevention. Depending on the sample, studies report from 5 to more than 10 injuries per 1000 h of football exposure (i.e., training and matches) [5,6]. There is a wide range of impact on different parts of the body, with the most commonly affected regions being the thigh, knee, and ankle [7]. Muscle injuries, particularly in the hamstring muscle group, are the most commonly injured tissue [6]. However, despite the aforementioned investments and advances in training technology and medical diagnostics and treatments, there has been no significant reduction in injuries in the recent period [8].

In addition to epidemiological studies, an important scientific contribution to injury prevention is the identification of risk factors. The occurrence of injury in football is influenced by numerous internal and external factors that interact in a complex manner. Intrinsic factors include previous injuries, age, gender, anthropometric characteristics (e.g., body mass index, limb asymmetry), and aerobic fitness levels [9,10]. On the other side, the environment and organizational system within which the player performs represent the origin of the external factors. This includes training load, match congestion, climate conditions (e.g., temperature, humidity), travel-related fatigue, and variability in pitch quality [9,11,12]. One notable external component is load control, particularly when changes to external factors occur. Often, intra-team leadership and structural shifts like the head coach being replaced tend to bring drastic changes to the team’s training approach, tactical requirements, and psychological focus, which can be very demanding [13,14,15,16,17]. While many studies aim to quantify load-dependent variables, very few focus on how organizational oscillations like changes in team personnel can affect injury occurrence. Therefore, coaching turnovers represent a potentially important yet overlooked externally dependent risk factor in professional football.

Although there is a perception in the football environment that a new coach means changes in the training method and an increased number of injuries, there are only a few studies that have attempted to investigate this. The study conducted on a sample of Turkish Super League football teams during one season explored whether coach dismissal may be related to muscle injuries within 1 month after the new coach takes over [18]. Results showed that the injury incidence increased from a season average of 2.3 muscle injuries per 1000 h of exposure to 5.3 in the 2 weeks after and decreased to 4.5 after 4 weeks after the coach’s dismissal [18]. Similarly, one of the recent studies within the UEFA Elite Club Injury Study project evaluated whether a change in head coach or other head staff before or during a season is correlated to hamstring injuries in male elite-level football in Europe [2]. The main findings highlighted that all staff changes, except for the change in the head coach, indicate an association with an increase in hamstring injury burden from 10% to 81%, while only changes in the fitness coach and team doctor were statistically significant [2].

Although the effect of coach turnover is often discussed within the football community, there is a very limited body of knowledge on this topic. Moreover, prior studies that have investigated how such transitions alter a player’s injury rate have focused on limited types of injury, in particular, muscle injuries or even more specifically, targeting just one muscle group. Hence, the broader impact of coaching changes on overall injury incidence needs to be analyzed. Therefore, the primary aim of this study was to explore to what extent are coaching turnovers associated with changes in injury incidence in an elite professional football club during the eight-season period. This study addresses current gaps in knowledge by analyzing both overall and muscle-specific injuries. Also, the longitudinal design in a consistent environment provides a valuable opportunity to explore how in-season coaches’ turnovers affect injury trends.

2. Materials and Methods

2.1. Study Design

This longitudinal study observed injury occurrence over eight competitive seasons, from 2016/2017 to 2023/2024, in an elite-level senior professional football team competing in the Croatian national top tier. All injuries were continuously monitored regularly recorded and saved in the club’s database by the club’s medical staff. All participants were fully informed about the nature and the aim of the study and signed an informed consent form agreeing to participate. The Faculty of Kinesiology’s Ethics Committee gave its approval for this study, which was carried out in compliance with the Declaration of Helsinki (Ethical Approval Number: 2181-205-02-05-23-0007) [19].

2.2. Participants

The study included all players who participated in the club’s first team during the observed 8-year period (N = 148, 24.67 ± 4.51 average age years old), and a total of 662 injuries were recorded. All participants had at least 7 years of specific football training and historical injury data were not used in this study. The inclusion criteria for players were that they had a professional contract with the club and that they spent at least one half-season fully with the senior team. The exclusion criteria were academy players with only occasional training participation. Players of all positions, including goalkeepers, were analyzed. In the observed period, the team played in 36 league matches, from 2 to 5 cup matches, and between 2 and 8 matches in European cups each season. Each season included summer and winter preparations of four to five weeks, a competitive part of the season for an average of 5 training sessions per week, and two breaks of two to three weeks, in summer and winter. The exception was the “Covid season” where due to the general lockdown in March and April 2020, all training was stopped for 4 weeks, and after that, training was continued individually or in smaller groups for another 3 weeks.

2.3. Variables

Club doctors recorded all players’ injuries daily and described them in detail according to the consensus statement of the FIFA Medical Assessment and Research Centre on injury definitions and data collection procedures in studies of football injuries [20]. The injury was classified as “any physical complaint resulting from football activity that leads to the player being unable to fully participate in future training or matches”. For this study, injuries were categorized into two groups: all injuries (including contact and non-contact) and muscle injuries. Overall injuries refer to all injuries leading to time loss, regardless of cause or type (contact and non-contact). Muscle injuries were defined as functional (DOMS) or structural (strains and ruptures), diagnosed by clinical examination, and recorded according to FIFA’s classification system. Exposure time was calculated for each season and period after coaching turnovers, and the injury incidence is shown as the mean number of injuries per 1000 h of football exposure, including both training and matches. The dependent variable was injury incidence per 1000 h, while the independent variables included periods (2-week periods, 4-week periods, a season). Contaminant variables such as player rotation and underreporting were acknowledged as potential limitations. Injuries were analyzed over three time frames: the number of injuries per 1000 h of exposure for each season; the number of injuries per 1000 h of exposure 2 weeks after the coach was sacked and the new coach arrived, and also for 4 weeks after the coaching change. Acute and subacute post-transition phases were represented by the 2-week and 4-week windows, respectively, where load and psychological stress are likely to peak. Also, only a similar study that explored these effects used the same time frames [18]. Coaches usually took full responsibility within 1–2 days of hiring and trained the team daily during these windows. During the observed period, a total of 16 coach transitions occurred.

2.4. Statistics

Statistical methods included descriptive parameters shown as the mean number of injuries per 1000 h of exposure for each observed period, with 95% confidence intervals and percentage changes between periods of coaching transition (i.e., two- and four-week periods) and seasonal averages. The normality of data distributions was assessed using the Shapiro–Wilk test before conducting paired sample t-tests. To compare these periods, a paired sample t-test was used separately for the overall injuries and specifically for the muscle injuries. All calculations were performed with a level of significance set at p < 0.05. To determine the magnitude of differences, Cohen’s d-effect size was calculated and interpreted as small (0.2), medium (0.5), and large (0.8) [21]. Past^® software (5.2.1 Version) was used for all calculations [22].

3. Results

The overall and muscle injury incidence for all eight observed seasons is presented in

Table 1. Injury incidence per 1000 h of exposure with 95% confidence intervals for all and muscle injuries across eight seasons.

Season	Overall Injuries	Muscle Injuries
2016/2017	17.935 (17.833–18.036)	5.978 (5.919–6.037)
2017/2018	14.265 (14.175–14.355)	6.029 (5.971–6.088)
2018/2019	11.534 (11.453–11.616)	4.194 (4.145–4.243)
2019/2020	9.208 (9.140–9.277)	3.469 (3.427–3.512)
2020/2021	8.036 (7.960–8.114)	4.592 (4.534–4.650)
2021/2022	15.637 (15.54–15.736)	5.584 (5.526–5.643)
2022/2023	13.182 (13.094–13.270)	6.06 (6.001–6.12)
2023/2024	10.883 (10.803–10.962)	5.743 (5.685–5.801)

with an average number of injuries per 1000 h of football-specific exposure with 95% confidence intervals. The results suggest an overall downward trend from the first observed season (17.935 injuries/1000 h in 2026/2017) until the COVID-19 period (8.036 injuries/1000 h in the 2020/2021 season) with a sudden spike in the following season (15.637 injuries/1000 h). Muscle injuries show fewer oscillations during the observed period, with the lowest rate of 3.469 injuries/1000 h in the 2019/2020 season and 6.06 injuries/1000 h in the 2022/2023 season.

Descriptive statistics for periods of two and four weeks following coaching transitions and season totals are presented in

Table 2. Descriptive statistics for overall and muscle injuries in three time frames.

	Overall			Muscle
	2 Weeks	4 Weeks	Season	2 Weeks	4 Weeks	Season
Min	0.00	0.00	8.04	0.00	0.00	3.47
Max	36.04	37.44	17.93	16.60	14.03	6.06
Mean	15.58	16.52	12.20	5.40	5.53	5.12
SD	10.95	10.21	3.36	5.32	4.15	0.94
Med	16.81	15.53	11.53	3.95	5.05	5.58
P25	6.63	11.46	9.21	0.00	1.85	4.29
P75	22.41	22.13	15.30	9.62	9.08	5.98

Legend: Min—minimum, Max—maximum, Mean—arithmetic mean, SD—standard deviation, Med—median, P25—25% percentile; P75—75% percentile.

. The results show an average of 15.58 injuries for 1000 h of exposure in the first 2 weeks and 16.52 after 4 weeks of coaching change, while muscle injuries had rates of 5.40 and 5.53 injuries for 1000 h in these periods.

The analysis of the difference in these observed time frames is presented in

Table 3. Paired t-test for comparison between observed periods.

Injuries	Period	Mean	95% CI	t-Test (Comparison with Season Average)			% Change
				t	p	Cohen’s D
Overall	2 WEEK	15.58	9.7461–21.416	1.18	0.25	0.42 (Small)	27.7
	4 WEEK	16.52	10.619–22.414	1.60	0.12	0.59 (Medium)	35.4
	SEASON	12.20	10.403–13.987
Muscle	2 WEEK	5.40	2.562–8.236	0.21	0.84	0.07 (Trivial)	5.5
	4 WEEK	5.53	3.135–7.925	0.39	0.70	0.14 (Trivial)	8.1
	SEASON	5.12	4.614–5.616

Legend: 95%CI—95% confidence interval, t- t value, p—statistical difference; Cohen’s D—effect size.

indicating no statistically significant changes, but with medium effect size when comparing overall injuries in 4 weeks and the season in general (Cohen’s D = 0.59).

Percentage changes in injury rates are graphically presented in Figure 1. Overall injuries had an increase of 27.7% and 35.4% in the two and four weeks after coaching turnovers, respectively, while for muscle injuries, these rates were lower and amounted to 5.5% and 8.1%.

4. Discussion

This study aimed to investigate whether the occurrence of injuries in elite football is affected by coaching turnovers during the season, specifically within 2 and 4 weeks after a change. In this regard, the most significant finding of this study is the 27.7% increase in total injuries in the first two weeks following the arrival of the new coach and a 35.4% increase in the first four weeks. While the differences did not reach statistical significance, the observed medium effect size indicates a potentially meaningful trend worth further investigation in larger or sample of multiple clubs. On the other hand, partially unexpected results indicate a slightly smaller increase in muscle injuries with 5.5% and 8.1% increases for the 2- and 4-week period, respectively.

4.1. Overall Injuries

The results of the analysis of the differences in the incidence of all injuries, between the entire season and the time immediately after the coaching changes at the club, suggest a general increase. Although these differences did not reach statistical significance, likely due to the limited sample size, the medium effect size for the 4 weeks after the change indicates a potentially significant negative impact of coaching shifts on players’ health. These findings are in line with the prevailing opinions of football coaches and practitioners, but also with previous studies on this topic [2,18]. Unlike previous studies focused on specific injury types such as hamstring strains or general muscle injuries, this study analyzed the overall injury incidence and the player’s perspective concerning availability [2,18]. Total injuries increased substantially following coaching changes, which is in partial alignment with previous observations of the transitional period. The dataset is inclusive of all injury types, whether contact or non-contact, suggesting multifactorial relations between tactical changes, changed physical load and psychosocial factors can complexly interact and affect injury occurrence. Although it was not directly measured here, this is in accordance with previous studies on this topic [2,23].

This suggests a more nuanced relationship among tactical shifts, loading changes, and psychological stressors.

As an example, a study by Ekstrand et al., 2023 focused specifically on hamstring muscle injuries and found that organizational instability, i.e., changes in coaches, medical staff, fitness coaches, and other staff members, caused an increase in injury burden [2]. The most obvious reason for this phenomenon can be found in the change in training methodology implemented by the new head coach. Specifically, in the desire to achieve an instant change in tactical aspect, atmosphere, approach, and consequently the result, new coaches often increase the frequency and intensity of training [17]. This can easily lead to disruption of the load monitoring process, especially if the change in head coach is accompanied by a change in fitness coach, which Ekstrand et al. point out in their study. The relationship between training load and injuries has recently been extensively studied, and many studies have linked the effect of acute and chronic load and sudden load spikes to which players are not adapted, with an increased injury risk [12,24,25,26]. Although this study is observational and does not directly investigate the causal interference of individual training factors with injuries, we can assume that coaching changes caused changes in the training process, and thus influenced the occurrence of injuries. This may also relate to a change in routine and implementation of prevention strategies that were established at the club under the previous staff [27]. Future studies should use load monitoring to better understand the causal mechanisms between coaching changes and injury incidence.

In addition to this, certain psychological factors could have increased the risk of injury in the period following coaching changes. Although not directly measured, informal observations by experienced practitioners, supported by peer-reviewed literature, suggest that psychological stress during coaching transitions could influence injury risk [28]. Namely, in such situations, players are uncertain, fearful of their place in the squad, and feel the pressure of proving themselves again to the new coach [28]. This can lead to increased fatigue, impaired cohesion and communication in the team, and uncontrolled motivation and aggressiveness in the training, which can result in a higher number of contact injuries [28]. However, this remains speculative and outside the scope of the present study.

It is also important to note that the injury incidence in this study is somewhat higher than is typically reported at the elite level of football. Specifically, as opposed to the 12.2 injuries found here, the studies that explored similar competitive levels usually report 5 to 8 injuries for 1000 h of exposure [6,8,29,30]. To contextualize this discrepancy concerning the aim of the present study, special attention should be paid to the number of coaching changes. Namely, in the club in this study, there were 16 coaching changes in eight seasons, or an average of 2 per season. On the other hand, in a previously cited study named the UEFA Elite Club Injury Study, only 24% of the clubs experienced a coaching shift throughout the season [2]. Also, a study on the top five European football leagues investigated the impact of coaching change on team performance and reported 411 coaching changes through eight seasons for a sample of 98 clubs per season, which represents a 52% coaching change rate among clubs at the elite football level [23]. These findings suggest that besides the usual shock effect and tactics and morale impact on the team, coaching shifts may have consequences on players’ health [31].

4.2. Muscle Injuries

Compared to overall injuries, the increase in muscle injuries after coaching turnovers was less pronounced. With a 5.5% and 8.1% rise in the first two and four weeks of the new coach’s term and a small effect size, muscle injuries appear to be less negatively affected by coaching changes. These results are not entirely consistent with the only study of its kind that investigated the effects of coaching changes on all muscle injuries [18]. Specifically, a 2020 study of all Turkish Super League clubs found an increase in muscle injuries of 2.3 times in the first 2 weeks (5.3 injuries/1000 h of exposure) and 1.9 times in the first 4 weeks (4.5 injuries/1000 h of exposure) compared to the seasonal average (2.3 injuries/1000 h of exposure) [18]. Although our study finding is somewhat counterintuitive, there are several potential explanations. First of all, muscle injuries, although affected by sudden spikes in training load, are also the result of chronic load and accumulated neuromuscular fatigue [31]. It is therefore possible that no adverse effects occurred in the short period observed in this study and that the muscle injuries are under the influence of a longer-term training load. Also, one of the important considerations is the possibility of underreporting injuries. As stated earlier, in the situations of coaching turnovers, players try to impress the new coach, and often push their limits in performance but also in the context of pain tolerance [16]. It is possible that in such situations, players feel certain issues, tightness, and even functional muscle injuries (i.e., delayed onset muscle stress, DOMS), but do not report them for fear of losing their place in the team. As several authors of this study have been involved in the work of football clubs for years, they can confirm that this phenomenon is very common and could have resulted in a lower recorded rate of muscle injuries. However, the small but still existing upward trend in muscle injuries in the period after coaching changes suggests that this type of injury should not be ignored. Special attention should be focused on communication between the staff members and players so that minor muscle issues are not ignored and later develop into more serious injuries [32].

4.3. Limitations and Strengths

The main limitation of this study is that we analyzed the injury incidence in a single team, which reduces the possibility of generalizing conclusions to a wider context with regard to the specifics of club culture, methodology, and other factors. Also, the study did not include direct parameters of training load, so causality could only be discussed hypothetically. Other organizational shifts, like changing fitness coaches, medical staff, and similar, were not analyzed in the study. However, their influence is acknowledged based on prior literature, and future studies should control for these variables. Finally, the aforementioned problem of underreporting is possible in situations where players do not want to report injuries. However, the use of a single elite team ensured consistency in the collection and recording of data, which certainly contributed to the reliability and validity of the results.

The most significant strength of this research is its longitudinal design, which spans eight competitive seasons, allowing for systematic monitoring of injuries within the same club environment. Also, an essential strength of the research is the fact that this is the first study to examine both muscle and overall injuries in situations of coaching changes, which offers a more clear and comprehensive understanding of how different types of injuries are influenced by coaching turnovers.

5. Conclusions

This study demonstrated that injury incidence increased by 35% in the four weeks following coaching transitions in elite senior football, while muscle injuries surprisingly showed a smaller upward trend. Although these differences did not reach statistical significance, the medium effect size provides valuable information about the magnitude of these changes. In the context of professional sport, it is important to highlight that even modest trends may have important practical value, as a small incremental increase in the number of injuries might severely impact performance in matches, squad rotation, and the overall health of the players in the long term.

Practical application of the findings of this study includes increased attention and injury prevention during times of coaching instability. Club coaching and medical staff should be especially careful in controlling the training load and recovery techniques in the transition periods while also maintaining the club’s preventive methodology. They should anticipate injury risk, ensure continuity in training load, and preserve preventive routines even after staff changes. By anticipating risk factors, the severity and frequency of injuries can be reduced, which will result in preserving player health and ultimately better team sports performance.

Future research should include multiple clubs across different leagues and incorporate biomechanical and physiological data to assess causal mechanisms of injury.