1. Introduction
Assessment of risk factors in previous longitudinal studies has been limited due to the release of information in professional clubs. The ability to have a full library of all reported injuries within the organization and not just what is reported publicly, enables a comprehensive analysis of injuries and risk factors. We can obtain detailed information about a player’s previous injury history, training protocols, and performance and strength testing continuously throughout each season. The aim of this study is to assess if specific characteristics of a soccer player such as position type, age, and history of previous adductor injury may be related to higher occurrences of adductor injury in a relatively controlled environment. We aim to understand the mechanisms that are related to adductor injury as well. We hypothesize that adductor strains will be one of the most common injury types endured in this population. Furthermore, we hypothesize that midfielders would have the highest incidence of injury due to the workload that they experience throughout their training and game schedules. Finally, we hypothesize that the MOI responsible for those injuries would predominantly be changes of direction, reaching, and kicking due to the stress these movements put on the musculature.
2. Materials and Methods
The data for this study were obtained through approval from the MLS M-Marc committee and a singular corresponding Major League Soccer (MLS) team. The inclusion criteria enveloped any player contracted by the MLS team between January 2016 to December 2022. A total of 86 athletes were contracted to the respective team during this period and all were included in this study. Of these individuals, 57 experienced at least one soft-tissue, non-contact lower extremity injury, and 20 were identified from training logs to have had at least one adductor strain during their time with the team. Descriptive analyses of injured and non-injured players were conducted. This includes assessing player position type, age, history of injury, and mechanism of injury (MOI). MOI was defined as the activity that took place at the time of injury. The MOI was determined by medical practitioners, employed by the team, communicating with the player as well as using video footage, if needed, to target what type of mechanism occurred at the instance of injury. If there was no consensus on the mechanism that occurred, it was categorized as unknown. Potential MOI can include kicking, reaching, running, overuse, change of direction, jumping, dribbling, diving, strength and conditioning events, and unknown. Practitioners assigned overuse as a MOI when a recurring complaint of discomfort resulted in injury with no known trauma. A player’s age was recorded at the time of injury. If an individual was not injured during the season, their age was reported from the beginning of each season. For the sake of this research and due to sample size, we will generalize athletes into the four position types, forwards, midfielders, defenders, and goalkeepers.
An injury was defined as any instance when a player noted an injury to medical staff, even if this did not result in time away from play. We analyzed the players’ history of injury and re-injury, where re-injury was defined as any reoccurrence of the same injury and laterality within their time at the club. All injuries were reported into a logging system by one of three medical staff from 2016 to 2022. Within the injury log, staff indicate when the injury happened, injury type, location, severity of the injury, and the mechanism in which the injury occurred. Additionally, it is noted whether the injury occurred during a game, practice, workout, or an off-duty event. If a player went to a specialist for an injury, the notes from the specialist were included. This information was used to give a more detailed understanding of the injury site and type. For example, the grade of strain and exact strain location through imaging, taken with the specialist, may be added to the injury log. Contact vs. non-contact injuries were also defined where a contact injury is considered if the mechanism in which the injury happened included a collision with a person or object. This does not include collision with a ball during a kicking or passing movement, as this is a fundamental aspect of the game. A non-contact injury would mean there was no contact with a person or object at the time of injury.
Descriptive statistics were used to characterize player demographics, positions, injury history, and mechanisms of injury. Generalized linear models (GLMs) utilizing GEEs were used to assess previous injury and position as risk factors for future injury among the cohort. The structure of the working correlation matrix was set as a first-order autoregression. This flexible class of models seeks to model population averages and is capable of handling both non-normal and longitudinal data, making it a highly suitable choice for our investigation. For this particular analysis, we assumed that individuals played the same position in subsequent half-seasons; we also controlled for player position in our model. This distinction was made to only compare the non-injured and injured individuals within the same season either in the first or last six months of the year.
3. Results
From 2016 to 2022, 86 male players made up the club population. A total of 57 individuals endured a soft-tissue, non-contact lower extremity injury during the time period of this study with 20 athletes experiencing 30 instances of soft-tissue, non-contact adductor strains. The total population ranged in age from 16 to 35 years old, with a mean and standard deviation of 26 ± 4.61; the injured population, including all injury types, ranged in age from 17 to 35 years old, with a mean and standard deviation of 27 ± 4.40; the adductor injury population ranged in age from 19 to 35 years old, with a mean and standard deviation of 27 ± 4.50. Adductor injuries made up 15.5% of all soft-tissue, non-contact lower extremity injuries (n = 194). The most common soft-tissue, non-contact lower extremity injury was hamstring strains (26.4%) and the third most common, after adductor strains, were quadricep strains (11.9%). Of the individuals who sustained a soft-tissue, non-contact adductor injury, 14 had one occurrence of injury during their time with the team, 4 had two injury occurrences, and 2 had three or more injury occurrences. By our definition of reinjury, we had a total of six reinjuries from four individuals. Thus, reinjury accounted for 20% of all adductor injuries. If we control for reinjuries within the same season, we see there were three reinjuries from three individuals. Adductor injuries were more common on their dominant side (54.8%). Of those that had an adductor injury, 70% occurred at practice with the remaining 30% occurring during a game. If we generalize to five practices a week in a ten-month season with 34 official league games, we can then see that an adductor injury occurred at an estimated 10.5% of practices and 26.5% of games respectively.
From our assessment of adductor injury through the utilization of GEEs, we found that the presence of a previous adductor injury had a strong association with the odds of adductor injury for any following half-season. Specifically, we observed that players with a previous adductor injury were estimated to have 167.2 times the odds of an adductor injury in a future half-season compared to someone with no previous injury (p < 0.01). Position type, season year, and season half were not found to have significant associations with the odds of injury.
4. Discussion
Overuse, change of direction, and running made up approximately 70% of the mechanisms of injury for adductor strains, partially confirming our hypothesis. Overuse is the general fatigue of soft tissue over time. Due to the nature of soccer and the number of high-intensity movements during training and games, soccer players can expose themselves to overuse injuries. For starting players in the games, these athletes can play 90 or more minutes of alternating low- to high-intensity activity during a game setting. This is a unique characteristic of soccer, whereas in many other sports, athletes will be substituted at some point throughout the game. We can see from our findings that adductor injuries occurred at an estimated 26.5% in games and only 10.5% in practices. This leads us to believe that in-game environments could elicit a greater risk of injury. Although practice exposure time is consistent between players, exposure time during games is highly varied and should be considered in future studies. Due to overuse being defined broadly by team practitioners as a recurring complaint with no known trauma, clarification of specific overuse mechanisms is needed before training regimens can be altered.
Coaches, medical staff, athletes, training schedules, and the environment greatly influence the type of workload and uncontrollable external factors an athlete may experience. While studies spanning across multiple teams lend themselves to greater external factor variability which could help diversify the population, these heterogeneous studies could fail to recognize specificities in training culture and should be considered when conducting any analysis. Injury characterization from a singular group with consistent training personnel and a team environment, as performed in this study, has provided a controlled environment in a chaotic system.
A notable limitation of this study is the size of the cohort included in this analysis. Ideally, we would include more subjects over the period of this study rather than limiting the sample to a single team over the course of seven seasons. Although this can be seen as a limitation, we should also acknowledge that this has led to more consistent training and rehabilitation methods, creating a more controlled group. Within this study, we did not specify the type of adductor strain relating to the grade of strain, specific adductor muscle, and location of the strain. The inclusion of these details in the future could warrant more refined results. Furthermore, we are only considering injury data from the time an athlete was contracted to the MLS top professional team. This means we do not take into consideration any previous medical history prior to them joining the team. Finally, we should acknowledge that inter-practitioner logging can create variability in how an injury is reported. However, the same practitioners logged injury for all seasons included in this study resulting in less variability relative to studies including multiple teams and changing practitioners. Applied research outside of the lab will have inherent uncontrollable external factors, however, we believe this research is an example of a well-controlled cohort active with a real-world professional team tracked over an extended period of time.
Our findings suggest there are certain position types, movement patterns, practice/game environments, side dominance, and previous adductor injury history that can put an athlete at a higher risk of injury. This information can aid sports scientists and practitioners by giving further knowledge surrounding at-risk categories and further discussion on knowledge of these factors can be integrated into monitoring and training. For a holistic view of injury risk for an individual athlete, future analysis will need to incorporate workload experienced in play, supplemented with motion capture technology, to understand which types of positions and movements are most dangerous with regards to adductor injury.
5. Conclusions
For a single MLS team spanning seven seasons, adductor injuries made up 15.5% of all soft-tissue, non-contact lower extremity injuries throughout the seven seasons of competition. It was found that midfielders (43.3%) and forwards (33.3%) had the greatest proportion of total injury thus the greatest incidence of injury, however, when normalizing to the number of players per position type, forwards (28%) and goalkeepers (25%) had the highest percentage of injured players. The most common mechanisms of adductor strain include overuse (30%), change of direction (26.7%), running (13.3%), and kicking (10%). Previous injury was found to be associated with 167.2 times the odds of future injury in a subsequent half-season as compared to a lack of a previous injury. Many studies previously conducted regarding adductor injury in soccer have utilized injury data from multiple teams across various levels of play and found similar results. These findings can give us a better understanding of who may be at higher risk of injury and further evidence to show the predominant mechanisms of adductor injury in a controlled group within the MLS. The scientific community and practitioners can use this information when addressing individuals who may fall into a similar category, indicating that they may have an inherently greater risk of injury.
Author Contributions
Conceptualization, R.D., J.H., M.H. and A.P.; methodology, R.D. and B.B.; software, R.D. and B.B.; validation, R.D., J.H., M.H., A.P., C.B.S. and B.B.; formal analysis, R.D. and B.B.; investigation, R.D.; resources, J.H., B.B., A.P., C.B.S. and M.H.; data curation, R.D.; writing—original draft preparation, R.D.; writing—review and editing, R.D., J.H., A.P., C.B.S., B.B. and M.H.; visualization, R.D.; supervision, J.H. and M.H.; project administration, R.D. and J.H.; funding acquisition, R.D. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the NSF GRFP and the University of Delaware Mechanical Engineering Helwig Fellowship.
Institutional Review Board Statement
IRB and MLS M-Marc Medical Committee approval was obtained for this research.
Informed Consent Statement
Informed consent and approval were obtained from the MLS M-Marc Medical Committee.
Data Availability Statement
Restrictions apply to the availability of these data. Data permissions are needed by the MLS M-Marc Medical committee and are not available for public sharing.
Conflicts of Interest
The author of this paper participated in a fellowship with a professional soccer team to gain greater sport analytic experience during the 2022 season. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Abbreviations
MLS | Major League Soccer |
MOI | Mechanism of injury |
GEE | Generalized estimating equation |
GLM | Generalized linear model |
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Soft-tissue, non-contact adductor injury mechanisms (n = 30).
Figure 1.
Soft-tissue, non-contact adductor injury mechanisms (n = 30).
Table 1.
Soft-tissue, non-contact adductor injury distribution across position types from 2016–2022.
Table 1.
Soft-tissue, non-contact adductor injury distribution across position types from 2016–2022.
Position | # Athletes | # Athletes with Injury | # Injuries | Proportion of Total Injury | Incidence Rate | % Athletes with Injury Per Position Type |
---|---|---|---|---|---|---|
Midfielder | 39 | 8 | 13 | 43.3% | 33.3% | 20.5% |
Forward | 14 | 3 | 4 | 13.3% | 28.6% | 21.4% |
Defender | 25 | 7 | 10 | 33.3% | 40% | 28% |
Goalkeeper | 8 | 2 | 3 | 10% | 37.5% | 25% |
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