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Abstract

Background:

Hamstring strain injuries are one of the most prevalent injuries in football (soccer). We examined the influence of accumulated match-play exposure on the occurrence of hamstring strain injury in professional football from 2 teams (Spanish 1st Division, LaLiga) over 3 seasons, and determined specific cut-off points as indicators of injury risk.

Hypothesis:

Overloaded players would be more likely to sustain a hamstring injury.

Study Design:

Prospective, controlled, observational study.

Level of Evidence:

Level 2b.

Methods:

Playing time, total running distance, and high-speed running (>24 km/h) distance during official matches of players that sustained a hamstring injury were compared with uninjured, paired controls. Cumulative playing time and running performance of 4 matches before the injury was computed. Relative risk (RR) of injury occurrence was estimated by generalized estimating equations. Diagnostic accuracy was determined by receiver operating characteristics and the area under the curve.

Results:

Thirty-seven hamstring strain injuries occurred, representing 23 ± 18 absence days per injury. Thirty-seven controls (uninjured players) were used as comparators. Low match-play exposures during 1 and 2 matches before injury were likely to explain injury occurrence (RR: 14-53%; P < 0.01). Metrics from the match before the hamstring muscle strain demonstrated the best accuracy to predict injury occurrence: high-speed running distance ≤328 m (sensitivity, 64%; specificity, 84%), playing time ≤64 min (sensitivity, 36%; specificity, 97%), and running distance ≤5.8 km (sensitivity, 39%; specificity, 97%).

Conclusion:

Relatively reduced competitive exposure in the previous 2 matches was associated with higher hamstring injury risk in professional football players.

Clinical Relevance:

Screening simple metrics such as the accumulated match exposure during official matches and considering specific cut-off points for some running variables may be good indicators of injury risk and may assist in better individual injury management in professional soccer players.

Keywords

external workload injury prevention musculoskeletal soccer team sports

Hamstring strain injuries are one of the most prevalent injuries in football (soccer).¹¹ Around 22% of players from elite teams suffer hamstring strain injuries each season, and the incidence increases overtime at a rate of 2.3% per year.¹² Furthermore, muscle injuries represent 18% to 37% of all time-loss injuries in elite football players,¹¹ which may impair the team’s performance²⁰ with a negative effect on the club’s finances.⁹

To suggest effective preventive injury strategies specific to football, several injury prevention models have been proposed to decrease the number and severity of injuries.^2,24 However, the referred models have been criticized because they do not account for the interactions and fluctuations between risk factors.^3,25 However, regardless of the interplay of risk factors or inciting biomechanical events, every athletic injury is sustained while athletes are exposed to training and competition exposures.³²

The incidence of muscle injury is 6 times higher during match play compared with training.^11,19 It is well known that a combination of the increased demands of matches in comparison with training sessions,⁵ together with insufficient recovery time in between matches¹⁰ might be responsible for those match-play injury rates. Specifically, the higher rates of hamstring strain injuries during competition are associated with greater exposure to high-speed running in matches.²⁰ In this regard, distances covered at high-speed running (ie, over 20 km/h) are several fold higher during official matches than during training sessions.^1,31 Hence, match exposure and running distance at high intensity during official matches may become a risk factor for hamstring strain injury. Previous studies found a relationship between metrics of high-speed running exposure during matches and hamstring injury risk in Australian football players.^7,27 Specifically, the referred studies observed that increased match exposure is predictive of future hamstring strain injuries, and the largest effects of high-speed running exposure on injury risk were observed in the 2-week interval preceding the exposure.^7,27 Conversely, Colby et al⁶ identified a link between the greater injury risks with the minimal exposure to high-velocity efforts (maximum speed exposure and sprint volume) in elite Australian football. To the best of the authors’ knowledge, no study has investigated the relationship between exposure to high-speed running in matches with the injury risk of sustaining a hamstring strain injury in professional football players.

Considering the increased rate of hamstring strain injuries in the last few years and the number of football players who sustained hamstring strain injuries during matches, there is a need to investigate the relationship between the demands of matches, particularly high-speed running, and the risk of hamstring strain injuries. Therefore, the present study aimed to examine the contribution of exposure and match-running demands (total distance covered and high-speed running) during official matches on the incidence of hamstring strain injuries in professional football players. According to current evidence, we hypothesized that excessive match workload would be associated with an increased risk of hamstring injury in professional football players.

Methods

Study Design

Professional football players from the Spanish 1st Division League (LaLiga) were prospectively followed during 3 consecutive seasons (2011/2012, 2012/2013, and 2013/2014). Data from the month preceding the hamstring strain injury (4 matches before injury) were collected for all the injured players and a paired control group of similar characteristics. Injured players were randomly matched (1:1) against control players according to the following criteria: players from the same team and playing position²⁸ with similar training activities, injury prevention strategies, quality of fields, and rehabilitation strategies.²² The controls were required to be starting players in the match in which the paired players suffered the muscle injury or the last match they participated in before getting injured. The procedures were approved by the institutional Ethics Review Committee. Furthermore, LaLiga has authorized the utilization of the data about match demands for the present study, and, following LaLiga’s ethical guidelines, this investigation does not include information that identifies football players.

Participants

In total, 144 professional outfield football players (99 players were listed in multiple seasons) from 2 teams competing in LaLiga agreed to participate and provided written informed consent before starting the study. Based on their movement demands, injury patterns and training characteristics, goalkeepers were excluded from the study to increase the heterogeneity of the sample. All the players followed a similar weekly routine consisting of approximately 14 h of training (4-6 sessions of combined football and 1-2 strength sessions) and 1 to 2 competitive matches. To be included in the study, participants had to take part in the 4 previous matches to obtain their running metrics.

Injury Data Collection

All hamstring strain injuries were diagnosed and recorded by the medical staff of the football team using the classification system developed by the International Soccer Injury Consensus Group.¹⁵ Recorded data were collated every week via an electronic database. A hamstring muscle strain was defined as any physical complaint sustained by the player in the competition, located in the posterior thigh, which prevented the injured player from participating in the following competition or normal training for at least 1 day.¹⁶ Injury recurrences (ie, an injury of the same type and at the same site) were considered as a new injury in cases in which the player returned to full training and (availability for) competitions for at least 1 week. Data about severity (absence days) were collected for each injury and reported to describe the injury context.

Match-play Exposure

Cumulative match-play exposure was determined by minutes played and running performance (total distance covered and high-speed running distance covered >24 km/h) during matches. These data were obtained by the validated multicamera player tracking system Mediacoach used by the football teams.¹⁴ Data files for the 4 matches prior injury in injured and control-paired players were imported into a spreadsheet.

Statistical Analyses

Means, SDs, frequencies, and percentages were calculated for each variable. Generalized estimating equation models were used to determine associations between match-play exposure with hamstring strain injury. Exponential coefficients were reported as a measure of relative risk (RR), interpreted as the multiplicative term to be used to estimate injuries when match-play exposure variables increased by 1 unit (ie, 10 min, 1000 m covered and 100 m at high-speed running). Receiver operating characteristic (ROC) curves were used to compare the diagnostic performance of each match-play exposure parameter. The true positive rate (sensitivity) was plotted against the false-positive rate (100-specificity) for different cut-off points so that each point represents a sensitivity/specificity pair corresponding to a particular decision threshold. The area under the ROC curve (AUC) was calculated as a measure of how well each parameter can distinguish between the 2 diagnostic groups (injured/noninjured). The Youden index (J) and associated criterion were calculated, being J = 1 a perfect discriminatory ability. An AUC >0.70 and J >0.30 were considered as a minimum level of acceptance.²³ Statistical calculations were performed using a custom Microsoft Excel spreadsheet, the SPSS Version 24 (IBM Corp., Armonk, NY) and MedCalc Version 18.2.1 (MedCalc Software bvba, Ostend, Belgium).

Results

A total of 37 noncontact hamstring strain injuries were screened in 23 players (age, 27 ± 3 years; body mass, 72.9 ± 5.9 kg; height, 1.79 ± 5.4 m), so 37 noninjured players with similar characteristics were randomly included as controls (Table 1). Most injuries occurred during match situations (86.4%) with the remaining 13.6% of injuries occurring during training. Most of the injuries happened during explosive actions, such as a sprint >20 m (29.7%), kicking the ball (27.0%), and accelerating (18.9%), with the rest occurring during football-related muscle stretching (16.2%) and change of directions (8.1%). Severity was moderate (56.7%), severe (27.0%), mild (10.8%), and minimal (5.5%). Injuries represented an absence of 23 ± 18 days absent from playing football, with a 10.8% recurrence.

Table 1.

Associations, diagnostic performance, and distribution of cumulative match-play exposure and running performance parameters to discriminate between injured and noninjured professional football players

Cumulative Match-play Exposure	Injured (n = 37)	Controls (n = 37)	Injury Risk^a	Accuracy
Cumulative Match-play Exposure	Injured (n = 37)	Controls (n = 37)	RR (95% CI)	AUC (95% CI)
Playing time (min)
1 Match before injury	65.4 ± 37.2	88.5 ± 13.2	1.41 (1.10-1.81)*	0.67 (0.55-0.78)*
2 Matches before injury	125.5 ± 68.3	164.1 ± 35.4	1.14 (1.05-1.24)*	0.66 (0.54-0.77)*
3 Matches before injury	194.4 ± 98.4	232.2 ± 55.6	1.06 (0.99-1.13)	0.58 (0.46-0.69)
4 Matches before injury	256.5 ± 134.1	292.2 ± 83.3	1.03 (0.98-1.07)	0.52 (0.41-0.64)
Distance covered (km)
1 Match before injury	6.9 ± 4.3	10.0 ± 1.7	1.41 (1.13-1.74)*	0.72 (0.60-0.82)*
2 Matches before injury	13.9 ± 7.5	18.6 ± 4.3	1.14 (1.04-1.24)*	0.69 (0.57-0.80)*
3 Matches before injury	21.4 ± 10.8	26.3 ± 6.8	1.07 (1.01-1.13)*	0.62 (0.50-0.73)
4 Matches before injury	28.5 ± 14.9	33.2 ± 9.7	1.03 (0.99-1.07)	0.56 (0.43-0.67)
High-speed running (m)
1 Match before injury	318.0 ± 244.5	509.3 ± 180.1	1.53 (1.18-1.98)*	0.77 (0.64-0.85)*
2 Matches before injury	667.4 ± 457.4	959.0 ± 404.2	1.17 (1.04-1.32)*	0.70 (0.57-0.79)*
3 Matches before injury	1062.3 ± 682.9	1352.7 ± 570.1	1.08 (1.00-1.17)	0.64 (0.52-0.75)*
4 Matches before injury	1399.1 ± 896.3	1678.7 ± 712.7	1.05 (0.99-1.11)	0.61 (0.49-0.72)

AUC, area under the receiving operator characteristic curve.

Relative risk (RR) is shown in units per 10 min (playing time), 1000 m (distance covered) and 100 m (high-speed running), ie, for a given increase in the exposure, the chances of injury increased the given RR times.

95% CI and P level from the generalized estimating equation (injury in the subsequent match: yes/no).

Significant values (P < 0.05).

Cumulative match-play exposure during 1 and 2 match time window before injury achieved the highest accuracy (ie, the highest true-positive and the lower false-negative rates) to discriminate hamstring strain injury cases from noninjured cases (Appendix Figure A1, available in the online version of this article). Low match-play exposure in the match before the injury was associated with higher risk of injury (Table 1 and Appendix Figure A2, available online). ROC analyses revealed the following thresholds: playing times ≤64 min (RR: 41%, P < 0.01; J = 0.33, P < 0.01), distances covered ≤5.8 km (RR: 41%, P < 0.01; J = 0.36, P < 0.01), and high-speed running distances ≤328 m (RR: 53%, P < 0.01; J = 0.48; P < 0.01). Low cumulative exposure for 2 consecutive matches was likely to increase subsequent hamstring injury occurrence (Table 1 and Appendix Figure A2, available online). ROC analyses revealed the following thresholds (when combining data of the 2 matches): times ≤95 min (RR: 14%, P = 0.01; J = 0.29, P = 0.01), distances covered ≤12.0 km (RR: 14%, P < 0.01; J = 0.31, P < 0.01), and high-speed running distances ≤901 m (RR: 17%, P = 0.01; J = 0.39; P < 0.01).

Discussion

The present study aimed to examine the contribution of exposure and match-running demands during official matches and subsequent hamstring strain injuries in professional football players. The rationale for this investigation was based on previous reports suggesting that the incidence of muscle injury is higher during match play compared with training^11,19 and that the higher rates of hamstring strain injuries during the competition are associated with greater exposure to high-speed running in matches.²⁰ However, to the best of the authors’ knowledge, no previous investigation has evaluated the contribution of exposure and match-running demands only during official matches on the incidence of hamstring strain injuries in professional football players. The current results showed a substantially lower accumulated match exposure (ie, min) and associated running performance (ie, total distance covered and high-speed running distance) in the hamstring strain injured players in the 2 matches before the injury occurred, at least when compared with control players that were involved in the same competition but had not suffered any injury. Interestingly, we found no significant differences between injured players and controls when comparing running metrics and time exposure for the 3 and 4 matches before the injury occurred. This suggests that match underexposure for only 2 matches may be a risk factor for hamstring injury in professional football players.

The findings of the current study are in line with those of previous researchers who reported that a low amount of total running distance accumulated over weeks resulted in an increased risk of sustaining a noncontact injury.²⁶ Specifically, in our study, players suffering hamstring strain injuries displayed lower running total distance covered 2 and 1 matches before the injury occurred in comparison with the control players (Table 1). In contrast, these results differ from previous studies^8,21 that reported no significant differences in total distance covered between injured and uninjured players. The lack of agreement between studies could be related to the methodological differences. In particular, although the present paper included exposure times and running performance during matches, others collected training data²¹ or a combination of training and match data.⁸ In addition, the current paper is specific for hamstring muscle injury which is an injury with a higher risk of happening during high-intensity actions in competition, whereas previous studies analyzed all types of muscle injuries sustained, including sometimes poorly associated with high-intensity running.^8,21

Concerning accumulated volumes of high-speed running distances, the current study reveals lower cumulative high-speed running distances (>24 km/h) in the 1 and 2 matches before the injury occurred (Table 1). In contrast, running metrics for the 3 to 4 previous matches had no significance to explain hamstring injury risk, suggesting that underexposure has not to be maintained long in time to produce a deleterious effect on the player’s likelihood to suffer a hamstring strain.

Our data support, at least partially, the findings of Colby et al⁶ that identified a link between greater injury risk and minimal exposure to high-velocity efforts (maximum speed exposure and sprint volume) in elite Australian football. However, these results partially differ from previous studies that reported no significant differences in the risk of injury in elite youth football players with different amounts of high-speed running and sprinting.⁴ The findings of the current study are also in disagreement with the results reported by previous studies conducted with Australian footballers that found associations between greater volumes of high-speed and very high-speed running and heightened soft tissue injury risk.^7,18,27 However, comparisons between studies are difficult due to the clear different demands and movement patterns that each of these sports requires. A potential explanation of the results obtained in the present study might be related to the fact that higher (specific) running distances during competition, and associated enhancements in fitness and tolerance to physical stress due to participation in matches, can have a protective effect. On the other hand, lower running distances during competition because low match exposure may be insufficient to induce adaptations or result in detraining,³⁰ thereby increasing the risk of injury when the player has to participate in the competition.¹⁷ In this sense, it is believed that participation in match-play itself is the most appropriate stimulus for preparing players for the physical demands of match-play,¹ which reinforces the idea that the regular involvement of the players during the competitive season (excluding scenarios with congested fixtures) is the best way to reduce the risk of hamstring muscle injury. In this regard, the use of training sessions with high-intensity actions may not be sufficient to avoid the injury, as in this study, the training characteristics of injured and control players was the same and they only differed in match exposure.

It is worth noting that, despite changes in high-speed running accounting for a better prediction of the injury risk, the collection of these data requires technology that may not be accessible for all teams. According to our findings, cumulative playing volume in minutes (which is easier to assess) stands as a practical alternative to identify the potential risk of hamstring injury in football players. According to our data, 4 of 10 injuries occurred when playing time was ≤64 min in the match before the injury or ≤95 min in the 2 matches before the injury occurred. Interestingly, less than 10% of injuries occurred when playing time was over these cut-off points (Appendix Figure A2, available online). Thus, coaches should consider the register of players’ exposure times during matches as a practical tool to estimate injury risk. This simple metric may help to identify over and underloaded players and manage training strategies accordingly. Owing to the ease of collecting playing minutes and its noticeable relationship with injury occurrence, our results encourage future studies to incorporate this measure to confirm these findings.

In addition to exploring the associations between cumulative match exposure and running metrics and hamstring strain injury occurrence, we conducted further analyses to explore the diagnosis accuracy (ie, ability to predict individual players who will incur hamstring injury) of such metrics. Diagnosis characteristics of the exposure and running performance metrics explored in the present study are listed in Table 1 and Appendix Figures A1 and A2 (available online). The outcomes of the present investigation show that monitoring simple metrics might offer some ability to predict hamstring injury in professional football players. In this regard, for some specific scenarios and variables, ROC curve analyses revealed AUC >0.70 (Table 1), which is the arbitrary cut-off value reported to establish an acceptable predictive ability.¹³ Similarly, for the same scenarios, the Youden index, which has been reported to assess the balance between sensitivity and specificity to quantify cut-off values,¹³ also showed moderate discriminative values greater than 0.30.²⁹ However, in practical “real-life” scenarios, the rather high number of false negatives (ie, low specificity, accounting for players with low match exposure that did not suffer an injury) present in most of the metrics and combinations of 1 to 2 matches before injury (Appendix Figure A2, available online) indicates a poor predictive ability to detect players that will go on to incur a hamstring injury. Nonetheless, that low specificity combined with the relatively low number of false positives (ie, high sensitivity, accounting for players with low match exposure that did suffer an injury) might suggest that individual management strategies designed to lower the risk could be implemented with players displaying reduced match-play exposure and associated competitive running performance during several consecutive matches.

Limitations

A significant limitation of this study is that training exposure and internal load data during training and competition were not recorded. Likewise, results must be interpreted considering that we included injuries occurring both in training and match scenarios. Within this context, our results indicate that match underexposure may increase the risk of hamstring injury, independently if this injury occurs during training and competition. Another limitation of this study is that the time elapsed to complete the 4 matches before the hamstring injury was not equal for all players. However, the time between the match before the injury and occurrence of the injury was between 3 and 7 days for all players. Further, the present outcome variables were obtained from 2 football teams, and extrapolation of these outcomes to other teams with different management of training and match exposure may be inappropriate.

Conclusion

In summary, professional football players that sustained a hamstring strain injury showed substantially less accumulated match-playing exposure, total running distance, and high-speed running distance than uninjured players occupying the same position within the team in 1 and 2 matches before the injury. High-speed running, the total distance covered, and simple metrics such as the playing time in the previous 2 matches showed the greatest ability to diagnose players at higher risk of hamstring injury. In this regard, underloaded players in terms of match exposure might need to be subjected to individual training strategies to minimize their potential risk of hamstring muscle injury.

Supplemental Material

sj-pdf-1-sph-10.1177_19417381231158117 – Supplemental material for Reduced Match Exposure in the Previous 2 Matches Accounts for Hamstring Muscle Injury Incidence in Professional Football Players

Supplemental material, sj-pdf-1-sph-10.1177_19417381231158117 for Reduced Match Exposure in the Previous 2 Matches Accounts for Hamstring Muscle Injury Incidence in Professional Football Players by Víctor Moreno-Pérez, Juan Del Coso, Roberto López-Del Campo, Ricardo Resta, José Romero-Sangüesa, Javier Courel-Ibáñez and Alberto Méndez-Villanueva in Sports Health: A Multidisciplinary Approach

Footnotes

The authors report no potential conflicts of interest in the development and publication of this article.

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Supplementary Material

Please find the following supplemental material available below.

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