Effectiveness of Injury Prevention Programs With Core Muscle Strengthening Exercises to Reduce the Incidence of Hamstring Injury Among Soccer Players: A Systematic Review and Meta-Analysis

Abstract

Context:

Muscles in the hamstring group are frequently injured in sporting activities. Injury prevention programs (IPPs), including eccentric training of the hamstrings, have proven to be of great value in decreasing the injury rate of hamstring muscles.

Objective:

To examine the effectiveness of IPPs that include core muscle strengthening exercises (CMSEs) in reducing hamstring injury rates.

Data Sources:

This systematic review with meta-analysis was based upon the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A systematic search was conducted for relevant studies published from 1985 to 2021 using the following databases: Cochrane Library, MEDLINE, AMED, PubMed, Web of Science, and Physiotherapy Evidence Database (PEDro).

Study Selection:

The initial electronic search found 2694 randomized controlled trials (RCTs). After removing duplicate entries, 1374 articles were screened by their titles and abstracts, and 53 full-text records were assessed, of which 43 were excluded. The remaining 10 articles were reviewed in detail, from which 5 studies met our inclusion criteria and were included in the current meta-analysis.

Study Design:

Systematic review and meta-analysis of RCTs.

Level of Evidence:

Level 1a.

Data Extraction:

Two researchers independently completed the abstract review and performed full-text reviews. A third reviewer was consulted to reach a consensus if any discrepancies were noted. Details were recorded about the participants, methodological aspects, eligibility criteria, intervention data, and outcome measures, including age; number of subjects in the intervention/control group; number of injuries in each group; and the duration, frequency, and intensity of the training conducted in the intervention.

Results:

The pooled results of 4728 players and 379,102 exposure hours showed 47% hamstring injury reduction per 1000 h of exposure in the intervention group compared with the control group with an injury risk ratio of 0.53 (95% CI [0.28, 0.98], P = 0.04).

Conclusion:

The results indicate that CMSEs incorporated with IPPs reduce susceptibility and risk of hamstring injuries in soccer players.

Muscle injuries account for approximately one-third of football (soccer) injuries and one-quarter of all football injury absences. ¹⁸ A study conducted on European soccer players revealed that muscle injuries such as hamstrings injury, caused by excessive contusions or strain, are, in fact, a frequent occurrence and happen mostly during periods of training or competition. ⁴⁸ Various sports epidemiologists have agreed on the definition of hamstring injury as an injury that causes an athlete to be unable to participate entirely in the subsequent training session or competition. ²² Hamstring injuries are most common and account for 12% of muscle injuries.^6,18 More specifically, data retrieved from 25 male professional football teams show that hamstring injury is responsible for 5 total impairments instead of the 3 quadriceps injuries witnessed in every season and a total of 130 missed days of football. ¹⁷

The risk factors for developing a hamstring injury are many. Schmitt et al ³⁷ and Caine et al ¹² state that the most common factors are a history of a previous hamstring injury, insufficient core stability, reduced flexibility and strength, bad lumbar posture, and absence of a good warm-up routine. Furthermore, Brukner et al ¹⁰ suggest that increasing age, higher level of competition, hamstring muscle strength imbalance, and an earlier hamstring injury are some of the strongest drivers toward developing a new hamstring injury. In contrast, Buckthorpe et al ¹¹ indicated that lumbopelvic and hip instability, motor patterns, and postural issues could contribute to hamstring injuries. More specifically, lordosis of the lumbar region puts the hamstrings at a mechanical disadvantage and, thus, prone to injury. ¹³

As advanced age and a history of injury are nonmodifiable factors, emphasis is placed on addressing the thigh muscles, gluteus, and core muscle imbalances and dysfunction. Various injury prevention exercises and programs have been conducted to avert or reduce the number of hamstring injuries among players. ⁴¹ Core muscle strengthening exercises (CMSEs) have emerged as a prominent part of the fitness paradigm in sports and medical rehabilitation. In preventing accidents and enhancing athletic activity, core stabilization has become a renowned practice.^38,44,49 In previous years, fitness specialists have progressively utilized CMSEs in sports conditioning programs. These exercises were designed primarily for patients with lower back complications in physical therapy units. ⁴⁹ Nowadays, CMSEs are also performed increasingly by healthy people such as those attending sports conditioning and fitness centers.

Muscles other than the abdominals have been included in CMSEs. These muscles have been defined recently as the complex of the lumbopelvic hip. ³⁴ Precisely, the core muscles include the posterior erector spinae, anterior rectus abdominis, transverse abdominis, multifidus, gluteus maximus, hamstrings, lateral gluteus medius, gluteus minimums, quadrates lumborum, medial adductor magnus, longus, brevis, and pectineus. ³⁵ Furthermore, 29 or more pairs of muscles constitute the lumbopelvic hip complex functioning to stabilize the pelvis, spine, and hips while executing functional movements. ³⁵ Core training of the lumbopelvic-hip complex should be acknowledged to optimize the biomechanics of lower extremities and enhance sports performance.^2,38 Core muscle recruitment during various medicine ball abdominal exercises,^20,33,43,47 and conventional abdominal exercises including bent-knee, sit-up, and crunch (abdominal curl-up) has been evaluated in recent studies.^{8,20,35,43,47} However, for this review, the lumbopelvic-hip exercises will be referred to collectively as CMSEs.

Training and recovery programs have incorporated many exercises to boost core strength and flexibility. For instance, exercises for training of gluteus maximus and hamstrings involve prone hip extension, roll-out, pike, knee-out, and skier, ²¹ with the aid of a Swiss ball. Planks, side bridge, single-leg stance, leg jumps, lunges, and bodyweight squat exercises are also considered part of the lumbopelvic-hip complex activation and strengthening protocols for injury prevention. ²⁹ Several of these exercises are selected depending on versatility or sports specificity. Such exercises involve complex mechanisms of muscle activation, neuromuscular control, and static and dynamic stabilization, which all assist in load transfer and injury prevention. ²⁹ Yet, the correct method of performing some abdominal strengthening exercises, such as the crunch and the bent-knee sit-up, is still unclear. ³¹

The idea of CMSEs and how they can improve sports performance and reduce hamstring muscle injury has been interpreted differently by specialists.^28,44,49 As these exercises often strengthen the lumbopelvic-hip complex muscles and not just the core muscles, they are used widely as part of injury prevention programs (IPPs), as supported by many researchers.^1,13,29 Consequently, this systematic review and meta-analysis of randomized controlled trials (RCTs) was conducted to investigate the effectiveness of IPPs that included CMSEs as part of the program in reducing hamstring injury rates among soccer players.

Methods

This systematic review with meta-analysis was based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. ³² The protocol was registered in the International Prospective Register of Systematic Reviews, PROSPERO (CRD42020186700).

Search Strategy

A systematic literature review was completed using multiple electronic databases, including the Cochrane Central Registry of Controlled Trials, Physiotherapy Evidence Database (PEDro), PubMed, Web of Science, EBSCO’s SPORTDiscus, and ScienceDirect. The search through these databases was conducted in the period from Monday, June 1, 2020 to Wednesday, March 31, 2021 for relevant studies published from 1985 to 2021. The search was carried out by 2 researchers who separately investigated the entitlement of the studies. The electronic search was performed based on the following keywords: “Hamstring injury” AND “Core exercise and prevention” OR “trunk exercise and prevention” OR “hamstring exercise and prevention program” OR “hamstring injury and prevention program” OR “FIFA11+.”

Criteria for Included Studies

Studies were considered in the review if they met the following criteria: cluster-randomized controlled trials (CRCTs), RCTs about programs for preventing sports injuries containing CMSEs and other exercises of an injury prevention protocol. The studies were required to report hamstring injury rates. Studies were included if conducted on soccer players, regardless of skill level, sex, and age group. Moreover, these studies were published solely in English until the end of search period (March 31, 2021). Studies of players actively engaged in preventive activities, which included CMSEs and contrasted to “different” or “regular” or “no program,” were also included. The outcome measures were hamstring injury rates.

Criteria for Excluded Studies

If the following elements were found in the articles, the study was excluded from the analysis: those that employed protective equipment for intervention; those that failed to indicate the players’ exposure time or injury rates; studies that had several exposure groups; those that were observational or cross-sectional, such as case reports or series; and finally studies that dealt with physical quantification only and did not assess hamstring injury rates. Studies based on prevention protocols consisting of only CMSEs or against regular warm-up protocols were excluded since they do not fall within the scope of this review.

Data Collection, Extraction, and Analysis

Two researchers independently completed the review of titles and abstracts. They then reviewed the full text and categorized the studies independently as suitable (included) or unsuitable (not included). When discrepancies arose, a third reviewer was consulted to reach a consensus. Details were recorded relating to the participants, methodological aspects of the studies, eligibility criteria, intervention data, and outcome measures, including age, number of subjects in the intervention/control group, number of injuries in each group, and the method/intensity of the intervention. Study authors were contacted when the hamstring injury rates were not reported or stated to provide missing details.

Administration

Organization, preservation, and screening of studies involved the employment of Endnote Version X7 (Thomson Reuters). Before data analysis using the Comprehensive Meta-Analysis (CMA) software Version 3 (CMA V3) (Biostat, Inc), primary findings from different studies were compiled and derived in MS Excel 365 (Microsoft Corp).

Meta-Analysis

The meta-analysis was based on data documented and evaluated using CMA software. The injury incidence rate was calculated by determining the ratio between total time-at-risk and the number of event injuries; this ratio was then multiplied by 1000. The figure obtained through this calculation represents the differences in the exposure periods of individual athletes. ¹² For the computation of injury risk ratio (IRR), we used the formula: IRR = injury incidence rate in the intervention group/injury rate in the control group. An IRR <1 implies a favorable result of the intervention. The CMA program was used for logging IRRs, variances, and SEs for the research outputs. A random-effect invariance model was used since the studies varied in sample size and, thus, true effect size. We assumed a heterogeneity value >50 and that studies drew on contexts and soccer communities, included several kinds of exercises, and evaluated alternative means for CMSEs. Our research also regulates sample size and CI for each study. For example, the IRR for raw numbers 57/114 was 0.5, and after meta-analysis the research sample size and CI were perceived to be modified, the IRR for the pooled estimate was 0.53. The workload of soccer players was identified as the cumulative number of active hours engaged in competitions or training, as mentioned in the studies included in the review. ⁵ The P value was set at 0.05, and P values <0.05 were considered statistically significant.

Publication Bias

Standard funnel plots were employed for the detection of publication bias. The Duval and Tweedie ¹⁵ trim-and-fill test methods were performed to indicate whether the value of IRR had to be modified to compensate for the missing studies. The Egger ¹⁶ test was used to validate asymmetry.

Methodological Quality Assessment

The same person assessed the methodological consistency of each sample study. Moreover, the probability of bias and quality parameters were assessed as per Furlan et al ²³ using 12 criteria, and the objects were scored as follows (“+” = 1 point, “-” or “?” = 0 points). Research that scored more than 60% for the 12 parameters involved a low risk of bias or excellent quality. The scores for each study are displayed in Table 1.

Table 1.

Methodological quality scores of the included studies

Criteria	Al Attar et al 3	Engebretsen et al 19	Silvers-Granelli et al 39	Soligard et al 40	van der Horst et al 46
Adequate randomization	+	+	+	+	+
Allocation concealment	+	-	+	+	+
Blinding patients	+	-	+	-	+
Blinding caregiver	+	-	+	-	+
Blinding/outcome assessors	-	-	+	-	+
Incomplete outcome data addressed/dropouts	+	+	+	+	+
Incomplete outcome data/ITT analysis	+	+	+	+	+
Free of suggestions of selective outcome reporting	+	+	+	+	+
Similar baseline characteristics	+	+	+	+	+
Cointerventions avoided or similar	+	+	-	+	-
Compliance acceptable in all groups	+	-	+	+	+
Similar timing of outcome assessment	+	+	+	+	+
Study score a	11	7	11	9	11
Percentage	91.6	58.3	91.6	75	91.6

ITT, intention-to-treat.

^aFor each question, only Yes (+) received a point. No points were awarded for No (-).

Results

Trial Flow

Of the 2694 articles that resulted from the database search, 1374 were examined depending on their title and abstracts. Studies not matching the primary criteria were eliminated. Of the remaining 53 complete text documents, 43 were excluded due to not meeting the inclusion requirements. Finally, 10 studies were selected for a thorough evaluation and analysis (Figure 1).

Figure 1.

Flow diagram of the systematic steps in articles screened for inclusion.

Of the 10 eligible studies, 5 publications were omitted for the following reasons: either IPPs did not use the CMSEs, the exposure time or hamstring injury rate was not reported, or only abstracts were available (ie, the abstracts were obtained from conference proceedings or free-paper presentations and no complete full articles were published). Consequently, the remaining 5 studies were included in the review and meta-analysis. A schematic description of the sequential stages involved in screening papers for inclusion using the PRISMA flow diagram approach is presented in Figure 1.

Study Characteristics

Of the 5 studies included in this meta-analysis,^{3,19,39,40,46} 4 were CRCTs,^3,39,40,46 and 1 was an RCT. ¹⁹ The 5 studies were conducted in Norway,^19,40 the Netherlands, ⁴⁶ the United States (US), ³⁹ and Australia. ³ Only male participants were recruited in 4 studies,^3,19,39,46 whereas 1 study recruited only female participants. ⁴⁰ One study included Norwegian first, second, and third division soccer team players aged 17 to 35 years, ¹⁹ 1 study involved soccer players aged between 14 and 35 years, ³ and amateur soccer players aged between 18 and 40 years were recruited for another study. ⁴⁶ Three papers applied the FIFA 11+ protocol,^3,39,40 whereas 1 study consisted of CMSEs. ⁴⁰ Regarding the intervention and control groups, the study of Al Attar et al ³ involved FIFA 11+ exercise programs in the intervention and control groups. However, it was applied before training only. On the other hand, the remaining studies applied a standard warm-up protocol in the control group.^19,39,40,46 In the studies by Al Attar et al, ³ Engebretsen et al, ¹⁹ Silvers-Granelli et al, ³⁹ Soligard et al, ⁴⁰ and van der Horst et al, ⁴⁶ the interventions periods were between 10 weeks and 8 months with compliance rates of 91%, 21%, 77%, 73%, and 83%, respectively.

In terms of methodological quality, the studies by Al Attar et al, ³ Silvers-Granelli et al, ³⁹ and van der Horst et al ⁴⁶ had excellent qualities and low risk of bias with a score 91.6%. They were followed by the study of Soligard et al, ⁴⁰ with a score of 75%. The study by Engebretsen et al ¹⁹ had the highest bias level, with a score of 58.3%, in which there was no allocation concealment and blinding of the patients, caregiver, or the outcome assessors. Table 2 reflects the characteristics of the 5 articles studied, and Table 3 indicates the frequency of postexposure injury rates in the study intervention and control groups.

Table 2.

Characteristics of the included studies

Study	Participants	Intervention	Control	Outcome Measures
Al Attar et al 3 CRCT Australia	N = 280 Age, 14-35 y Sex, male Compliance, 83% Amateur soccer	FIFA 11+ program performed before and after training (includes CMSEs) 2-3/week × 6 mo	FIFA 11+ program performed before training only 2-3/week × 6 mo	Hamstring injuries
Engebretsen et al 19 RCT Norway	N = 209 Age, 17-35 y Sex, male Compliance, 28% First, second and third division soccer	Targeted exercise program (includes CMSEs) 2-3/week × 2.5 mo	Neuromuscular training, NH lowers, and groin strength training 2-3/week × 2.5 mo	Hamstring injuries
Silvers- Granelli et al 39 CRCT US	N = 1525 Age, 18-25 y Sex, male Compliance, 73% Collegiate soccer	FIFA 11+ program (includes CMSEs) 3/week × 6 mo	Aerobic warm-up (eg, running exercises), static and/or dynamic stretching and soccer skills practise (eg, cutting and short passing drills) 3/week × 6 mo	Hamstring injuries
Soligard et al 40 CRCT Norway	N = 1892 Age, 13-17 ySex, female Compliance, 77% Youth soccer	FIFA 11+ program (includes CMSEs) 3/week × 8 mo	Standard warm-up, typically including running exercises to warm up and static stretches 3/week × 8 mo	Hamstring injuries
van der Horst et al 46 CRCT the Netherlands	N = 579 Age, 18-40 ySex, male Compliance, 91% Amateur soccer	Targeted exercise program (includes CMSEs) 2/week × 4 mo	Standard warm-up	Hamstring injuries

CMSEs, core muscle strengthening exercises; CRCT, cluster-randomized controlled trial; FIFA, Fédération Internationale de Football Association; NH, Nordic hamstring; RCT, randomized controlled trial; US, United States.

Table 3.

Injury rates per 1000 h exposure in the intervention and control groups of the included studies

Study	Intervention Subjects	Intervention Hamstring Injuries	Intervention Exposure Hours	Intervention Hamstring Injuries per 1000 h	Control Subjects	Control Hamstring Injuries	Control Exposure Hours	Control Hamstring Injuries per 1000 h
Al Attar et al 3	160	2	35,802	0.055	184	9	31,616	0.284
Engebretsen et al 19	193	23	41,856	0.55	195	17	40,913	0.416
Silvers-Granelli et al 39	675	16	35,226	0.146	850	55	44,212	0.902
Soligard et al 40	1055	5	49,899	0.5	837	8	45,428	0.179
van der Horst et al 46	292	11	26,426	0.37	287	25	27,724	3.115
Pooled data	2375	57	189,209	0.301	2353	114	189,893	0.600

Pooled Injury Estimates

A total of 171 hamstring injuries were found because of pooled data from 5 included studies among a total of 4728 players. Altogether, these players had 379,102 exposure hours.

Meta-Analysis Results

Reduction in Hamstring Injuries

The pooled results of 4728 players and 379,102 exposure hours showed a 47% reduction in hamstring injury risk per 1000 h of exposure in the intervention group compared with the control group with an IRR of 0.53 (95% CI, 0.28, 0.98; P = 0.04) (Figure 2).

Figure 2.

Forest plot illustrating the effect of CMSEs versus controls on hamstring injury risk ratio. CMSE, core muscle strengthening exercises; CON, control; df, degrees of freedom; INT, intervention.

Publication Bias Results

The asymmetry evident in the funnel plot eliminated the possibility of any bias (Figure 3). The symmetry (intercept = -1.26, SE = 2.64, P (1-tailed) = 0.33, P (2-tailed) = 0.66 was confirmed by the Egger ¹⁶ test. The general IRR was not modified by the trim and fill procedure proposed by Duval and Tweedie. ¹⁵ The value of the point estimate for included studies was found to be 0.55 (95% CI, 0.40, 0.78) fixed effects; 0.52964 random effects; n = 0 imputed studies); therefore, it was not essential to adjust the overall point estimate.

Figure 3.

Funnel plots based on the study SE and log risk ratio in assessing publication.

The circles in Figure 3 represent the study point estimates. The study points estimates had been determined through the process proposed by Duval and Tweedie. ¹⁵ The summary measure used for this research is pooled IRR, indicated in Figure 3 by a diamond. The vertical tip represents the overall result at the center of the diamond, while the horizontal tip of the diamond indicates the corresponding CIs.

Discussion

This systematic review and meta-analysis was conducted to investigate the impact of CMSEs in reducing the incidence of hamstring injuries among soccer players. The findings confirmed the added benefits of including CMSEs in reducing the risk of hamstring injuries for soccer players when used independently and combined with IPPs.

Blasimann et al ⁹ conducted a systematic review in 2018 that examined the effect of CMSEs on the general injury rate. However, their review included the overall injury rates in male adult soccer players only compared with hamstring injuries in both sexes in the current study. Another methodological difference resembled the number of included studies. They included 7 studies compared with 5 studies in the current review even though their conclusion is like ours, in which the inclusion of CMSEs can positively impact the injury rates among soccer players.

Many studies highlight the importance of compliance rates in reducing hamstring injuries.^14,46 In our meta-analysis, the study by van der Horst et al ⁴⁶ had a high compliance rate of 91% and reported that 36 players (6.2%) had hamstring injuries during the registration period of 579 players. Moreover, 11 (31%) hamstring injuries from 292 players were recorded in the intervention group and 25 (69%) from 287 players in the control group. However, there was a statistically insignificant difference between the intervention and control groups (P < 0.05). In contrast, the study by Engebretsen et al ¹⁹ reported little beneficial effect on the hamstring injury rate with a compliance rate of 21.1%. Goode et al ²⁵ reported a 65% decrease in the hamstring injury risk when adherence to eccentric hamstring exercises was high. The amount of compliance with sports injury prevention interventions significantly affects outcomes.

Nordic hamstring (NH) exercises are the leading exercises in reducing the rate of hamstring injuries. ⁷ Greater compliance has been observed when a specific IPP such as FIFA 11+ was performed in addition to NH exercises.^{24,26,27,40,42,45} Consistent with our results, Petersen et al ³⁶ found that the use of NH exercises in both new and chronic hamstring injuries decreased the overall injury risk. Our meta-analysis found a reduction of 50% of hamstring injuries in the intervention group when CMSEs were included in the protocol compared with the control group who implemented the usual exercise program. Furthermore, in players with previous episodes of hamstring strains, they found that the injury rate decreased by around 86% relative to players using regular training programs. ³⁶

With access to only a single study with female athlete participation that met our inclusion requirements, ⁴⁰ it was impossible to perform a separate analysis based on sex difference. With the knowledge that female players have a substantially lower peak torque-to-body mass ratio for the quadriceps and hamstring muscles than male players, it would be helpful to examine the effectiveness of CMSEs in minimizing hamstring injury rates in female players. ³⁰ In their meta-analysis analyzing the effects of the FMARC IPP, Al Attar et al ⁴ observed that the proportion of injury reduction in female players was not significant relative to male players. For lower extremities, male players were stated to have a substantial decrease in potential injuries. It is crucial to provide a managed CRCT to evaluate the effectiveness of CMSEs interventions on hamstring injury rates in female players.

In sports other than soccer, such as rugby and Australian football, which also have a high rate of hamstring strains, a consensus in favor of the efficacy of CMSEs in the reduction of hamstring injuries has yet to be established. Therefore, studies investigating the effect of CMSEs in different sports are essential.

Strengths and Limitations

High methodological standards, including the injury rate relative to exposure hours, were used to analyze the data in this research. In any case, there are certain limitations. First, this meta-analysis included only studies of hamstring injuries in soccer players. Second, we only included studies that investigated the effectiveness of CMSEs as part of the hamstring IPP. We did not investigate the effect of other exercises from the included studies, which may have had a possible effect on reducing the incidence of hamstring injuries.

To answer the question conclusively, it is imperative to study the effect of CMSEs in isolation. Three studies^3,39,40 included in this review performed FIFA 11+ (which included CMSEs), and 1 study ¹⁹ used targeted exercises including core muscles. Another study used self-initiated preventative strategies, including CMSEs and plyometric exercises. ⁴⁶ Consequently, our methods aimed to include the studies that have CMSEs in isolation or as a part of other exercise programs.

Conclusion

This meta-analysis established evidence that CMSEs, when performed in conjunction with other IPPs during the warm-up protocol, may reduce hamstring injury rates by 50% compared with teams that do not include such programs in their warm-ups.

Clinical Recommendations

It is recommended that CMSEs be used not only for prevention but also in rehabilitation to prepare the soccer players for future stress and protect them from further injuries.