Hamstring strain injury (HSI) is the most prevalent injury in football (soccer), representing 12% of all injuries in high-level players.1 A professional team can expect 5 to 6 HSIs per season,2 and these injuries typically have persistent symptoms3 and high recurrence rates.2 Players “off the pitch” because of injuries compromise the entire team performance4 and negatively affect the club finances5; hence, prevention is the primary goal. Although the etiology of HSI is complex and multifactorial, a few specific intrinsic factors have been associated with higher injury rates. Previous HSIs and advanced age are accepted as the main nonmodifiable risk factors,6,7 while prospective studies have found a higher incidence of HSIs in football players with poor flexibility,8,9 low muscle strength,10–12 short muscle fascicles,12 and deficits in central stabilization (ie, core stability).13
Researchers have advanced in understanding HSI risk factors6,7 and have proposed effective preventive actions.14,15 At the same time, football clubs have invested in laboratory and field tests for screening players who might be prone to injuries.16,17 Even in the face of this progress, HSI rates have increased by 4% annually among professional football players,18 emphasizing the gap between science and practice in the sports field. Practitioners who work with high-performance football operate at a frenetic pace, and they use their experience and limited data to support decision-making processes. However, data assessment and analysis by practitioners often do not follow the same standards researchers do.19 That is why football clubs have been looking for universities and/or researchers to achieve a competitive advantage using evidence-based approaches. The success of such a partnership depends on efficient communication between both sides; thus, scientists should create reports with objective information that can be quickly understood and applied by whoever is working in the field.20
In this study, we designed and applied a testing protocol for screening HSI risk factors supported by prospective cohort studies. The objective was to describe the prevalence of the main intrinsic risk factors for HSI in professional and under-20 male football players.
Methods
Study Design
This was a cross-sectional study. All evaluations were carried out during the first 2 preseason weeks. Each player was submitted to all tests in a single visit to the laboratory. After data processing and analysis, individual reports were generated and delivered to the coaching staff (Supplementary Material [available online]). A detailed explanation about the report and its interpretation was also given to the team physiotherapist and/or conditioning trainer. The study was approved by the institutional ethics committee, and all volunteers provided informed consent before starting study participation.
Participants
Two football clubs agreed to participate in this study. A total of 101 male football players (52 from professional teams and 49 from under-20 teams) were assessed (Table 1). Both clubs played in a state premier league in Brazil, and all players had professional work contracts with their clubs. Professional and under-20 players follow a similar training routine, which usually encompasses 2 daily training sessions, 3 to 5 days per week, according to each weekly schedule (ie, training, matches, and trips).
Players’ Characteristics
| Total (N = 101) | Professionals (n = 52) | Under-20 (n = 49) | P value | |
|---|---|---|---|---|
| Age, y | 21 (3) | 22 (4) | 18 (1) | <.001 |
| Weight, kg | 75 (9) | 76 (8) | 74 (9) | .16 |
| Height, cm | 179 (7) | 179 (7) | 179 (8) | .97 |
| Leg preference | ||||
| Right | 84 (83.2%) | 41 (78.8%) | 43 (87.8%) | .23 |
| Left | 17 (16.8%) | 11 (21.2%) | 6 (12.2%) | |
| Playing positions | ||||
| Goalkeepers | 12 (11.9%) | 6 (11.5%) | 6 (12.2%) | .72 |
| Backs | 24 (23.8%) | 10 (19.2%) | 14 (28.6%) | |
| Midfielders | 41 (40.6%) | 23 (44.2%) | 18 (36.7%) | |
| Forwards | 24 (23.8%) | 13 (25.0%) | 11 (22.5%) | |
Note: Data presented as mean (SD) or n (%). P value refers to professionals compared with under-20 players.
Procedures
Assessments were performed in the morning, and the coaching staff was informed that the players should not perform vigorous training sessions on the previous day. In addition, players received the following recommendations: (1) not to perform high-intensity physical activities 24 hours before the tests, (2) not to take any kind of analgesic and/or anti-inflammatory drugs 48 hours before the tests, and (3) not to consume stimulant substances (eg, caffeine) on the day of the test. In approximately 1 hour each player completed the 5 steps in the following order: (1) anamnesis, (2) ultrasonography, (3) passive straight-leg raise (PSLR) test, (4) functional movement screen (FMS), and (5) isokinetic dynamometry.
Anamnesis
A specific questionnaire was used to record personal data and injury history. Specific questions were made for hamstring and other muscle strains (eg, quadriceps and calves), groin/pubis pain, ligament injuries (eg, knee and ankle), meniscus and/or cartilage injuries, and tendon injuries (eg, Achilles and patellar tendon). Body mass and height were also measured.
Ultrasonography
A B-mode ultrasonography system (Vivid I; GE Medical Systems, Fairfield, CT) with a linear-array probe of the same manufacturer (GE 8L; frequency: 13 MHz, depth: 5 cm, and width: 4 cm) was used to assess muscle architecture of biceps femoris long head (BFLH) by an experienced examiner.21 Briefly, after a minimum of 10 inactive minutes, subjects were assessed in prone position with the hips in neutral position, the knees fully extended and relaxed muscles. The BFLH length was determined as the distance from the ischial tuberosity to the superior border of the fibular head, and the ultrasound scans were taken at 50% of the muscle length. The probe was positioned perpendicular to the skin in the muscle’s longitudinal axis, and slight adjustments in the probe orientation were made by the examiner to optimize the aponeuroses and fascicle identification. Three ultrasound images from each subject were taken and stored for analysis with the ImageJ software (National Institutes of Health). From each image, muscle thickness (MT), pennation angle (PA), and aponeurosis angle (AA) were measured, while fascicle length (FL) was estimated using the following validated equation22: FL = sin (AA + 90°) × MT/sin [180° − (AA + 180° − PA)]. The average value of the 3 collected images was considered the player’s BFLH fascicle length. Fascicle length was reported in absolute terms and relative to BFLH length (fascicle length/muscle length).
PSLR Test
Two examiners executed the PSLR test with a gravitational inclinometer (Sanny, São Bernardo do Campo, Brazil).23 Players were placed in supine position with their legs straight. One examiner kept the contralateral leg straight to avoid external rotation and fixed the pelvis to minimize posterior pelvic tilt. Another examiner lifted passively the tested leg into hip flexion. The knee was fixed in full extension and the ankle in a relaxed position to minimize gastrocnemius participation. At least one of the 3 following criteria determined the end point: (1) the examiner’s perception of consistent resistance, (2) palpable onset of pelvic rotation, or (3) the participant’s feedback on discomfort/pain sensation. Three attempts were performed with each leg, and the largest range of motion value was used for data analysis.
Functional Movement Screen
Players were assessed using the 7 movement patterns comprised by the original FMS24,25: (1) deep squat, (2) hurdle step, (3) in-line lunge, (4) shoulder mobility, (5) active straight-leg raise, (6) trunk stability push-up, and (7) rotary stability. Specific clearing tests were performed after tests #4, #6, and #7. Evaluations were carried out by a certificated (level 2) examiner. Each movement pattern was thoroughly explained to the players, who had 3 trials to execute the movement in the most suitable way. The highest score among the 3 trials was recorded. Scores obtained from each test, composite score (sum of 7 scores), and bilateral asymmetry (ie, asymmetry between right and left side in unilateral tests) were used for data analysis.
Isokinetic Dynamometry
After a general warm-up (5 min of cycle ergometer exercise), players were positioned in the isokinetic dynamometer (Biodex System 4; Biodex Medical Systems, Shirley, NY) according to the manufacturer’s recommendations. Players performed 10 submaximal concentric knee flexion/extension repetitions at 90°/s for specific warm-up and familiarization with the equipment. Thereafter, 2 attempts of 3 consecutive maximum contractions were executed in the concentric–concentric (60°/s; 0°–90° of knee flexion) and eccentric–eccentric (60°/s; 30°–90° of knee flexion) modes. A 1-minute resting period was allowed between attempts. Quadriceps and hamstring peak torques were used for data analysis. Hamstring-to-quadriceps (H/Q) conventional ratio (hamstring concentric peak torque/quadriceps concentric peak torque) and H/Q functional ratio (hamstring eccentric peak torque/quadriceps concentric peak torque) were also calculated.26
Screening for HSI Risk Factors
Only HSI risk factors supported by at least one prospective cohort study were considered in our screening protocol. Data collected in the battery of tests allowed us to screen 11 possible HSI risk factors for each leg, besides the player’s age as a systemic risk factor (see “Summary of hamstring injury risk factors” in the Supplementary Material [available online]).
A previous HSI is the main nonmodifiable risk factor for a new HSI7, while players with a history of quadriceps and calf muscle injuries, groin/pubis injuries, and anterior cruciate ligament reconstructions seem to be more prone to sustain a hamstring injury.7 Thus, previous HSIs and a history of injuries in adjacent regions amount to 2 independent HSI risk factors in this study.
The reference values for BFLH fascicle length of professional football players were extracted from Timmins et al12 (ie, the 95% confidence interval of absolute BFLH fascicle length in uninjured players was added to the report as reference values). Because players with fascicle length relative to BFLH length <0.25 present a 3.7 higher risk to sustain an HSI during the season compared with those with ≥0.25 relative fascicle length,12 we used this cutoff point.
There are inconsistencies concerning the acceptable bilateral asymmetry of hamstring flexibility tests; therefore, our report suggests a “between-leg” difference up to 10% as acceptable, but we did not use this cutoff point as an HSI risk factor in our summary. The passive flexibility was checked as a risk factor when players had <90° in the PSLR test.8 The FMS test #5 (active straight-leg raise) was used to assess the active flexibility of hamstring muscles, and we considered this outcome to be a risk factor when the player scored <2 points.
Recent findings support that reduced electromyographic activity of glute and trunk muscles during explosive running is associated with HSI occurrence in football players.13 The assessment used by Schuermans et al13 requires a specific setup and equipment (ie, synchronized 3-dimensional kinematic analysis and electromyography systems, respectively) and is time consuming. Thus, we assessed the players’ movement kinematic and the central stabilization through the FMS, which is a reliable27 and frequently used test by football premier clubs to detect noncontact injury risk.16,17 When a player scored <2 in tests #1, #2, or #3 of the FMS,24 a deficit in functional movements was checked as a risk factor. Similarly, deficits in core stability were checked when a player scored <2 in tests #6 or #7 of the FMS.25
Typical peak torque values of professional football players were included in the report.26 The isokinetic data were used to check 4 hamstring injury risk factors: hamstring concentric strength deficit, hamstring eccentric strength deficit, H/Q conventional ratio, and H/Q functional ratio. Deficits in hamstring concentric or eccentric strength were marked when a leg presented a deficit >15% compared with the contralateral leg (bilateral asymmetry).10,11 The normative value of 0.6 for the H/Q conventional ratio has been widely used for identifying strength imbalance in football players,26 and scores <0.6 were marked as a risk factor. Values from 0.810 to 1.028 have been used as cutoff points for identifying strength imbalance through the H/Q functional ratio; thus, we checked H/Q functional ratio as a risk factor when the player scored <0.9.
Lastly, age has been considered a significant and nonmodifiable risk factor for HSI in sport.7 We considered age as a progressive risk factor according to the player’s aging; thus, we checked 1 of 4 range categories accordingly: players aged 26–30, 31–35, 36–40, and older than 40 years.
Statistical Analysis
Mean values, SDs, and percentage distributions were used to describe the results. Independent t tests were used to compare professional and under-20 groups, as well as to compare left and right legs. Chi-squared tests were used for categorical variables. A priori significance level of 5% was assumed (α < .05).
Results
Despite the obvious difference in age, professionals and under-20 groups presented no significant difference (P > .05) for weight, height, and distribution of leg preference or playing positions (Table 1). Professionals presented higher prevalence (P = .02) of previous HSIs compared with under-20 players: 21 players (40.4%) versus 9 players (18.4%). We found no difference between age groups for the other screening tests; thus, data of professional and under-20 football players were presented as a single sample.
Ultrasonography, isokinetic dynamometry, and PSLR data from left and right legs are presented in Table 2. No significant difference (P > .05) was found between left and right legs. Players’ performance during each FMS test is presented in Table 3. They obtained a mean FMS composite score of 14.15 (2.30), with values ranging from 6 to 18 points. Twenty-eight players (27.7%) had at least one bilateral asymmetry during FMS unilateral tests.
Ultrasonography, PSLR Test, and Isokinetic Dynamometry
| Left leg (N = 101) | Right leg (N = 101) | |
|---|---|---|
| Ultrasonography | ||
| BFLH fascicle length, cm | 10.85 (3.17) | 10.97 (2.80) |
| BFLH fascicle length/muscle length | 0.27 (0.08) | 0.27 (0.06) |
| PSLR test, deg | 85.93 (12.23) | 86.60 (12.68) |
| Isokinetic dynamometry | ||
| Quadriceps CON peak torque, N·m | 276.00 (40.64) | 270.95 (40.86) |
| Hamstring CON peak torque, N·m | 148.76 (22.14) | 151.27 (23.00) |
| Quadriceps ECC peak torque, N·m | 317.26 (65.67) | 315.20 (55.51) |
| Hamstring ECC peak torque, N·m | 208.03 (40.12) | 207.74 (39.18) |
| H/Q conventional ratio | 0.54 (0.06) | 0.56 (0.07) |
| H/Q functional ratio | 0.76 (0.12) | 0.77 (0.13) |
Abbreviations: BFLH, biceps femoris long head; CON, concentric; ECC, eccentric; H/Q, hamstring to quadriceps; PSLR, passive straight-leg raise. Note: Data presented as mean (SD).
Functional Movement Screen
| Test score (% of players) | |||||
|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | Asymmetry (% of players) | |
| #1 Deep squat | 4.0 | 24.8 | 66.3 | 5.0 | – |
| #2 Hurdle step | 1.0 | 13.9 | 83.2 | 2.0 | 1.0 |
| #3 In-line lunge | 1.0 | 1.0 | 94.1 | 4.0 | 2.0 |
| #4 Shoulder mobility | 6.9 | 8.9 | 24.8 | 59.4 | 14.9 |
| #5 Active straight-leg raise | 0.0 | 20.8 | 51.5 | 27.7 | 12.9 |
| #6 Trunk stability push-up | 0.0 | 14.9 | 35.6 | 49.5 | – |
| #7 Rotary stability | 1.0 | 20.8 | 78.2 | 0.0 | 2.0 |
Note: Data presented as percent distribution. Asymmetry was checked as different scores between left and right sides.
Figure 1 illustrates the proportion of players who presented each HSI risk factor in left and right legs. Among the 101 football players, 29.7% had already sustained at least one HSI; 58.4% had a history of injuries in adjacent regions; 48.5% had short BFLH fascicles; 66.3% and 20.8% had poor passive and active flexibility, respectively; 41.6% and 28.7% had deficits in functional movements and core stability, respectively; 6.9% and 25.7% presented bilateral imbalance for hamstring concentric and eccentric strength, respectively; and 87.1% and 94.1% obtained low values for H/Q conventional and functional ratios, respectively. Among the 52 professional players, advanced age was found in 13 players (25%): 10 (19.2%) older than 25 years, 3 (5.8%) older than 30 years, and no player older than 35 years.
—Hamstring injury risk factors in football players. CON indicates concentric; ECC, eccentric; FMS, Functional Movement Screen; H/Q, hamstring to quadriceps; PSLR, passive straight-leg raise.
Citation: Journal of Sport Rehabilitation 29, 3; 10.1123/jsr.2018-0084
Figure 2 illustrates the proportion of players who presented one or more HSI risk factors. Sixty-six percent of players had 3 to 5 risk factors for the same leg. None of the 101 football players was fully free of HSI risk factors.
—Number of simultaneous risk factors in football players.
Citation: Journal of Sport Rehabilitation 29, 3; 10.1123/jsr.2018-0084
Discussion
The battery of tests provided a series of data regarding athletic medical history, hamstring structure, and muscular/functional performance, which allowed us to describe the prevalence of intrinsic risk factors for HSI found in well-trained male football players. Most relevant information related to HSI risk factors was compiled into a single report that was sufficiently informative and accessible to the coaching staff.
The prevalence of 30% of players with a history of HSI further supports the high incidence of this injury in competitive football.1 Considering that almost half of participants were under 20 years, a considerable number of players (18%, according to our data) start their professional career already presenting a strong and nonmodifiable risk factor for HSI.6,7 Thus, clubs should consider the adoption of evidence-based screening tests and injury prevention programs in youth categories as a medium- to long-term investment in their players’ health and career.
Among the modifiable risk factors assessed in this study, attention is drawn to the proportion of players with low values of H/Q torque ratios. In fact, there is no consensus on cutoff values used in this study for H/Q conventional and functional ratios. Nonetheless, our findings agree with the previous evidence that showed that football players present clear deficits in hamstring eccentric strength.26 Although no association has been found between isokinetic measures and HSIs in a large cohort study,29 prospective studies with football players10–12 and other athletic populations30 have supported that athletes with weak hamstrings are more likely to sustain muscle injuries. Therefore, eccentric strengthening of the hamstring muscle seems to be advisable for most football players.
Training programs comprising the Nordic hamstring exercise (NHE) are very popular among football teams,16,17 and significant reductions on HSI rates have been found in amateur and professional football players.14 Although the optimal training periodization remains unclear and the preventive effect may depend on intervention compliance,31 football players have increased the hamstring eccentric strength by 11% to 12% after 10 to 12 weeks of NHE.32,33 Unfortunately, this strengthening magnitude would be insufficient for many players to reach the desired levels of H/Q functional ratio. However, NHE is also able to lengthen BFLH fascicles by ∼2 cm.21,34,35 Such magnitude of change provides a considerable preventive effect against HSIs in football players.12 In other words, the protective effect of NHE training programs is perhaps more attributable to the muscle structure adaptations than to muscular strengthening, but this issue needs further investigation.
Fascicle length is significantly reduced in previously injured BFLH muscles compared with uninjured contralateral legs,36 as well as the presence of short BFLH fascicles in preseason increases the risk of hamstring injuries during the season.12 A short fascicle means a reduced number of serially arranged sarcomeres in the muscle fiber, which enhances the chance of a muscle being overstretched and having damage caused by powerful eccentric contractions,37 such as those performed during the terminal swing phase of high-speed running.38 Approximately one-quarter of football players assessed by Timmins et al12 and almost half of those assessed in our study had BFLH fascicles shorter than the cutoff point of 0.25 (fascicle length/muscle length). These data provide arguments in favor of introducing the ultrasonography to the screening tests performed at the football clubs. In addition, we recommend that players with short BFLH fascicles should be engaged in eccentric-overload training programs that also focus on lengthening muscular fascicles, such as NHE.21,34,35
Prospective studies involving football players have demonstrated that those with less flexibility are at greater risk of sustaining a hamstring injury throughout the season,8,9 which suggest caution with two-thirds of players assessed in our study. Although stretching programs usually enhance the joint range of motion,39 adoption of a stretching program may not affect the hamstring injury rate in elite football teams.40 Therefore, issues like training volume, intensity, weekly frequency, and compliance must be further investigated to ensure effective stretching interventions.
Athletes with FMS composite scores ≤14 points have been usually classified as high injury risk, but there is strong criticism on the predictive value of this cutoff point.41 Thus, increasing attention has been given to individual test scores and bilateral asymmetries, as performed in the current study. The functional movements tests (deep squat, hurdle step, and in-line lunge) are considered primary athletic movements, and a score ≥2 is expected for competitive athletes. The poor performance (score <2) found in 42% of players may be attributed to deficits in flexibility, balance, and/or neuromuscular control. Similarly, deficits in core stability are the main factor to explain the poor performance of 29% of players on push-up and rotary stability tests. Ideally, these deficits should be analyzed case-by-case to provide individual corrective interventions, but this is sometimes difficult in team sports. It is imperative that collective prevention programs in football should comprise balance stimulus and core stability exercises, such as those comprised by the “FIFA 11+.”15
Teams assessed in the current study were not monitoring the players’ exposure hours in matches and practice sessions. Therefore, we were unable to follow the HSI rates throughout the season and investigate their relationship with the risk factors assessed during preseason assessments; this is the main limitation of the current study. Another limitation is the self-report regarding the players’ injury history; this method does not allow a precise classification of the type and severity of the previous injuries. Nevertheless, our study adds to the literature an important piece of information: professional and under-20 football players usually present multiple HSI risk factors, and a comprehensive approach is mandatory to screen the most vulnerable players. In addition, the current study describes a successful partnership between the university and professional football clubs. Accessible reports are an effective way to make laboratory tests more useful to practitioners in the pitch, thus reducing the gap between science and practice.
Conclusion
Our findings support that most football players present multiple risk factors for sustaining an HSI, so a comprehensive and evidence-based testing protocol is recommended for screening the most vulnerable players. Hamstring eccentric weakness is the most prevalent risk factor, but the teams should also be aware of deficits in flexibility, core stability, functional movements, and hamstring fascicle length. Players’ injury history is a nonmodifiable risk factor that cannot be neglected, so players with previous injuries should further minimize possible deficits on the other (modifiable) risk factors.
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