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Hamstring-and-Lower-Back Flexibility in Male Amateur Soccer Players

van der Horst, Nick PT, MSc; Priesterbach, Annique MD, MSc; Backx, Frank MD, PhD; Smits, Dirk-Wouter PT, PhD

Clinical Journal of Sport Medicine: January 2017 - Volume 27 - Issue 1 - p 20–25
doi: 10.1097/JSM.0000000000000294
Original Research
Free
SDC

Objective: This study investigated the hamstring-and-lower-back flexibility (HLBF) of male adult amateur soccer players, using the sit-and-reach test (SRT), with a view to obtaining population-based reference values and to determining whether SRT scores are associated with player characteristics.

Design: Cross-sectional cohort study.

Setting: Teams from high-level Dutch amateur soccer competitions were recruited for participation.

Participants: Dutch male high-level amateur field soccer players (n = 449) of age 18 to 40 years. Players with a hamstring injury at the moment of SRT-measurement or any other injury that prevented them from following the SRT protocol were excluded.

Main Outcome Measures: Sit-and-reach test scores were measured and then population-based reference values were calculated as follows: >2SD below mean (defining “very low” HLBF), 1SD-2SD below mean (“low” HLBF), 1SD below mean to 1SD above mean (“normal” HLBF), 1SD-2SD above mean (“high” HLBF), and >2SD above mean (“very high” HLBF). Whether SRT scores were correlated with player characteristics was determined using a Pearson correlation coefficient or Spearman rho.

Results: Sit-and-reach test scores ranged from 0 to 43.5 cm (mean 22.0 cm, SD 9.2). The cutoff points for population-based reference values were <3.5 cm for “very low”, 3.5 to 13.0 cm for “low”, 13.0 to 31.0 cm for “normal”, 31.0 to 40.5 cm for “high”, and >40.5 cm for “very high”. Sit-and-reach test scores were significantly associated with players' height (ρ = −0.132, P = 0.005), body mass index (r = 0.114, P = 0.016), and history of anterior cruciate ligament surgery (P < 0.001).

Conclusions: This study is the first to describe the HLBF of amateur soccer players. The SRT reference values with cutoff points may facilitate evidence-based decision making regarding HLBF, and the SRT might be a useful tool to assess injury risk, performance, or for diagnostic purposes.

Department of Rehabilitation, Nursing Science & Sports, Rudolf Magnus Institute of Neurosciences, University Medical Center Utrecht, the Netherlands.

Corresponding Author: Nick van der Horst, PT, MSc, Department of Rehabilitation, Nursing Science & Sports, Rudolf Magnus Institute of Neurosciences, University Medical Center Utrecht 85500, 3508 GA Utrecht, the Netherlands (n.vanderhorst-3@umcutrecht.nl).

The authors report no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's Web site (www.cjsportmed.com).

Received December 05, 2014

Accepted October 03, 2015

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INTRODUCTION

The sit-and-reach test (SRT) is one of the most commonly used instruments to measure hamstring-and-lower-back flexibility (HLBF) and is often used to diagnose or assess the risk of injury and to evaluate performance.1,2 The classical SRT was first described by Wells and Dillon in 1952.3 Since then, the SRT has been incorporated in many HLBF and fitness test protocols such as the Eurofit Test of Physical Fitness.1,4–9 The SRT has a high intrarater reliability and test–retest reliability.2–4,8,10 For practical use by clinicians (eg sports physicians, physical therapists, sports masseurs, etc.), the SRT is quick and simple to perform and requires little skill and training, both for administering the test and interpreting the scores.5,11 Furthermore, the SRT is particularly useful in large-scale evaluation of HLBF in the field setting such as team monitoring of HLBF over time.11,12

Hamstring-and-lower-back flexibility is an important modifiable risk factor for injuries and is easy to measure in clinical practice using instruments such as the SRT. As such, HLBF deserves attention in sports injury-related research.13–15 Hamstring-and-lower-back flexibility is an integral part of the current cause–effect model for hamstring injury, although research contains controversial findings regarding the contribution of hamstring flexibility on increased injury risk.16 Mendiguchia's new conceptual model for hamstring injury suggests that hamstring flexibility could particularly turn into a risk factor when combined with other risk factors, such as strength, and increase the likelihood of injury.16 Regarding the relationship between strength and flexibility, fundamental research has shown that the ratio of the change in resistance to the change in length of the muscle, termed stiffness, is associated with an increased risk of injury.17–19 As a less stiff muscle can extend to a greater length, it can better absorb applied forces.17,20 Sports requiring optimal use of the stretch-shortening cycles of the hamstring muscles generally involve rapid acceleration and deceleration, such as is seen in rugby, American football, and soccer, all of which are high-risk sports for hamstring injuries.21,22 Therefore, evaluating hamstring muscle flexibility, using instruments such as the SRT, is a regular assessment in sports medical evaluation because reduced HLBF has been proposed as a predisposing factor for increased risk of hamstring injury.23

A reduced HLBF can be a risk factor not only for sports injuries,24–28 such as acute hamstring injuries,27 muscle damage after eccentric exercises,26 patellar tendinopathy,28 anterior knee pain,28 low back pain,25 but also for reduced performance.24 Competitive soccer players have a reduced HLBF compared with recreational athletes,29,30 possibly as a result of the long-term impact of soccer training on the muscle-tendon system. It potentially makes soccer players more susceptible to hamstring injuries.29,31,32 Indeed, the highest rate of hamstring injury is seen in soccer,33 accounting for 47% of all muscle strains in the sport and the most lost playing time when compared with all other injuries.22 Hamstring injuries are characterized by a high recurrence and substantial lost playing time.22,34 Therefore, identifying soccer players with a reduced HLBF, measured with the SRT, might facilitate identification of those players at risk of injury.35

Accurate diagnostics and valid prediction rules for HLBF can improve the effectiveness of treatment, prevention, and training. However, the interpretation of outcomes for many clinical tests, such as the SRT, is still highly subjective.36 The lack of population-based reference data with appropriate cutoff points makes it difficult to use the results of clinical tests for evidence-based decision making or research.37,38 To our knowledge, no studies have measured HLBF in soccer players. The aim of this study was to measure HLBF in male adult amateur soccer players, with a view to establishing population-based reference values and to determining whether HLBF in this population is associated with specific player characteristics.

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METHODS

This cross-sectional study was part of the Hamstring Injury Prevention Strategies study, a study of interventions to prevent hamstring injuries in male adult soccer players in the Netherlands (trial number NTR3664).39 Baseline data, including SRT scores, of 616 soccer players were available for this study.

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Subjects

Soccer teams from Dutch high-level amateur field soccer competitions (“1e Klasse”) were invited to participate. Teams were included if the coaches and medical staff agreed on participation and players were willing to sign informed consent. Male players aged between 18 and 40 years were eligible for inclusion.

Teams were regarded as dropout if the medical staff did not return the player questionnaires or SRT scores of the team or did not follow the SRT testing protocol. Individual players were excluded if they suffered from a hamstring injury at the moment of inclusion or any other injury that prevented them from following the SRT protocol. Individual players were regarded as dropout if they were not available for flexibility testing.

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Procedures

Instruction meetings for the medical staff of participating teams were organized in each district 2 to 6 weeks before the study started in January 2013. During these meetings, the aims of the study and the SRT for flexibility measurements were explained. Team medical staff were provided with a SRT box, written instructions, and intake questionnaires and were responsible for collecting and returning the questionnaires and SRT scores. Data were collected during the first soccer team activity after the winter break in January 2013.

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Instruments

Intake Questionnaires

Information about player characteristics (date of birth, self-reported height and weight, nationality, years of soccer experience, dominant leg (ie, kicking leg), field position, current injury status, and soccer injury history) was obtained with a questionnaire. This questionnaire defined soccer injuries, in accordance with Fuller's consensus statement, as any physical complaint sustained by a player that results from a soccer match or soccer training, irrespective of the need for medical attention or time loss from soccer activities.40

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Sit-and-Reach Test

Flexibility was measured using the classical SRT protocol as described by Ayala et al.5 Meta-analysis has shown that the classical SRT protocol has a better criterion-related validity than modified versions of the SRT protocol.12 For this test, a standard SRT box (30.5-cm-high) with a sliding reach indicator on top of a measuring scale (0–50 cm) was used. The 35-cm mark was aligned with the foot panel of the box. The test has a high intrarater reliability [intraclass correlation coefficient (ICC) = 0.92-0.98] and test–retest reliability (ICC = 0.92-0.95) for the SRT.1,2,6,10

The SRT was performed before normal training, and the player was not allowed to do any warm-up or stretching exercises before the test. The player was tested while sitting on the floor, with the legs together, the knees extended, and the soles of the bare feet placed against the foot panel of the test box (Figure 1 see Video, Supplemental Digital Content 1, http://links.lww.com/JSM/A104). He was instructed to place his hands on top of each other with the hand palms facing downward and to reach forward slowly, pushing the reach indicator as far as possible along the measuring scale. Throughout the test, a member of the medical staff made sure that the knees of the player remained extended; the knees could be fixed during the test. The maximum position had to be reached gradually and maintained for 2 seconds. Two measurements were taken, with a 30-second interval, for each player.5 In between the 2 measurements, the player had to sit up straight so that the hip extensor muscles were returned to a neutral position; the player was not allowed to stand up or stretch. Test scores were recorded to the nearest 0.5 cm. If a player could not reach the zero mark on the box, the test score was reported as zero.

FIGURE 1

FIGURE 1

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Statistical Analysis

All statistical procedures were performed using SPSS 22.0 (IBM Corp. 2011, Armonk, NY). Player characteristics were reported as mean and SD values for continuous variables [age, height, weight, body mass index (BMI), and soccer experience] and as the number of players and percentages for ordinal or categorical variables (nationality, leg dominance, field position, and injury history).

Sit-and-reach test scores are reported as mean values in centimeters, SDs, range, and quartiles.5 Population-based reference values of HLBF were calculated as: >2SD below mean (defining “very low” HLBF), 1SD-2SD below mean (“low” HLBF), 1SD below mean to 1SD above mean (“normal” HLBF), 1SD-2SD above mean (“high” HLBF), and >2SD above mean (“very high” HLBF).

Pearson correlation coefficient or Spearman rho was calculated to determine whether player characteristics were correlated with SRT scores. Continuous variables were checked for normal distribution by using the Kolmogorov–Smirnov test. Subsequently, differences in player characteristics among subgroups of HLBF were analyzed with χ2 tests and analyses of variance. Statistical significance was accepted at the P = 0.05 level.

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Ethical Considerations

This study was approved by the Medical Ethical Committee of the University Medical Center Utrecht, the Netherlands (File number 12–575/C). All players were asked to provide written informed consent before the start of this study. Players unwilling to do so were excluded from the trial.

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RESULTS

Baseline data of 449 soccer players from 29 teams were available for analysis. A flow chart of the study population is presented in Figure 2.

FIGURE 2

FIGURE 2

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Player Characteristics

Player characteristics are summarized in Table 1. Their mean age was 24.5 years (SD 3.8), and they had played soccer for a mean of 18.1 years (SD 4.2). The right leg was dominant in 68.1% of the players. Field positions were proportionally represented, although some players reported multiple field positions (eg midfielder and forward). Almost one in 4 players (23.3%) had had one or more hamstring injuries in the previous year.

TABLE 1

TABLE 1

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Sit-and-Reach Test Scores

Sit-and-reach test scores are presented in Table 2. The mean overall SRT score of all players was 22.0 cm (SD 9.2; range 0-43.5 cm). Fifteen players (3.3%) scored 0 cm in both tests. The lower and upper limits of the normal range of SRT scores for this population (mean ± 1SD) were 13.0 and 31.0 cm, respectively. The lower and upper critical limit values for HLBF (mean ± 2SD) were 3.5 and 40.5 cm, respectively. The population-based reference values for the SRT in male adult amateur soccer players are presented in Table 3.

TABLE 2

TABLE 2

TABLE 3

TABLE 3

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Player Characteristics Associated With SRT Scores

Player height was negatively correlated with SRT scores (ρ = −0.132, P = 0.005), whereas BMI was positively correlated with SRT scores (r = 0.114, P = 0.016). Players with a history of anterior cruciate ligament (ACL) surgery had a higher SRT score (mean 7.6 cm) than players without such a history (P < 0.001). Age, weight, soccer experience, leg dominance, field position, and previous hamstring injury were not associated with HLBF. There was a difference in BMI between the “very low HLBF” and “high HLBF” groups (Δ −1.36, P = 0.045) and for “history of ACL surgery” [χ2 (4, n = 422) = 25.424, P = 0.000].

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DISCUSSION

This study investigated the HLBF of male adult amateur soccer players, with a view to establishing population-based reference values for the SRT and to determining whether player characteristics are associated with SRT scores.

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Population-Based Reference Values

The mean SRT score was 22.0 cm, and normal values ranged from 13.0 to 31.0 cm in male adult amateur soccer players. These soccer players had a substantially lower flexibility than other athletes.4,5,41 Using the same protocol, Ayala et al5 found SRT scores of 35.9 (±10.1) cm and 38.1 (±9.7) cm among 243 recreationally active young adults (mean age 21 years). Soccer training reduces muscle flexibility in both the short and long terms,42 and, because our participants had played soccer for an average of 18 years, this could explain why they had lower SRT scores than the recreationally active young adults described by Ayala et al.5 Moreover, the study population of Ayala et al contained both men and women and it is recognized that women generally have higher SRT scores than men.8 The SRT scores of Spanish male professional futsal players were reported as 44.1 (±7.8) cm and 42.4 (±7.5) cm.1 The difference in SRT scores between these studies and our study might be due to more extensive stretching protocols during the training sessions of professional players, which could increase their overall muscle flexibility.31 Furthermore, unlike in our study, in the other studies participants followed a 5-minute warm-up and stretching protocol before testing. This could have affected hamstring flexibility, because it has been shown that 120 to 150 seconds of stretching results in changes in the viscoelastic properties of muscles that last 20 minutes.43,44 Therefore, most SRT protocols, including the original protocol by Wells and Dillon, do not recommend a warm-up before testing and this study adhered to these guidelines.3

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Associations With Player Characteristics

To our knowledge, no previous study has reported associations between HLBF and player characteristics. We found HLBF (SRT score) to be significantly associated with the height of adult soccer players. Our results show that taller players have lower hamstring flexibility than shorter players. However, the methodology of the SRT might have contributed to this correlation because of differences in the proportional length of the arms and legs, as tall adolescents with longer legs relative to their arms have a poorer performance on the SRT.45 Clinicians could therefore consider using a modified version of the classical SRT protocol to establish a relative zero point for each person, thereby solving this methodological problem of tall players.45

We also found HLBF to be significantly correlated with a history of ACL surgery, with SRT scores being substantially higher in players who had undergone surgery. In contrast, Ekstrand and Gillquist29 reported no difference in lower extremity muscle tightness between players with and without soccer injuries in the previous year. In their study, Ekstrand and Gillquist analyzed all knee injuries but did not report additional analyses for players with a history of ACL surgery. In players with recent ACL surgery, this increased flexibility might be a result of rehabilitation during which extensive (hamstring) stretching exercises are combined with a period without soccer. Unfortunately, we do not know whether a hamstring tendon autograft was used for ligament reconstruction, because this could potentially help explain the change in hamstring length or flexibility in this subgroup.46 However, because ACL surgery should, theoretically, not directly influence HLBF, this finding suggests that ACL surgery influences SRT test scores in some other way. In conclusion, the normal range of SRT scores presented here should not be applied to players with a history of ACL injury.

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Methodological Considerations

The main strength of this study is the large representative population of 449 soccer players, all playing at the same amateur level, with similar training and competition loads. All player characteristics, such as age, field positions, and injury history, were well represented among the players. Moreover, all players performed the SRT following a standardized, easily executed protocol with the same measuring device, for which the members of medical staff of the teams had received identical instructions. The test protocol used in this study is simple and requires little skill or training, both with regard to test administration and data interpretation.1 This enabled a team of players to be tested in a short time, which increases the practical usability of this test for both research purposes and in the field. This supports the representativeness and relevance of the reported population-based SRT reference values.

A potential study limitation is the lack of a criterion standard. Several different tests to measure hamstring flexibility or HLBF are available, such as the knee extension angle, sacral angle, straight leg raise, toe touch test, and different versions of the SRT.4,6 However, no criterion standard has yet been established and these tests do not possess sufficient concurrent validity to assume that they each measure solely hamstring flexibility or HLBF.1,6 In this study, we chose to measure flexibility with the classical SRT, which measures a combination of hamstring and lower back flexibility.3

It has been argued that the SRT score may be influenced by other anthropometric and physical factors,4,6,11 such as limb and trunk length, gastrocnemius length, and flexibility of the shoulders, spine, and ankles. We did not correct for these factors, because the test was performed in a way that is clinically and easily applicable on the soccer field. The SRT reference values we determined represent statistically determined limits for HLBF in soccer players. Given that the standard sit-and-reach box has a range of 0 to 50 cm, a normal range of 13.0 to 31.0 cm leaves more room for distinguishing between players with a high than low HLBF. In total, 3.3% of players scored 0 cm on the SRT in this study and these individuals may be at risk of hamstring injuries because of a limited HLBF.

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Future Research

Future research will have to determine whether the SRT reference values can indeed identify players at increased risk of hamstring injuries due to reduced flexibility. If this is the case, then the reference values can be further refined to identify not only players with very poor and very high flexibility, but also the intermediate categories of flexibility with differing injury risk.

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PRACTICAL APPLICATIONS

Because a reduced HLBF is often suggested to be a modifiable intrinsic risk factor for soccer injuries and diminished performance, identification of players with reduced HLBF is essential.34,47–49 The SRT is a preferable test for clinicians to measure HLBF because it is reliable, quick, and simple to perform and easy for group measurements in the field setting.11,12 Normal values of the SRT for male players— the largest subgroup in soccer— provide a basis for targeted injury prevention or performance-enhancing strategies.

This study provides population-based reference HLBF values (measured with the SRT) for male amateur soccer players as follows: “very low” (<3.5 cm), “low” (3.5–12.5 cm), “normal” (13.0-31.0 cm), “high” (31.5-40.5 cm), and “very high” (>41.5 cm). With a mean SRT score of 22.0 cm, male adult amateur soccer players have a lower HLBF than other groups of sportsmen. Coaches and practitioners should be aware of population-specific differences when using the SRT for diagnostic purposes or to assess injury risk and/or performance and remember that the HLBF references values are not appropriate for players with a history of ACL injuries.

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REFERENCES

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Keywords:

sit-and-reach test; hamstring; flexibility; reference values

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