Adequate flexibility of the hamstring muscle is basic for the correct functioning of the musculoskeletal system, mainly because of its dominance in postural activity and the tension continually experienced by this muscle group.1
Shortening of the hamstring muscles has repercussions on the lumbar spine and pelvis,2–8 as well as leading to a greater predisposition to leg lesions, mainly due to the resulting muscle decompensation,9,10 giving rise to a high incidence of these lesions and days off sports.11–15
The practice of some sports, like soccer, is associated with an increased prevalence of the short hamstring syndrome (SHS).9 It would therefore be interesting to review stretching programs, particularly in young children and adolescents, as the SHS reduces flexibility and has possible consequences.11,16–19
Many studies have analyzed techniques for hamstring muscle flexibility. These techniques can be dynamic or static, the latter being more effective.20–25 Two of the static methods involve either active or passive stretching, with varying results between 1 and the other.11,26,27 Several studies have examined different techniques for proprioceptive neuromuscular facilitation versus static techniques, although the differences were not significant.28–31 No uniform agreement exists about the duration of the stretching; some studies reported greater effectiveness after 15 seconds,32–34 and others after 30 seconds.31,35 In addition, no significant differences have been found concerning the stretching position.32
However, we have only found studies involving samples of 12 or fewer adults that examined the effectiveness of stretching versus stretching with the application of electrical stimulation (stretching + TENS) (Table 1). Accordingly, we undertook a study to assess and compare the effectiveness of an 8-week program of active stretching (AS) versus stretching + TENS in young soccer players. We used AS because it is the technique that best respects the neutral lumbar lordosis position and achieves the greatest gains in joint range. We gave 1 session per week for 8 weeks as this combination produces modifications in viscoelasticity,37,44 with a duration of 15 seconds each stretch, as recent studies have considered this time to be adequate to gain muscle flexibility.6,32,33
Young federated soccer players in the town of Jumilla, in the region of Murcia (Spain). Jumilla has a population of 25 711 inhabitants,45 and it is usual for children and adolescents to play soccer there.
Study Design and Participants
Randomized, controlled, single-blind parallel clinical trial with 3 arms and a 2-month follow-up. Assignment ratio: 1:1:1. All the study boys were aged 10 to 16 years, registered with the Soccer Federation of the Region of Murcia, and members of the Jumilla Municipal Soccer School (JMFS). In addition, they had to have a result in the straight leg raise (SLR) test <70 degrees and/or >15 degrees in the passive knee extension (PKE) test. The children stated that they were not afraid to receive electrotherapy,46 had not had any lumbar pain during the previous 3 months, and had no current lesion. Children were excluded if they had any acute muscle or ligament lesions, recent or nonconsolidated fractures, ligament hyperlaxity, Marfan syndrome or functional shortening.
To obtain these subjects, we contacted the management of the JMFS and then explained the study to the soccer trainers. We then explained it to the children and their parents who attended the training sessions and were interested in the project, resolving any doubts. After this, the informed consent form was sent to the parents. This was followed by the initial evaluation phase (pretest), with the evaluation taking place on those days when the children had no training or extracurricular activities. The final sample comprised those children who wished to take part and who had SHS. This whole procedure was undertaken during April 2012.
Each participant was randomly assigned to one of the 3 groups: (1) stretching + TENS, (2) AS, or (3) control. The randomization was done with Epidat 3.1. The participants were able to change the day of their sessions if necessary or if they failed to attend on the specific day. Any participant who did not receive all 8 treatment sessions was excluded from the final analysis; the rest were reassessed after 8 weeks (posttest).
(1) Stretching + TENS (see Video, Supplemental Digital Content 1, https://links.lww.com/JSM/A65, which demonstrates the technique for stretching + TENS)—the participants underwent AS with low-frequency electrical stimulation. The participant lay on the examination table in a supine decubitus position and adhesive electrodes were attached. Two 5 × 5-cm electrodes were placed on the ends of the belly of the femoral biceps, and 2 electrodes on the ends of the semitendinosus and semimembranosus muscles.47 The instrument used was the TENS-MED 931 (ENRAF-NONIUS) that has 2 leads (1 for each 2 electrodes). The participant was then asked to flex the hip 90 degrees until the end of the test. To maintain this position, the physiotherapist used the inclinometer at the start of the test and made all necessary corrections during the test. This was followed by the participant extending the knee until he noticed tightness in the back of the thigh or behind the knee. At this point, the intensity of the low-frequency electrical stimulation was increased until there was contraction of the hamstring muscles. The electrical current was an alternating symmetrical biphasic rectangular waveform and was present throughout the whole exercise. When the participant ceased noting the tightness, he was asked to extend the knee again until it was again tight, at which point, the electrical stimulation was again intensified. This process was repeated until such time as the feeling of tightness continued even after increasing the intensity. As a general rule, this procedure is satisfactory after 3 repetitions and does not usually last more than 45 to 60 seconds.38,40,48 The technique is performed on both legs. Muscle contraction was achieved using transcutaneous electrical nerve stimulation (TENS) at a frequency of 50 Hz with impulse trains of 1 second. A towel was used to keep the leg raised and achieve knee extension with isotonic contraction of the quadriceps. The towel was placed around the sole of the foot and its only purpose was to hold the leg up, so that the subject did not have to exert any effort.
(2) AS [see Videos, Supplemental Digital Contents 2, (https://links.lww.com/JSM/A66), 3 (https://links.lww.com/JSM/A67), and 4 (https://links.lww.com/JSM/A68), which demonstrate the stretching exercises] (Figure 1)—these participants performed 3 specific exercises of static AS of the hamstring muscles, maintaining the maximum tightness that was tolerable without pain for 15 seconds.17,47 These exercises were performed twice, with a total of 6 repetitions.1,49 The rest period between stretching was 15 seconds. All the participants received precise instructions about each stretching exercise on the day before the stretching session.
(3) Control group—these participants continued their usual clinical practice of stretching under the direction of their respective trainers.
Following the final evaluation, all the participants in groups 1 and 3 received a personal demonstration of the same exercise of AS of the hamstring muscles as that performed in group 2, to be able to continue with this program independently at home or during their soccer practice.
Groups 1 and 2 underwent 1 session weekly for 8 weeks, that is, a total of 8 treatment sessions.41 These sessions were given in the Jumilla Physiotherapy Office.
Variables and Measurements
Differences achieved in range of joint movement 8 weeks after starting the study (posttest minus pretest). The joint movement parameters measured were SLR, PKE, and toe-touch test (TT). The participants were categorized according to the results of these tests. A patient was considered to have improved his joint movement if the progression was above the mean of all the subjects involved. This grouping was decided as we were unable to find any reference defining improvement. For the 3 tests, improvement was considered to be a greater SLR and TT, and a lower PKE.
The test procedures were measured as follows. The SLR was measured using a Uni-level inclinometer (ISOMED Inc, Portland, Oregon) on the tibial crest, starting from a horizontal position of 0 degrees (Figure 2).43 One evaluator took the measurement, whereas the other controlled for pelvic rotation, hip abduction/adduction, hip rotation, knee flexion, ankle, and foot position; as well as placing a towel in the lumbar area to prevent pelvic retroversion. The PKE was measured using a 30-cm long-arm goniometer. The result was taken according to the method of Norkin and White,50 considering 0 degrees as complete knee extension (Figure 3). This measurement was controlled as in SLR. In addition, the virtual knee axis corresponded with the goniometer axis. The TT was first described by the American Academy of Orthopedic Surgeons in Chicago in 196528; it is also known as the Cureton test, Krauss and Hirsland test, or the “toe-touch” test (Figure 4). The TT measurement was controlled by 1 evaluator watching to make sure it was done correctly, whereas the other took the measurement. No warming-up was allowed before these tests. All the evaluations were done under the same ambient conditions, and at the same time and day of the week as their respective pretest evaluations.
All the participants were evaluated bilaterally before starting the interventions (pretest). At the end of the 8 weeks of treatment, given as 1 session per week, all the participants were again assessed 5 minutes after the last stretching (posttest). The same researcher directed the stretching sessions in group 2, with a maximum of 5 children per session, to obtain correct supervision and performance of the stretching, as well as controlling the correct development in the intervention group 1. Two physiotherapists (blinded) with over 10 years work experience, especially in sports physiotherapy, and duly trained undertook the pretest and posttest measurements.
All the participants received instructions not to undertake any stretching other than that recommended by the trainers and that given by the physiotherapist, as well as not to modify the intensity and frequency of physical exercise during the study.
Secondary variables were laterality, body mass index (BMI), age, number of years federated, and degree of shortening. These variables were measured to verify the correct randomization. The data about age, laterality, and number of years federated were collected by personal interview with each participant. The BMI [weight (kg)/height2 (m)] was calculated using calibrated devices.
Final sample was 51 boys. Differences were contrasted in independence (qualitative improvement: χ2 test) and comparison of mean values [quantitative improvement: analysis of variance (ANOVA)] to evaluate the SLR interventions. Ten children were randomly chosen to obtain an estimation of the contrast parameters considered. In both legs, with α = 5%, we obtained a power for both contrasts of almost 100%.51
To describe the variables, we used frequencies (absolute and relative) for the qualitative variables. For the quantitative variables, we used mean values plus standard variations (normal variables) and medians plus interquartile ranges (non-normal variables).
To verify the homogeneity of the randomization, we used χ2 tests (Pearson or Fisher), ANOVA (normal variables), and median comparison (non-normal variables). The same procedure was used to verify that the participants who did not finish the study had similar characteristics to those who remained.
The improvement was analyzed quantitatively using ANOVA (normal variables) and comparison of medians tests (non-normal variables). In the event of significant differences in the ANOVA models, we used the Tukey post hoc test to determine where the differences existed. In the case of non-normality, we made box charts.
Qualitatively, we assessed the improvement by calculating clinical relevance indicators, such as relative risk (RR), absolute risk reduction (ARR), relative risk reduction (RRR), and number needed to treat (NNT).
To check for normality, we used the Shapiro–Wilk/Kolmorov–Smirnov test (</≥30 items of data). All the analyses were made for both legs in all the outcomes and α = 5%. The confidence intervals were calculated for the most relevant parameters. The analyses were done using IBM SPSS 19.0 (Armonk, New York).
The study posed no risk to the participants and a direct benefit was expected, since the results could reflect improved hamstring flexibility in the children with SHS. The study was undertaken in accordance with the principles of the Declaration of Helsinki and complied with the European Union norms of good clinical practice. The participants, their parents, their coaches, and the Soccer Federation were informed of the nature of the study and the data needed. In addition, the participants undertook to attend all the programmed sessions. The study was approved by the Ethics Committee of Miguel Hernández University. The study was registered in the clinical trials database (clinicaltrials.gov), with registry number NCT02100241.
Figure 5 shows the number of participants in each study phase. Of the 85 children who wished to participate, 23 were excluded as they did not fulfill all the inclusion criteria. Thus, the study comprised 62 children who underwent the randomization process. After 2 months, 11 participants abandoned the study [3 in group 1 (2 fractures and 1 withdrew from the study), 5 in group 2 (all withdrew), and 3 in group 3 (all withdrew)].
Table 2 shows the descriptive characteristics of the initial sample, as well as the homogeneity of the groups. No significant differences (P > 0.05) were seen between the groups. Table 2 also shows that the participants who abandoned the study (n = 11) had similar characteristics to those who remained (n = 51) (P > 0.05).
Table 3 shows the quantitative analysis. Comparison between groups 1 and 2 showed a significant mean reduction of 5.5 degrees in the right SLR. For the PKE, we found a significant mean increase of 10.2 degrees in the right and 6.2 degrees in the left legs. No significant differences were seen in either the TT or the left SLR.
Comparison between groups 1 and 3 showed significant mean differences in all the tests. In the SLR, the mean reduction was 12.3 degrees in the right and 10 degrees in the left. For the PKE, there was a mean increase of 12.9 degrees (right) and 8.6 degrees (left), and for the TT, the mean reduction was 8.8 cm.
Comparison between groups 2 and 3 showed a significant mean reduction in the SLR (right, 6.8 degrees; left, 5.8 degrees), and for the TT, a mean reduction of 6.7 cm. No significant differences were seen in the PKE.
The patients who improved their joint movement experienced: left SLR, 22 (43.1%); right SLR, 25 (49.0%); left PKE, 25 (49.0%); right PKE, 21 (41.2%); and TT, 23 (45.1%).
Table 4 shows the clinical relevance analysis. Comparison between groups 1 and 2 showed significant variables for the RR (0.35-0.38), RRR (0.62-0.65), ARR (0.32-0.39), and NNT (3-4). The SLR and left PKE were not significant.
Comparison between groups 1 and 3 showed all the variables to be significant: RR (0.12-0.28), RRR (0.72-0.88), and ARR (0.60-0.83). All the NNT had a value of 2.
Comparison of the significant differences between groups 2 and 3 showed the following: RR, 0.44 to 0.53; RRR, 0.47 to 0.56; ARR, 0.40 to 0.56; and NNT, 2 to 3. The right PKE was not significant.
In general, all the outcomes in this study showed a better improvement in group 1 compared with the other 2 groups. In turn, group 2 also experienced generally better improvements than group 3. A search of the literature revealed 2 studies comparing the effectiveness of stretching + TENS.22,38 These articles, however, had very different study designs and study populations in comparison with our study. These studies found a positive mean increase of about 5 degrees in the PKE after application of electrical stimulation instead of AS. We may have obtained a significant improvement with the use of electrical stimulation as the other studies were undertaken in reduced samples, as well as being undertaken in adults and non-sportspersons. Other studies have evaluated different types of stretching that cannot be compared with our study (Table 1).
During the follow-up, there were 2 fractures, both in group 1 (Figure 5). As this number was very small, we consider it due to chance. Nonetheless, this should be corroborated with larger studies.
When we started the study, we expected to find that the stretching + TENS contributed significantly to the improvement in the state of extensibility of the hamstring muscles in the children with SHS. However, an unexpected finding was that stretching + TENS was not significantly different to AS in the left SLR and the TT. This may be because the dominant leg for most of the children was the right, with this leg being more often stretched when kicking than the left leg. Regarding the TT, although, this dominance may not have been influential as this test not only evaluates hamstring extensibility but also involves multiple joints and muscle chains, and it may be altered by the different leg length.
Physiotherapy services should play an active part in the fight against SHS in children, integrating techniques to minimize this problem. Our results indicate that the use of stretching + TENS is a relevant technique for habitual clinical practice that could be systematically included for children who play soccer and who have this syndrome. We base this suggestion not only on the significant differences found between the different interventions but also on the excellent results as analyzed with the indicators of clinical relevance.
Strengths and Limitations of the Study
The main strength of the study concerns the lack of other studies analyzing the repercussion of stretching + TENS in children with the SHS who play soccer. This is therefore a pioneering study in the analysis of this technique in this particular population. Additionally, the results are clinically useful for physiotherapy offices. Another important point is the high statistical power in the analyses made, which enables us to infer our results with a minimum possibility of error. Finally, this study was a randomized, single-blind, clinical trial, so that, all the possible systematic errors were minimized as the 3 groups were equally divided.
Regarding study limitations, we have to consider that these results are limited to children aged 10 to 16 years who play soccer.
We believe that this study is ideal to compare the technical differences examined. In summary, in children who play soccer who have the SHS, stretching + TENS results in greater improvement than AS alone, and these 2 are both better than the conventional stretching commonly performed in training. We should also not forget to recommend stretching of all the muscle groups, concentrating on the hamstring muscles in children with these characteristics. Thus, one of our maxims is to raise awareness of the importance of stretching so that it becomes a habit.
This study was conducted with the involvement of children belonging to the Jumilla Municipal Football School. The authors thank the physiotherapists, M. D. Bleda and A. Valero for doing all the tests and their support, as well as Ian Johnstone for the translation and his helpful comments.
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