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Eccentric strengthening effect of hip-adductor training with elastic bands in soccer players: a randomised controlled trial
  1. Jesper Jensen1,
  2. Per Hölmich1,
  3. Thomas Bandholm2,
  4. Mette K Zebis3,
  5. Lars L Andersen4,
  6. Kristian Thorborg1
  1. 1Arthroscopic Centre Amager, Copenhagen University Hospital, Copenhagen, Denmark
  2. 2Clinical Research Center and Departments of Orthopedic Surgery and Physical Therapy, Copenhagen University Hospital, Hvidovre, Denmark
  3. 3Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark
  4. 4National Research Centre for the Working Environment, Copenhagen, Denmark
  1. Correspondence to Dr Kristian Thorborg, Arthroscopic Centre Amager, Copenhagen University Hospital, Italiensvej 1, Copenhagen S 2300, Denmark; kristianthorborg{at}hotmail.com

Abstract

Background Soccer players with weak hip-adductor muscles are at increased risk of sustaining groin injuries. Therefore, a simple hip-adductor strengthening programme for prevention of groin injuries is needed.

Objective We aimed to investigate the effect of an 8-week hip-adductor strengthening programme, including one hip-adduction exercise, on eccentric and isometric hip-adduction strength, using elastic bands as external load.

Methods Thirty-four healthy, sub-elite soccer players, mean (±SD) age of 22.1 (±3.3) years, were randomised to either training or control. During the mid-season break, the training group performed 8 weeks of supervised, progressive hip-adduction strength training using elastic bands. The participants performed two training sessions per week (weeks 1–2) with 3×15 repetition maximum loading (RM), three training sessions per week (weeks 3–6) with 3×10 RM and three training sessions per week (weeks 7–8) with 3×8 RM. Eccentric hip-adduction (EHAD), isometric hip-adduction (IHAD) and isometric hip-abduction (IHAB) strength, and the IHAD/IHAB ratio were measured assessor-blinded preintervention and postintervention, using reliable hand-held dynamometry procedures.

Results In the training group, EHAD strength increased by 30% (p<0.001). In the control group, EHAD strength increased by 17% (p<0.001), but the increase was significantly larger in the training group compared with the control group (p=0.044). No other significant between-group strength-differences in IHAD, IHAB or the IHAD/IHAB ratio existed (p>0.05).

Conclusions 8 weeks of hip-adduction strength training, using elastic bands, induce a relevant increase in eccentric hip-adduction strength in soccer players, and thus may have implications as a promising approach towards prevention of groin injuries in soccer.

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Introduction

Hip and groin injuries are a major problem in soccer players, constituting up to 20% of all injuries.1 Adductor-related groin injuries are the second-most common type of muscle injury in soccer,2 and the most common injury in the hip/groin region, constituting 63% of all injuries to this region.3 With respect to this, decreased muscle strength of the hip-adductors has been shown to be a risk factor for groin injuries,4–7 which is why strengthening of the hip-adductors seems relevant in the prevention of adductor-related groin injuries.

Soccer players have increased eccentric strength in the hip-abductors, but not in the hip-adductors, compared with activity-matched controls.8 This indicates that soccer play may substantially increase eccentric hip-abduction strength, but not hip-adduction strength. The hip-adductor muscle–tendinous complex is stressed substantially in soccer, especially during kicking,9 ,10 and increased muscle–tendinous strength in the hip-adductors may protect relevant structures in the groin when exposed to large eccentric forces. Also, increased muscle strength may contribute to increase the energy absorption in the tissues,11 possibly decreasing stresses at the tendons and the insertion sites. Therefore, increasing the hip-adduction strength may decrease the stress put on the hip-adductor muscle–tendinous complex, and prevent overuse injuries and acute tears. Some support for this notion exists, as strength training with an eccentric component is effective in the prevention and treatment of muscle injuries in soccer,12–14 and in the treatment of different tendinopathic conditions.15–17

Strength training using elastic bands is often used in rehabilitation of sports injuries, and the simplicity of this type of strength training makes it possible to perform it at the training field, in connection with everyday soccer training. Importantly, strength training using elastic bands seems as effective as more traditional types of strength training.18–21 However, the effect of strength training performed with elastic bands on hip-adductor strength gains has not been investigated earlier.

Therefore, the purpose of the present study was to investigate the effect of 8 weeks of hip-adduction strength training, performed with elastic bands, on hip-muscle strength in healthy soccer players. We hypothesised that an eccentric and isometric hip-adduction strength increase would be induced.

Methods

Participants

Thirty-four healthy male soccer players gave their informed consent in accordance with the declaration of Helsinki, and participated in the study. The study was conducted in the mid-season break, between January and April 2011. The participants were recruited from five sub-elite soccer teams, with a soccer training frequency of 2–4 sessions a week, and 1–2 weekly matches. Included participants had to be at least 18 years old. A participant was excluded if he, in the preceding year before the study, had had an injury in the low back, in the lower extremity and/or in the hip/groin with a duration of more than 6 weeks, and/or groin pain during soccer play with a duration of more than 1 week, which resulted in a loss of soccer training/game within the last 3 months before the study was initiated, and/or if the participant in the preceding month had had hip/groin pain, regardless of the duration. Participants, who had performed systematic strength training of the hip-adductors for more than once a week in the preceding 6 months before the study, were also excluded.

The participants were stratified by club to account for the club-differences in playing level and training frequency, and were randomly assigned to either a training group or a control group (figure 1). The allocation process was blinded, and randomisation was performed by drawing lots.

Because the clubs started their mid-season soccer training on different dates, the pretesting and allocation of the participants were done for each club separately, in accordance with the club's starting date. If an uneven number of participants were included in one club, one of the two groups would have an extra lot (participant). Which group (training or control) that was to have the extra lot was decided by coin toss, and the next time an uneven number of participants was present in a club, the other group (training or control) would have the extra lot and so forth. This procedure was performed to avoid large differences in group size. Ethical approval was obtained from the Danish Ethics Committee of the Capital Region (ID: H-4-2010-119), and the Danish Data Protection Agency (ID: 2007-58-0015), and the trial was registered at clinicaltrials.gov (ID: NCT01318616).

Training intervention

All training sessions in the training group were supervised by a physiotherapist, who ensured that the exercise was performed correctly and according to the protocol. The training sessions were performed as group training in each club in connection with the normal soccer training sessions. The exercise was performed with both legs. The exercise consisted of dynamic and isometric hip-adduction in a standing position, and was performed with elastic bands (Thera-Band) as external loading. In the standing position, the elastic band was attached to a fixation point, and the participants had the elastic band around the ankle of the leg nearest to the fixation point (the training leg). The pelvis was in the horizontal plane; the upper body was straight and fixated by holding on to a stationary object with both hands. This was the starting position (figure 2A).

Figure 2

(A) The intervention exercise, starting and ending position, full hip-abduction position. (B) The intervention exercise, full hip-adduction position.

The hip-adductor strengthening programme, constituting the training intervention, is documented and described below concerning load magnitude, number of repetitions, number of sets, rest in between sets, number of sessions per week, duration of the intervention period, fractional and temporal distribution of the contraction modes per repetition, and duration of one repetition, rest in between repetitions, time under tension, range of motion and anatomical definition of the exercise, as suggested by Toigo and Boutellier.22

The exercise was performed in full range of motion from hip-abduction to hip-adduction. A repetition was performed from the starting position (figure 2A), where a 3-s concentric hip-adduction movement—starting from full hip-abduction—was performed. The concentric hip-adduction was performed until the distance between the training leg and the standing leg was one foot width in the frontal plane, and at the same time, the training leg was a half foot length behind the standing leg in the sagittal plane (figure 2B). In this position, 2 s of isometric hip-adduction was performed, followed by 3 s of eccentric hip-adduction, until full hip-abduction range of motion was reached. Before the next repetition, a 2-s pause in maximal hip-abduction was performed. Time under tension was supervised by the physiotherapist. Throughout the exercise, the training leg was extended and the standing leg was slightly flexed at the hip, knee and ankle. One training session lasted 10–15 min, and consisted of three sets performed until contraction failure for each leg, shifting between legs. One-minute rests separated each set. Both legs were trained, since the strength training intervention most likely activates both the ipsilateral and contralateral hip-adductors during training of one leg, such as seen in the contralateral hip-adductors during kicking.9 ,23 Two training sessions were performed per week (weeks 1–2) with a relative load of 15±2 RM (repetition maximum), three training sessions per week (weeks 3–6) with a relative load of 10±2 RM and three training sessions per week (weeks 7–8) with a relative load of 8±2RM (table 1). A minimum of 24 h separated each training session. An external load corresponding to a relative load of 10 RM, which was used in the present study, is typically defined as heavy strength training (HSR).24 The physiotherapist in each club determined the correct RM load at the first training session and adjusted the load at every training session on a set-by-set basis, to reflect the intended RM load. The initial RM determination and subsequent adjustments were done with combinations of different elastic bands (Thera-Band colours: blue, black, silver and gold) and by altering the distance from the fixation point to the standing leg. In this way, an increased relative load could be obtained by using a heavier elastic band, increasing the distance from the fixation point to the standing leg or by adding an extra elastic band.

Table 1

Duration of intervention, external load, number of repetitions, sets and training sessions during the intervention period

Control group

In the control group, participants were instructed not to perform systematic strength training of the hip-adductors during the study, and to continue to play soccer as usual.

Outcome measures

The primary outcomes were maximal eccentric (EHAD) and maximal isometric (IHAD) hip-adduction strength. As a secondary outcome, maximal isometric hip-abduction (IHAB) strength was measured to obtain the ratio between maximal isometric hip-adduction and hip-abduction strength (IHAD/IHAB ratio).

The strength measurements were performed in an examination room using a standardised setup, consisting of a transportable examination table and a hand-held dynamometer (HHD) (Powertrack II Commander JTECH Medical, Salt Lake City, Utah, USA). Strength measurements were only performed on the dominant leg, defined as the leg preferred for kicking a ball. The maximal strength measurements were performed 1 week preintervention (pretest) and 3–6 days postintervention (post-test), to avoid delayed onset muscle soreness (DOMS) during testing. Moreover, the measurements were planned so that no participants were involved in game play or strength training for the lower extremity the day before. A single assessor, blinded to group allocation, performed all the strength measurements with HHD, a valid measure of muscle strength.25–27 The HHD procedures, used in the present study, have been reported to have substantial intratester, interday reliability and no systematic test–retest bias.8 ,28 The strength measurement procedures have previously been reported in detail.8 ,28 They consisted of measurements of IHAD and IHAB strength in the supine position using a make test, and EHAD strength in the side-lying position using a break test, as these test positions have shown to be the most reliable.8 ,28 The order of testing was the same for all participants and the experimental protocol lasted 30 min for each participant.

For each of the isometric strength tests (IHAD and IHAB strength), four maximal contractions, separated by 30 s pauses, were performed to reduce a possible learning effect.28 The highest value obtained in the four contractions was used for comparison as this has shown to be reliable with a SE of measurement (SEM) of 6% for the IHAD strength test, 3% for the IHAB strength test and 8% for the IHAD/IHAB ratio.28 If the last contraction elicited the highest value, an additional contraction was performed. For the EHAD strength test, maximal contractions were separated by 60 s pauses, and were performed until a force plateau of <5% between two consecutive measurements was attained. A mean of these values was used for comparison as this has shown to be reliable with an SEM of 6%.8

Maximal hip strength for all hip actions was expressed as torque per kilo body weight (Nm/kg), using the external lever arm and the body weight of each participant. The external lever arm was measured from the most prominent point of the anterior superior iliac spine in the supine position to 5 cm proximal of the proximal edge of the medial malleolus.

Besides the strength measurements, the physiotherapist at each club recorded training compliance, DOMS and perceived loading at all training sessions in the training group, and number of soccer training sessions and games in both groups. DOMS was quantified using a numerical rating scale, and perceived loading was quantified using the Borg CR10 scale.18

Statistical analyses

As the training intervention consisted of concentric, eccentric and isometric hip-adduction, we expected that both EHAD and IHAD strength would increase in the training group. Concentric hip-muscle strength cannot be measured using HHD. Since the study had an exploratory character, we consequently chose both EHAD and IHAD strength as primary outcomes. Only the dominant leg was analysed to reduce the number of statistical analyses. A strength difference between the groups of 15% was chosen as a realistic and relevant minimal real difference (MIREDIF) for both EHAD and IHAD strength.29 Sample size estimation was performed using the equation n=2×(Ζα+Ζβ)2 × (SD/MIREDIF)2, with 80% power and a significance level of 5%. The SD (standard deviation) was based on previous studies.8 ,30 For the EHAD strength, eight participants were required in each group and for the IHAD strength, 14 participants were required. To account for a potential drop-out of 20%, 34 subjects were included.

Data were analysed using SPSS V.18.0. Variability around mean values is presented as ±1 SD. The dependent variables demonstrated a normal distribution (Shapiro-Wilk W-test), and parametric statistics were applied. Paired and unpaired Student's t-tests were used to determine statistically significant (p<0.05) within- or between-group differences, respectively. Analyses were performed per-protocol (n=13 for the training group and n=11 for the control group; figure 1). Intention-to-treat analysis was not performed, since we were interested in the potential efficacy of the exercise, and therefore only in participants, who complied with the specific intervention and test setup exactly as planned and defined prestudy execution.31 Specifically, only data from participants in the training group having a compliance of 70% or more were analysed. Moreover, participants in both the training and the control group were not analysed if they sustained an injury to the hip/groin region, or any other injury that kept them from participating in training and/or game (figure 1), as previous research has shown that hip-muscle strength is affected in individuals with lower-limb injury.4 ,32 Also, participants were not analysed if they experienced hip and/or groin pain during testing (figure 1), as this has been shown to influence hip-adduction strength.30 A person, blinded to name and group-allocation, excluded participants from the analyses, according to the criteria aforementioned. Grubb's test was used to detect any outliers within the groups pretesting and post-testing, but no outliers existed.33

Results

Compliance, DOMS, perceived loading and complications

The study profile is shown in figure 1. Of the 22 training sessions planned for each participant during the 8 weeks of training, the total compliance for all participants (n=13) was 19.4 (±1.4) sessions. Eight participants reported DOMS in the hip-adductors at the first training session, likely as a result of the pretest. DOMS was reported throughout the intervention period, with no significant differences between the dominant and non-dominant leg (p=0.570; figure 3A). Average perceived loading during training was 7.3 (±0.5) Borg CR10-points for the dominant leg and 7.5 (±0.4) Borg CR10-points for the non-dominant leg, throughout the intervention period. No significant differences existed between the dominant and non-dominant leg (p=0.394; figure 3B). One participant experienced low-back pain at two training sessions. This could be avoided in the following training set by modifying the training position slightly. One participant experienced groin pain at one training session. This could be avoided by lowering the resistance in the following set. These minor complications, apparently related to the training intervention, were the only reported complications during the intervention period.

Figure 3

(A) Delayed onset muscle soreness (DOMS) registered for the training group. (B) Perceived loading registered for the training group. RM, repetition maximum.

Drop-outs

Four participants in the intervention group were not included in the analyses (figure 1). Two participants were lost to follow-up because of club transfers during the intervention period. Two participants were excluded from the analyses because of an exercise compliance of <70%.

Six participants in the control group were not included in the analyses (figure 1). Two participants were lost to follow-up because of club transfers during the intervention period. Two participants were excluded from analyses because of an ankle injury, which kept them from participating in training and/or game for more than 3 weeks. One participant was excluded from analyses because of groin pain during the postintervention test and one participant was not able to perform a maximal hip strength test because of an acute ankle injury (application of the HHD above the malleoli was too painful) sustained 2 days prior to the postintervention test.

Baseline analyses

The number of soccer training sessions during the intervention period was not different between groups (p=0.696), and neither was the number of games (p=0.267).

Effects of training

Within-group changes

In the training group, 8 weeks of strength training of the hip-adductors, using elastic bands as external load, increased EHAD strength by 30% from 1.93 (±0.4) Nm/kg at baseline to 2.46 (±0.3) Nm/kg after 8 weeks (p<0.001; figure 4); increased IHAD strength by 14% from 2.12 (±0.5) Nm/kg at baseline to 2.40 (±0.6) Nm/kg after 8 weeks (p=0.017; figure 5A) and increased IHAB by 17% from 1.67 (±0.3) Nm/kg at baseline to 1.94 (±0.3) Nm/kg after 8 weeks (p=0.001; figure 5B). In the training group, the IHAD/IHAB ratio was 1.27 (±0.2) at baseline and 1.24 (±0.2) after 8 weeks (p=0.518; figure 5C).

Figure 4

The maximal eccentric hip-adduction muscle strength (EHAD) in the training group (heavy strength training, HSR) and the control group (CON) before (black columns) and after (white columns) the intervention. * Denotes statistically significant (p<0.05) within-group difference. ** Denotes statistically significant (p<0.05) between-group difference.

Figure 5

(A) The maximal isometric hip-adduction muscle strength (IHAD), (B) the maximal hip-abduction muscle strength (IHAB) and (C) the IHAD/IHAB ratio in the training group (heavy strength training, HSR) and the control group (CON), before (black columns) and after (white columns) the intervention. * Denotes statistically significant (p<0.05) within-group difference.

In the control group, who did no strength training of the hip-adductors, EHAD strength increased by 17% from 1.86 (±0.3) Nm/kg at baseline to 2.16 (±0.3) Nm/kg after 8 weeks (p<0.001; figure 4), IHAD strength was 1.68 (±0.3) Nm/kg at baseline and 1.80 (±0.4) Nm/kg after 8 weeks (p=0.193; figure 5A), IHAB increased by 21% from 1.46 (±0.3) Nm/kg at baseline to 1.74 (±0.2) Nm/kg after 8 weeks (p<0.001; figure 5B) and the IHAD/IHAB ratio decreased by 10% from 1.17 (±0.2) at baseline to 1.03 (±0.2) after 8 weeks (p=0.044; figure 5C).

Between-group differences

The between-group difference for EHAD strength increase was 0.22 (±0.1) Nm/kg, corresponding to a 13% significantly greater EHAD strength increase in the training group (p=0.044; figure 4). No significant between-group differences were observed for IHAD (p=0.243), IHAB (p=0.840) or for the IHAD/IHAB ratio (p=0.152; figure 5A–C).

Discussion

The primary finding of the present study was that hip-adduction strength training, using elastic bands as external load, significantly increased maximal eccentric hip-adduction strength in the training group compared with the control group.

Eccentric hip-adduction strength training effect and prevention of muscle–tendon injuries

In the present study, EHAD strength increased 30% after 8 weeks of strength training. Since the control group had a 17% increase during the 8 weeks, around 13% of the EHAD-increase in the training group appear to be an effect of the strength training, which indicates that a relevant strength improvement has occurred. A 13% increase in EHAD strength is comparable with that reported by Mjolsnes et al,34 who found an 11% increase in eccentric strength of the hamstrings after 10 weeks of the Nordic Hamstring exercise, an exercise programme that subsequently has been shown to decrease the number of hamstring injuries and re-injuries in soccer, substantially.14 An increased EHAD strength may therefore be proposed to have a similar effect on adductor-related groin injuries. Previously, a randomised controlled trial (RCT) have shown that a comprehensive strengthening programme, supervised by physiotherapists, including focused hip-adductor strengthening is highly effective in the treatment of long-standing adductor-related groin pain.35 The preventive effect of a programme based on this treatment, supervised by the team coach, has also been investigated in an RCT and found a 31% reduction of groin injuries in a soccer population, however, not significant.36 The lack of a significant effect of this programme have been suggested to be the result of different factors, such as an insufficient sample size, and a possible lack of compliance of the soccer players to perform the programme as prescribed, regarding the intended frequency and intensity.36 To document the exact compliance and ensure correct performance of the hip-adductor strengthening programme, in the present study, the programme was supervised by a physiotherapist, in each of the participating clubs. In conjunction with the Borg CR10 data, DOMS data in the present study indicate that the hip-adductor strengthening programme was progressive in nature, which together with the high compliance (88%) substantiates why the exercise had such a large effect on EHAD strength, and consequently may be effective in the prevention of adductor-related groin injuries, if performed as prescribed. Compared with the Nordic Hamstring exercise programme,34 DOMS in the present hip-adduction strengthening programme, was somewhat worse and did not level off, indicating that the hip-adductor strengthening programme seems more progressive than the Nordic Hamstring exercise programme, where the absolute load is not adjusted on a set-by-set basis.34

The underlying physiological mechanisms, as to why strength training, including an eccentric component, may protect against muscle–tendon injuries are currently unknown.37 ,38 One theory suggests that increased muscle strength may contribute to increased energy absorption in the trained tissues,11 thereby possibly decreasing stresses at the tendons and insertion sites. With respect to decreased tendon stress, Kongsgaard et al16 have shown that heavy slow-resistance training, consisting of a substantial eccentric component, increases the cross-sectional area and density of human tendons.39 ,40 Furthermore, heavy slow-resistance training is clinically effective in treating patella tendinopathy, and seems to have a healing effect on the tendon by adding more fibrils.40 A third theory is that a shift in the optimum angle for torque generation, to longer hamstring muscle lengths, as a response to eccentric strength training, can protect against hamstring injuries and re-injuries.41

Strength increases as a response to resumed soccer play?

For the teams involved in the present study, the competitive season started in August and ended in June the following year, with a mid-season competition break from November until March/April. In this mid-season break, the teams had a soccer-free period of up to 2 months (November–January). The intervention used in the present study, started after this soccer-free period, at the same time as the teams had their first soccer training session in January 2011. Although no studies have investigated changes in hip strength of soccer players during periods with neither soccer training sessions nor any games, one could speculate that the 30% and 17% increases in EHAD strength found in the training and the control group, respectively, were strength gains, partly induced by starting the soccer training sessions and games in the intervention period. Especially, if a loss of hip-muscle strength occurs during the preceding soccer-free period. A study by Hortobagyi et al,42 where a 12% decrease in eccentric force was observed after 14 days of training cessation, has previously indicated that eccentric strength is specifically affected by training cessation, and possible hip-strength deconditioning could explain why a low level of off-season sports-specific activity has been shown to be a risk factor for groin injuries.43

Isometric hip-adduction strength gains?

The increased IHAB strength in both the training group and the control group indicates that soccer play induces strength gains. It has previously been indicated cross-sectionally that soccer play increases hip-abduction strength.8 This adaptation to soccer play seems to be muscle-group specific, as no changes in hip-adduction strength seems to occur in response to soccer play.8 In the present study, the within-group analyses indicate that only the training group had a substantial IHAD strength increase, and since both groups had an increase in IHAB strength, this resulted in a decreased IHAD/IHAB ratio in the control group only. However, the changes in IHAD strength and in the IHAD/IHAB ratio were not significantly different in the between-group comparison. Small between-group differences for IHAD strength and for the IHAD/IHAB ratio may certainly exist, however, because of the sample size, the present study was not able to show these between-group differences, and whether such small differences are of any clinical value, remains uncertain. With respect to the IHAD/IHAB ratio, a ratio of <0.8 seems to increase the risk of ice hockey players sustaining a groin injury.7 Similarly, it is conceivable that having a large IHAD/IHAB ratio may protect against adductor-related groin injuries in soccer players. However, this needs to be prospectively investigated in future studies.

Implementation of simple hip-adduction strength training using elastic bands

As soccer players are likely to lose hip-muscle strength during the soccer-free period, this period seems to be ideal for a hip strength training intervention. This would allow players to return after the soccer-free period, having increased their EHAD strength. However, placing the intervention in the soccer-free period may reduce training compliance, as the sessions will then have to be home-based and unsupervised. Placing the intervention sessions as a part of the soccer training sessions—as in the present study—likely solves the potential compliance problem, since the exercises can be incorporated in either the warm-up or after training sessions. This approach has previously been used with success in preventing hamstring injuries by implementing the Nordic Hamstring exercise.14 Therefore, it is our opinion that a hip-adduction strength training intervention can successfully be carried out during a seasonal break, in either the soccer-free period or in the period with resuming soccer play. Pragmatic and logistical considerations can further determine the approach chosen by individual teams. Furthermore, it is recommended to continue with a weekly training session throughout the season, to maintain the obtained strength gain.

When introducing possible preventive measures, one should always consider the possibility of this modality having either no or even a negative effect. Fredberg et al44 have shown that soccer players with subclinical pathology, as evaluated by sonography, were more likely to develop tendinopathy than soccer players without subclinical pathology. A similar situation should be considered when introducing the hip-adduction strengthening exercise using elastic bands, due to the hereby increased adductor-training load.45 There were, however, no indications of provocation of subclinical pathology in the present study, as no hip and/or groin injuries were observed in the training group.

Study limitations

During the intervention period we—as expected—lost a relatively large number of participants (figure 1). In clinical intervention studies, this would be a major limitation; however, in the present study we were only interested in the physiological effect of the exercise, if performed as prescribed. We knew from experience that a great number of participants were likely to sustain an injury related to soccer, which would preclude them from finishing the intervention. Therefore, we predefined acceptable levels of compliance (70%) and had an assessor blinded to name and group allocation, who excluded participants on the basis of physiological factors that have been shown to influence hip-adduction and hip-abduction strength. We therefore included a sufficient number of ‘excess’ participants to account for this.

In the present study, we only measured hip-adduction strength on the dominant leg, even though the hip-adductor strengthening programme is aimed at both legs. However, as the primary purpose of the study was to investigate whether an intervention using elastic bands could induce relevant hip-adduction strength effects, measuring one leg was sufficient. Whether a strength effect on the non-dominant leg is different from the effect on the dominant leg therefore remains unknown.

Generalisability and clinical implications

The hip-adductor strengthening programme in the present study was performed with elastic bands, which is a simple strength training modality requiring only a fixation point for the elastic band (eg, a bench in the locker room or a fence surrounding the training pitch) and a rack or fence for the player to hold on to and fixate the upper body with. This makes it possible to perform the hip-adductor strengthening programme at the training field, as a part of the soccer players’ everyday soccer-training sessions. Adequate quality of the hip-adductor strengthening programme can be obtained by supervising the training sessions, which can be easily performed  as group-sessions. It is recommended to introduce this hip-adductor strengthening programme as a preventive approach towards adductor-related groin injuries in soccer players, especially in players with previous injury and decreased hip-adductor muscle strength.5 The inclusion of sub-elite soccer players only limits the generalisability to this group, bearing in mind that this group constitutes a large part of all soccer players. Whether introduction of this hip-adductor strengthening programme will influence the risk of adductor-related groin injuries in soccer players should be investigated in a randomised clinical study.

Conclusion

The present study showed that 8 weeks of hip-adduction strength training, performed with elastic bands as external load, increased maximal eccentric hip-adduction strength substantially in healthy, sub-elite soccer players, compared with a control group that did no strength training. As decreased hip-adduction strength is a risk factor for developing groin injuries, the hip-adduction strengthening exercise using elastic bands may have implications as a promising approach towards prevention of groin injuries in soccer.

What is already known on this topic

  • Adductor-related groin injuries are the most common type of groin injury in soccer.

  • There is evidence for increased risk of groin injury in soccer players who have decreased muscle strength in the hip-adductors, but no simple hip-adductor strengthening programme for prevention of groin injuries exists.

What this study adds

  • Eight weeks of hip-adduction strength training, performed with elastic bands as external load, increases maximal eccentric hip-adduction strength in soccer players compared with a control group doing no hip-adduction strengthening.

  • This simple hip-adductor strengthening programme can be performed at the soccer field, and thus may have implications as a promising approach towards prevention of groin injuries in soccer.

Acknowledgments

The authors thank the participating clubs, physiotherapists and players for their time and trouble during the trial.

References

View Abstract

Footnotes

  • Contributors In accordance with the Vancouver guidelines, all six authors have contributed to conception and design, acquisition of data or analysis and interpretation of data; drafting of the article or revising it critically for important intellectual content; and final approval of the version to be published.

  • Ethics approval The Danish Ethics Committee of the Capital Region (ID: H-4-2010-119) and the Danish Data Protection Agency (ID: 2007-58-0015).

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • ▸ References to this paper are available online at http://bjsm.bmj.com

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