By Ashokan Arumugam1, Kavitha Vishal2
1Department of Community Medicine and Rehabilitation – Physiotherapy Section, Umeå University, SE-901 87, Umeå, Sweden
2Department of Physiotherapy, School of Allied Health Sciences, Manipal Academy of Higher Education, Karnataka, India
PEDro synthesis on the following paper: “Al Attar WSA, Soomro N, Sinclair PJ, et al. Effect of injury prevention programs that include the nordic hamstring exercise on hamstring injury rates in soccer players: a systematic review and meta-analysis. Sports Med 2017;47:907-16.”
Hamstring strain injuries (HSI) are common among sportspeople participating in high-speed running and sporting manoeuvres causing excessive stretching of the hamstrings (e.g. slide tackling or kicking a ball).1 2 The chances for HSI exist in the terminal swing phase of high-speed running3 owing to eccentric loading and lengthening of the hamstrings. Therefore, hamstring strengthening using an eccentric loading exercise, such as the Nordic hamstring exercise (NHE), has been recommended to prevent HSI. 2
This study aimed at corroborating evidence on the effectiveness of the NHE as a sole intervention or in combination with other training programmes on preventing HSI in soccer players while considering player exposure hours (training or matches).
Searches and inclusion criteria
A systematic search of the following electronic databases was conducted from January 1985 to December 2015: the Cochrane Central Register of Controlled Trials (OvidSP), AMED (OvidSP), CINAHL, PubMed, EMBASE, SPORTDiscus, AusSportMed, MEDLINE, and Web of Science. The search was targeting studies published in English mainly based on the key words/terms addressing the NHE, sports injury prevention programmes (by the Federation Internationale de Football Association [FIFA] Medical and Research Centre [FMARC]), sports/athletic injuries, and variations of these terms combined with Boolean operators (OR and AND). Randomized controlled trials (RCTs), cluster-RCTs, and prospective cohort studies evaluating the effects of the NHE alone or its combination with other training programmes in soccer players and reporting player exposure hours (workload) and injury rates were included. Papers having other study designs that did not address the research question of the review were excluded. Risk of bias associated with the included studies was evaluated using a 12-point criteria (a score > 60% indicated low risk of bias) recommended by the Cochrane Back Review Group.4
The eligible interventions were injury prevention programmes using the NHE either as a sole intervention or in combination with other exercises focusing on neuromotor control, muscle strength, balance and/or kinaesthetic awareness of the body (e.g. FIFA 11/11+) for decreasing HSI rates and, at the same time, measuring athlete workload. Players in comparator groups (if included) must have received conventional warm-up or training programmes.
Main outcome measures
The primary outcome measure was HSI incidence rate which was defined as the number of HSI divided by the total exposure time (training and matches) multiplied by 1000. Additionally, the injury risk ratio (IRR) expressed as a ratio of HSI incidence rate in the intervention group to that of the control group was calculated. A low IRR (< 1) indicated a positive effect of the intervention. For instance, an IRR of 0.20 denotes an 80% reduction in HSI.
HSI incidence rate and IRR along with 95% confidence intervals were used to estimate the effects of intervention. Meta-analysis was performed with an invariance random effects model to account for the heterogeneity in study populations, exercise dosage and the execution of the NHE. Publication bias was identified using standard funnel plots, the Egger test to detect asymmetry of funnel plots, and the ‘trim and fill” method to assess the need for adjusting the overall IRR estimate owing to missing studies.
Of the 3242 studies retrieved, only five studies (one RCT,5 three cluster-RCTs6-8 and one cohort study9) with a total of 4455 participants (2256 in the intervention group and 2199 in the control group, including collegiate, youth, amateur or elite soccer players) were included in the review. The included players were males5 7-9 and females6 aged between 13 and 40 years. Four studies employed the NHE along with FIFA 11+6 7 9 or other targeted exercise programmes (of the groin, knee and ankle)5 and one study employed only the NHE for the intervention groups while the control groups received usual5-8 or conventional dynamic warm-up training.9 In the cohort study,9 the same group of soccer players served as their own controls (season 1 [control] compared to season 2 [intervention]).The duration of the studies ranged from 10 weeks to 8 months with a training frequency varying from 1-6 times per week. Compliance with the training programmes varied from 21-100%.
The risk of bias scores of the included studies ranged from 7/12 (58.3%)5 to 11/12 (91.6%)7 8 indicating a low risk of bias for nearly all the included studies. The meta-analysis for HSI indicates that soccer players in the training programmes incorporating the NHE showed a statistically significant reduction in HSI by 51% (IRR: 0.49 [95 % CI, 0.29–0.83], p = 0.008), indicating a positive effect of the intervention, when compared to the controls. Funnel plot analysis revealed symmetry and the ‘trim and fill’ procedure reported 0.49 as the overall IRR, which imply a lack of publication bias.
Strengths and limitations
The main strength of this study is the use of robust statistical methods, and almost all the included studies had a low risk of bias. Despite that the review included only five studies, 4455 participants, 166 HSI, and 315,992 exposure hours were available for the analysis. Nevertheless, the study has some limitations. Four of the five included studies focused on males and this precluded the calculation of gender-specific HSI rates and IRR. Further, the study findings need cautious interpretation for female soccer players. This systematic review included only soccer players undergoing the training programmes employing NHE compared to usual or conventional warm-up training. Hence, the results cannot be implied to other forms of eccentric hamstring training (e.g. a YoYo™ flywheel ergometer)10 and the prevention of HSI in other sports. Moreover, only one study evaluated the benefit of the NHE on its own8 while the other studies included the NHE with other exercises which further limits isolating the effects of the NHE on reducing HSI.
It is evident from the meta-analysis that hamstring injury prevention programmes employing the NHE as a sole intervention or in combination with other exercises focusing on lower limb neuromotor control, muscle strength, balance and/or kinaesthetic awareness prevent HSI among soccer players when compared to usual or conventional warm-up programmes.
- Askling CM, Malliaropoulos N, Karlsson J. High-speed running type or stretching-type of hamstring injuries makes a difference to treatment and prognosis. Br J Sports Med 2012;46:86-87. doi: 10.1136/bjsports-2011-090534
- Brooks JH, Fuller CW, Kemp SP, et al. Incidence, risk, and prevention of hamstring muscle injuries in professional rugby union. Am J Sports Med 2006;34:1297-306.
- Chumanov ES, Heiderscheit BC, Thelen DG. Hamstring musculotendon dynamics during stance and swing phases of high speed running. Med Sci Sports Exerc 2011;43:525.
- Furlan AD, Pennick V, Bombardier C, et al. 2009 updated method guidelines for systematic reviews in the Cochrane Back Review Group. Spine (Phila Pa 1976) 2009;34:1929-41.
- Engebretsen AH, Myklebust G, Holme I, et al. Prevention of injuries among male soccer players: a prospective, randomized intervention study targeting players with previous injuries or reduced function. Am J Sports Med 2008;36:1052-60.
- Soligard T, Myklebust G, Steffen K, et al. Comprehensive warm-up programme to prevent injuries in young female footballers: cluster randomised controlled trial. Br Med J 2009:95-99.
- Silvers-Granelli H, Mandelbaum B, Adeniji O, et al. Efficacy of the FIFA 11+ injury prevention program in the collegiate male soccer player. Am J Sports Med 2015;43:2628-37.
- van der Horst N, Smits D-W, Petersen J, et al. The preventive effect of the nordic hamstring exercise on hamstring injuries in amateur soccer players: a randomized controlled trial. Am J Sports Med 2015;43:1316-23.
- Grooms DR, Palmer T, Onate JA, et al. Soccer-specific warm-up and lower extremity injury rates in collegiate male soccer players. J Athl Train 2013;48:782-89.
- Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports 2003;13:244-50.