Background: Hamstring injuries are the most common non-contact injuries in sport1 and account for 40% of muscle injuries in football,2 where up to one-third recur the following season.3-5 The rehabilitation of players with hamstring injuries usually lasts more than a month, restricting the athlete`s performance and causing a financial impact on clubs.3 4 6 The Nordic hamstring (NH) exercise is a promising option to decrease the risk of hamstring injuries in athletes. However, results of randomised controlled trials have been conflicting.6-8 Aim: The aim of this review was to assess the effectiveness of injury prevention programmes that included NH exercise on reducing the incidence of hamstring injuries among football players. Searches and inclusion criteria: Searches were conducted on Cochrane Central Register of Controlled Trials, PubMed, Medline, Embase, CINAHL (the Cumulative Index to Nursing and Allied Health Literature), AMED (Allied and Complementary Medicine Database), SPORTDiscus, AusSportMed and Web of Science databases up to December 2015. Randomised controlled trials or prospective cohort studies investigating the effectiveness of prevention programmes that included NH exercise for preventing hamstring injuries among football players were included. Studies that did not report player exposure hours or injury rates were not included. Only articles published in English were included. Interventions: The interventions were studies reporting sports injury prevention programmes that included NH exercise compared with `usual` or `different` or `no program`. Main outcome: The main outcome is hamstring injury incidence rates. A hamstring injury was defined as an injury that causes a player to be unable to completely participate in the following match or training session. Statistical methods: Incidence rate ratios (IRR) and 95% CIs were used to estimate the treatment effect, taking into account exposure time. A random-effects meta-analysis was used to pool the estimates of treatment effect. Publication bias was assessed using funnel plots, the Egger test and the `trim and fill` method. Results: Five studies (4455 participants) were included in this review. Four studies were randomised controlled trials and one was a cohort study. All studies, except one, comprised male participants. The included studies covered different levels of participants (youth, college, adult professional and amateur players). The duration of the injury prevention programmes ranged from 10 weeks to 8 months. Three studies used the prevention programme `FIFA 11+`, which includes NH exercise performed in three different stages, and another study used a targeted exercise prevention programme that included NH exercise also performed in three different stages. Only one study solely used NH exercise as the intervention. The comparison groups used in most of the studies were the `usual` warm-up programmes. The meta-analysis, including the five included studies, demonstrates that the prevention programmes that included NH exercise reduced the hamstring injuries by 51% (IRR 0.49, 95%?CI 0.29 to 0.82). The inspection of funnel plots indicates a slight asymmetry, failing to suggest publication bias. The `trim and fill` method did not suggest that adjustment was necessary. Limitations and strengths: The results of this review should be interpreted with some caution. First, it would have been preferable to confine studies to randomised controlled trials and use the GRADE (Grading of Recommendations Assessment, Development and Evaluation) system to judge the overall quality of the available evidence. However, we reanalysed the data excluding the cohort study from the meta-analysis, and the pooled treatment effect did not change substantially (IRR 0.54, 95%?CI 0.33 to 0.90). Second, there was variation in the prevention programmes evaluated, and as most of the included studies used cointerventions in addition to the NH exercise, it is not clear that NH is solely responsible for the injury reduction rate. The strengths of this review include the statistical analysis used. The adoption of IRR rather than a simple injury incidence (eg, OR or risk ratio) gives a better understanding of the effect of the intervention, as the IRR adjusts for exposure time and also enables the collection of multiple injuries per athlete. Clinical implications: The findings from this review suggest that prevention programmes that include NH exercise are effective in preventing hamstring injuries among football players. Given that this review mainly included multiattribute injury prevention programmes, the isolated effect of NH exercise in preventing hamstring injuries is unclear. Despite this, football teams should consider adopting the NH exercise alone or in combination with a broader prevention programme in order to decrease hamstring injury rates.
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