Discussion
The principal finding of the current study was the low adoption and implementation of the full NHE programme among teams, even lower than what was reported in the 2012–2014 seasons. Noteworthy was the discrepancy between the positive attitude to the NHE among the medical staff and the low implementation of the exercise in the teams. The second main finding was that teams that used the NHE in their team training had significantly lower match hamstring injury rates, less severe injuries and lower recurrence rates than teams that only used the NHE for individual players with a history of a hamstring injury.
Can we explain why the implementation of the NHE is low despite the positive attitude of medical staff?
The positive attitude of team medical staff towards the NHE is understandable since there are well designed controlled studies showing that the NHE programme effectively reduces injuries.5 6 Our study highlights the difficulty with applying an intervention in a controlled vs a clinical environment. Preventive measures proven highly efficacious in clinical trials do not necessarily work in real life on the football field.2 16 17 These measures have to be successfully implemented in the club, but if the players, coaches and officials are not motivated to use the measures, then the preventive efforts will fail.18
There are many potential barriers to implementing preventive measures at the club level. First, the coaching staff usually decide on training content and coaches might not always be prepared to devote training time to preventive programmes.2 19 Coaches may be concerned about a lack of football specificity or the validity of the suggested preventive measures from the club medical staff since many coaches want the training sessions to mimic actions during matches.20 Second, lack of time for recovery might be another obstacle. Most professional clubs have a very congested playing calendar with multiple days and nights of travelling.2 21 22 A tight schedule has a negative effect on the injury situation and the availability of players for training sessions, with increased muscle injury rates found in periods of match congestion.23 Consequently, coaches could be reluctant to include any time-consuming preventive measures in their regular team training schedule. Third, another obstacle might be negative opinions from players. The majority (81%) of teams using the NHE in this study reported complaints from players, mainly in the form of muscle soreness or stiffness.
Can we explain why NHE seems to reduce hamstring injuries and recurrences effectively?
A main finding in this study is that the hamstring injury rate was significantly lower in teams that implemented the NHE in the ordinary team training and used it for all team players compared with teams with non-use or only individual use of the NHE. An explanation could be that the players understand that the exercise is important if included in team training. Further, the coaches will supervise the exercise session and take action if it is not done with seriousness and quality.
Can we explain why teams that used the NHE in team training had a lower incidence of hamstring injuries at matches only?
The incidence of hamstring injuries at matches was only one-third in the team group compared with the individual training group while the difference between groups were not as profound (and statistically non-significant) for the incidence in training. This difference is most likely explained by the difference in high-intensity actions between training and matches. Most teams have two matches a week during the competitive season, and the training sessions between matches are often focused on recovery, with few opportunities to have high-intensity training sessions. During recovery sessions, the high-intensity actions are fewer, meaning less risk of hamstring injuries and, in turn, less effect of hamstring prevention programmes.
How can our findings be of practical value for players, clinicians and clubs?
A key for medical staff is to convince the coaches about the benefits of using the NHE in the team training. The results from this study might be helpful since it provides clear practical information. Our findings indicate that teams that implemented NHE in team training and used it with most players had fewer hamstring injuries than teams that only used the NHE for individual players with current or a history of a hamstring injury. We acknowledge that the generalisability of our findings outside of the study cohort is uncertain although a previous study on top-tier clubs in Norway showed similar adoption rates as in the ECIS clubs,10 and preventive efficacy of the NHE has mainly been documented in subelite teams.5–8
Player availability is a key indicator for team performance,24 and absence due to hamstring injuries is a big problem for most teams. Our study shows that teams that implemented NHE in team training had a lower injury burden 100 days fewer injury absence days per team and season on average.
Future improvements
Bahr et al10 reported 83% of teams as non-compliant to the original NHE programme, as suggested by Mjølsnes et al.15 This study found 94% non-compliant. The current study indicates that positive benefits were also seen from a modified NHE programme as long as the NHE is included in the team training for most players.
We suggest three areas of possible improvements be evaluated in future studies:
Get the coaches on board. Decision-makers (eg, coaches and managers) could play important roles in injury prevention and be decisive for the overall injury situation in a club.2 The coaches decide the content of training sessions and the load on players. Medical staff can only realise suggestions for preventative training implementation across the whole team if the coach and coaching staffs are positive about the suggestions.
Establish the minimum effective dose. As pointed out by Bahr et al,10 we do not know the NHE programme’s minimum effective dose.10 The current study indicates that even a modified programme might work if used by all or the majority of players in the team and used primarily as a maintenance programme once a week throughout the whole season. The obvious question from coaches to medical staff might be: What is the minimum time that we need to devote to the NHE in our team to reduce our hamstring injury incidence significantly? There are some indications from the Australian football where Presland et al25 found that 2 sets of 4 per week maintain the architectural adaptations (provided they had the 6 weeks of building up strength start of the season). This could be a good starting point for clinicians.
Widen the horizon and look at alternative risk factors. Injuries are multifactorial. Preventive strategies targeting player-related risk factors may not, on their own, be sufficient to reduce injury rates at the top-level significantly.2 26 Alternative risk factors need to be investigated to determine whether there is an association with injury rates and to provide guidance on the most appropriate preventive measures to be adopted.27 We have studied some risk factors suggested by chief medical officers of the clubs participating in the ECIS to contribute to injury risk, and we have found associations between overall injury rates (not specifically hamstrings injuries) and the leadership style of the head coach,27 the quality of the internal communication within a team,28 and during periods of match congestion.22 It is likely that a better understanding of how these risk factors may be associated with injuries could help us reduce hamstring injury rates even further.
Methodological considerations
The study is limited by a relatively small sample size and the short observation period. It would have been beneficial to have a study over several consecutive seasons to establish a more robust association between NHE use and hamstring injury rates, that is, being less likely to be influenced by seasonal flukes in injury occurrences or changes in training practices. Another limitation is the lack of detailed information about the teams’ training practices, including strength training sessions and general conditioning or individual training performed by players outside the team. This is especially important as part of the study period was atypical due to the COVID-19 pandemic restart of league play in Europe. However, in a separate study, we found neither any apparent differences in the percentages of muscle injuries between the pandemic 2020 and the previous 5 years nor between the different periods (prelockdown, lockdown and restart) of 2020.29 On the other hand, another recent, but smaller, study on 84 male elite players from three Belgian teams identified a fivefold higher muscle strain injury rate in players with COVID-19 infection, in particular within the first month of football resumption.30 To date, there are no hamstring-specific data published and this need to be explored in future studies. Finally, as this is a descriptive ecological study, we cannot infer any causality between NHE use and injury rates since teams may have differed in other aspects of importance for hamstring injury occurrence (confounding factors) unknown to us.
