Discussion

This is the first study to report injury incidence and injury patterns in League of Ireland football. During the 2014 season, 152 injuries were recorded from six clubs, which provided a total injury incidence of 9.2/1000 hours. Moderate injuries occurred most often, and injuries were approximately five times more likely to occur during a match compared with training. The thigh region was most commonly injured followed by the ankle. Therefore, unsurprisingly, muscle strains and ligament sprains were the two most common injuries. League of Ireland injuries were more likely to occur in non-contact situations and during the final 15 min of the second half.

The overall injury incidence is higher than the northern (7.89/1000 hours), southern (7.02/1000 hours)4 and combined (8.0/1000 hours)3 European region injury incidence for professional footballers. Our incidence rate is approximately that reported in 23 amateur Dutch squads (9.6/1000 hours).11 The same research group also highlighted a lower incidence rate (6.2 per 1000 hours) in professional Dutch footballers.5 The fact that semiprofessional footballers in Ireland have an injury incidence rate akin to amateurs in Holland may be due to Ireland’s relatively low ranking in the UEFA club coefficients. Holland is ranked 13th of 54 compared with Ireland who is ranked 46th of 54, and it may be that amateur players in Holland are akin to semiprofessional in Ireland. The similarities with amateur Dutch football do not appear to be consistent when compared with amateur players in Spain,12 Italy13 and Sweden14 who demonstrate a heterogeneous but consistently lower training and match injury incidence. Differences are likely due to the definition of amateur. Although convenient to separate elite from amateur, the gradations in playing level in the amateur ranks are vast. The retrospective study of Spanish amateurs,12 although including a robust sample (n=1 34 570), includes players up to age 55 years who played in national but also regional leagues. Training (0.49/1000 hours) and match (1.15/1000 hours) injury incidence were substantially lower compared with our cohort, and in contrast to our study, in which major injuries were lowest, major injuries accounted for almost half of all injuries in Spanish amateurs (47.4%). This may suggest that amateurs of this level may play at a low enough intensity whereby less injuries occur, players play on through minor injuries (and therefore injuries go unrecorded) and more major injuries result as a result of a lower skill level or physical conditioning. It is also possible that the retrospective study design and the arbitrary assignment of exposure (one match, two training) lead to a lower injury incidence being reported. Amateurs restricted to those competing within the sixth tier of Swedish14 or Italian13 football demonstrate a closer approximation in trends of training (2.0–2.7/1000 hours) and match injury (12.3–14.0/1000 hours) incidence but are still substantially lower. By contrast, the Dutch amateurs we have compared our data to are part of the second tier of Dutch amateur football and are likely of a higher skill level compared with the cohorts reported on from Spain, Sweden and Italy. It should be acknowledged that the studies from Holland, Sweden and Italy appear to follow the consensus guidelines for injury definition and classification used in our study but the Spanish study did not. This may add to the discrepancy seen in injury trends between the Spanish footballers and the other European amateurs.

The injury patterns in our study were akin to European professional footballers in that upper leg muscle strains were most common followed by ligament sprains.3 By contrast, the Spanish amateurs discussed above12 reported ligament injury (32.1%) and contusions (23.5%) as occurring more often than muscle/tendon injuries (17.4%). It is plausible that less high-intensity running, a willingness to play on through minor strains and a greater proportion of traumatic injuries due to a lack of physical conditioning may account for some of these differences. The amateurs in Swedish14 and Dutch11 football report muscle injuries to account for 48% and 37.9% of injuries, respectively, followed by ligament sprains accounting for 14% and 23.4% of injuries, respectively. Although the prevalence of injury in these studies follows the same order as ours, it does appear as though our proportions are more akin to professional football. It may be that the level of football in the League of Ireland sits somewhere in between Dutch amateur football and European professional football. The definition of amateur is an important consideration for future research in football. We have identified from the second highest tier of Irish football, which is considered semiprofessional, injury data which are in line with high-ranking amateurs in traditional football nations but very different to amateur footballers in its broadest sense. Definitions may be made easier to compare if all researchers work from the highest division (usually professional) downward when classifying leagues in order to give the reader a clearer sense of playing standard. The measurement of anthropometrical and physiological data may also prove useful toward this aim.

To the aim of comparing our injury data with that of Ireland’s nearest and most climatically similar region, one study15 prospectively followed four English professional football clubs for 3 years and reported an injury incidence of 8.5/1000 hours. This may highlight demographic similarities between the regions. However, given the breadth and depth of evidence available from northern Europe, this suggestion is unlikely, and the rates reported are more likely linked to playing standards. To the best of the authors knowledge, there is only one other study7 highlighting injury incidence from teams in northern Europe using the summer soccer season format. Our incidence is higher than that reported in professional footballers from the Swedish first division (9.2 vs 7.65/1000 hours). This provides further support for the hypothesis that differences in injury incidence emerge from playing standard rather than season format. Sweden, like Holland, are also ranked considerably higher (18th of 54) than Ireland in the UEFA coefficient rankings. The reasons for a greater injury incidence at lower levels are suggested to be linked to less high-quality medical staff being available, lower quality or less time for rehabilitation, physiological differences16 and a smaller competitive squad to rely on.6 17 A smaller competitive squad has greater potential for those with minor injuries to play and suffer a moderate or major injury.6

League of Ireland players were approximately five times more likely to receive an injury during a match compared with training, which is in keeping with the 3.5–8 times greater risk associated with match play across Europe.3 4 7 15 The predominate cause of this ratio is thought to be match intensity.18 In keeping with previous literature, we report muscle strains to account for half of all injuries and the thigh region, especially the posterior compartment, to be particularly affected.5 19 20 This finding provides further support to recent data which suggest that although the incidence of ligament injury has reduced, muscle injury remains unchanged.2 The ankle region was the second most frequent site of injury, and perhaps unsurprisingly, ligament sprains were the second most prevalent injury. Injuries occurred most often during non-contact activity, mainly running, which is the primary activity whereby muscle injuries occur.21 Injuries were three times more likely to occur in the second half. Unlike previous literature which suggests injuries occur most often in the final 15 min of each half, we only report this phenomena to occur in the second half. The next ‘high-risk’ time period for injury occurrence in this study was the first 15 min after half-time. In contrast to the overall prevalence of injury in this study, ankle sprains were most common (29%) followed by hamstring strains (18%) during this period. Although injuries occurring in the final stages of the second half fit with the fatigue hypothesis,22–24 our findings in the first 15 min of the second half may suggest inadequate warm-up after the half-time break. Weston et al25 reported a reduction (−1.5°C to 2.0°C) in muscle temperature during the half-time break. Furthermore, Salgado et al26 and Mohr et al27 have reported improvements in joint position sense and sprint performance following a warm-up in footballers. However, as intensive warm-ups are not common place after half-time, even in professional football, our findings may be coincidental, and so our interpretation remains speculative.

In League of Ireland footballers, moderate injuries (8–28 days) occurred most often, followed by minor, slight and then major injuries, which is in agreement with previous literature from northern Europe4 and England.15 A large-scale study17 on top-level and elite European (n=8970) footballers suggested a more even distribution of injury among slight, minor, moderate and major injuries. This may be due to a more representative European sample but also, as discussed above, could be contributed to by more informed clinical decision making and a larger competitive squad at the elite level.28 Linked to this and in line with the greater injury incidence reported in our study, recurrent injury was also greater (16% vs 12%) than that reported by Ekstrand et al3 in professional European footballers. As previous injury is a significant risk factor for future injury, these findings are somewhat unsurprising.29 It is also possible that in smaller squads, there is a greater risk of returning a player to play too soon and subjecting the player to a sudden change in loading, which is associated with a dramatic increase in injury risk.21 Furthermore, the time loss due to injury for our players may have been greater due to the impact that non-football-related professional or educational commitments may have had on recovery time.