This study provides novel insights into the training practices and RRI in a population of competitive adolescent distance runners. The key findings were that: 1) the number of training sessions per week (frequency) increased with chronological age; 2) for “all RRI,” the IP was 126/100 participants/year (95% CI: 113 to 138), and the IR was 6.3/1000 participation hours (95% CI: 5.3 to 7.4); 3) the most common injury sites were the knee, foot/toe, and lower leg, with overuse being the most common cause of injury; and 4) “sessions per week” and a “higher level of specialisation” were associated with a lower risk of all RRI and time loss RRI, respectively.
4.1 Training Practices:
The results highlight that the number of training sessions per week (frequency) are significantly different between age-groups, with a higher number of weekly sessions being recorded in the older 17–18 age-group, compared to the 13–14 age-group. This finding supports that reported in a cohort of elite Australian youth track and field athletes, and the notion that performance athletes in ‘centimetres, grams, or seconds’ sports increase their training practices during late adolescence [13, 19]. However, no other significant differences were found between age-groups, in relation to the training practices of these athletes. This might be because a large proportion (58%) of these adolescent distance runners had higher levels of specialisation and broadly similar training ages, regardless of sex.
The fact that both the volume and intensity of training practices do not differ much between age-groups is concerning, especially considering that distance running is a late-specialisation sport [19, 20]. For example, the average number of training months per year (11.0) in this cohort exceeds current evidence-based recommendations, having been associated with injury history in youth athletes [22]. However, it is important to recognise that self-reported intensity was based on perceived exertion. Therefore, this does not mean that other physiological or biomechanical measures of intensity did not vary between participants and the different age-groups.
In further contradiction to the training recommendations [22], the exploratory analyses highlight that the more training sessions that an adolescent runner completes per week, were associated with a lower risk of injury. When only including time loss RRI, a higher level of specialisation was also associated with a lower risk of injury, compared to those with a lower level of specialisation. Whilst there are several confounding factors to consider when interpreting these results, they highlight that the current training recommendations may need revaluating for specific sports. However, analytic epidemiology is required to detail what this would involve within adolescent distance running, having quantified the association between training exposure and RRI.
4.2 Running-Related Injuries:
For “all RRI,” the reported injury IP was high in this population (126/100 participants/year), when compared with other youth sport studies [33]. When considering exposure time, the reported injury IR (6.3/1000 participation hours), for “all RRI,” was slightly higher than that found in a two different cohorts of elite Swedish adolescent distance runners (4.0 to 5.3/1000 participation hours) [8, 11]. Yet, these rates are lower than those found in recreational (7.7/1000 participation hours) and novice (17.8/1000 participation hours) adult runners [4]. Overall, these IP and IR results indicate that this cohort of competitive adolescent distance runners may have a greater training volume (exposure) than in other sports, whereby the higher IP may be a result of this greater training volume.
The injury IR for male participants (5.9/1000 participation hours), for “all RRI,” is higher than that reported in several other youth sports, with a similar injury IR to that found in youth football studies [32]. A similar pattern was found in terms of the injury IR for female participants (6.6/1000 participation hours), indicating a lack of significant sex differences. This is further supported by a non-significant rate ratio between the male and female participants in this study (rate ratio = 0.89; 95% CI: 0.68–1.33). Notably, the studies reporting injury IR in a population of youth cross-country runners calculated this outcome according to the number of injuries per 1000 athletic exposures, making comparison of results difficult [9, 10].
The injury IP for those RRI that required medical attention (52/100 participants/year) was higher than that reported in athletes aged 11–18 years (35/100 participants/year), across a range of sports [16]. The largest proportion of RRI incurring time loss were categorised as “serious” (25%). Although this is an interesting finding, highlighting that a quarter of RRI resulted in more than 28-days of time loss (up to 6-months), this may be due to recall bias. On the contrary, the large proportion of RRI incurring “no time loss” (22%) was due to participants being able to register “any physical complaint” when self-reporting RRI, whereby 45% (n = 30) of self-reported ‘pains or discomforts’ did not result in time loss.
RRI were most commonly reported in the lower extremity, with the knee, foot/toe and lower leg being the most frequently injured sites. These injury sites are comparable to those reported in elite adult and adolescent track and field athletes [17]. Likewise, the most common self-reported cause of injury was overuse, which supports previous findings [45]. These data indicate that injury prevention interventions for adolescent distance runners should predominantly focus on reducing the risk of lower extremity RRI caused by overuse.
4.3 Methodological Limitations:
The main limitation of this study is the use of a cross-sectional study design. As a result, it is not possible to determine temporal relationships between potential injury risk factors and RRI. Recruitment difficulties resulted in a limited sample size is, also considered a study limitation. While this is not a limitation for the descriptive data, the limited sample size meant that only exploratory univariate analyses were conducted when investigating potential correlates of RRI. The convenience sampling method may also have led to a non-representative sample.
Recall bias is a further study limitation, whereby the accuracy of data was dependent upon self-report. This type of bias often results in participants under-reporting minor injuries, leading to an artificially greater proportion of severe injuries. While there was a high proportion of serious time loss RRI, the proportion of RRI that incurred no time loss was also elevated. Nonetheless, research has shown that participants can accurately recall the total number of injuries and injury sites when providing a 12-month self-reported history [46]. However, as that research was based in a different sporting context, the effect of recall bias remains unclear within this study. Social desirability bias is also possible, whereby participants could have over-reported their training practices (frequency, volume, and intensity).