Sixty percent of sports club members and 30 % of non-members reported at least one acute or overuse injury during the past twelve months. The probability for sustaining an acute or overuse injury increased with the increasing number of training hours and competitions among sports club members. However, having less than three resting days per week did not increase an injury. Acute injuries were reported more often in contact and team sports than in individual sports among sports club members.
The sports club members and non-members formed a representative sample of the different regions of Finland and the sports club sample covered the 10 most popular sports in Finland. Both individual and team sports and summer and winter sports were equally represented [
28]. Both groups should have similar accuracy in injury data and recall. The terms “acute injury” and “overuse injury” were defined in the questionnaire—as validated earlier [
19,
25,
26] —to make the answers more reliable, and acute and overuse injuries were looked at separately.
One limitation of our study is the cross-sectional nature of this study. Also the self-reported questionnaire may cause recall bias and sports club members may remember their injury history better. However, the questionnaires used in these surveys were compiled from previously validated questions [
19,
23‐
26]. The non-respondents were 9th grade students as were those who took part in the study, we do not have further details concerning the non-respondents. We believe that the risk for either participating or not participating is the same for both injured and non-injured adolescents. However, there may be a risk that injured athletes had withdrawn themselves from sports club activities because of an injury/injuries and would be non-members with an injury sustained in a sports club setting. Although, at the age of 15 only a few injuries are so severe that they cause withdrawal from sports participation. There may also be a risk that sports club members understood the terms an acute and an overuse injury more clearly than non-members, but the definitions were written clearly and their differences were underlined to minimize the risk of misunderstanding. In the self-reported questionnaire there is always a risk that adolescent is not aware of the name of the anatomic location that is injures, but this is the case in the both study groups, and thus, the results should be comparable. A further limitation is that training details were only inquired from the sports club member group, since reporting training hours may have a different meaning and be less valid among non-members. We also carried out an additional analysis of the occurrence of injuries in both sports club members and non-members using an identical question on leisure-time physical activity volume for both groups.
Injury occurrence and profile
Different sports disciplines have their specific typical acute and overuse injuries. For optimal preventive measures, sports-specific injury profiles need to be known. Taking into account the intensity and type of sports performed by the Finnish adolescents, our results concerning athletic injuries in young people who are participating in sports are in line with previous findings [
6,
29‐
32].
Leppänen et al. [
6] studied overuse injuries among adolescent basketball and floorball players using a self-reported questionnaire. Among floorball players the knee was the most common site for an overuse injury, the same finding was made in our study. Bostrom et al. [
29] studied seven different sports and found that nearly half of the young adults had been injured during the past 12 months, which is close to 44% of adolescents having sustained at least one acute injury in our study. In both studies boys were more often injured than girls. However, in the study by Bostrom et al. [
29] an acute injury was defined by an absence time of one week from sports compared to one day in our study. In a study of high school athletes by Kahlenberg et al. [
30] those with a higher total number of hours per year of sports participation had a higher risk of injury. In our study, both acute and overuse injury was significantly increased with 15 h or more of training per week.
In our study, the acute injury incidence rate was significantly higher in team and contact sports compared to individual sports, which is in accordance with previous studies [
4,
5,
33,
34], and acute injuries occurred more frequently to boys than to girls. This reflects the sports type distribution among Finnish adolescents, which explains why this result deviates from the findings seen in specific sports with a similar degree of competitiveness, where girls have a higher incidence of injury than boys [
35‐
37]. In our study individual sports that were more often performed by girls were orienteering, riding, dancing, gymnastics, and skating.
In our study, sports club members reported on average one overuse injury per year. This was more than in non-members, but the reported 0.7 overuse injuries per year among non-members also needs to be recognized. Our result is in line with that reported by Malisoux et al. [
31], but again the risk for an overuse injury depends on the sport type [
6].
The most common acutely injured location for both sports club members and non-members was the wrist and hand combined. The most common more specific location among sports club members were the ankle and foot, again in accordance with previous research [
35,
38]. In our study, the most common site for an overuse injury was the knee for sports club members and the wrist and hand for non-members.
Earlier studies have reported that the knee is commonly injured in adolescents [
39] and 18–35 year old adults [
40] especially in ball games and running. The knee was second most common acute injury location in non-members whereas in sports club members it was the 5th most common injury location. Acute knee injuries may be relatively less common among sports-club members as a result of neuromuscular training targeted at injury prevention [
41]. Acute knee injuries were reported by 12% of all participants, there was no difference between boys and girls but a greater number of acute knee injuries were reported by sports club members than non-members. Most acute knee injuries occurred in ball games and among girls in synchronized skating and gymnastics. In a previous study by Pasanen et al. [
42] 18% of acute injuries in floorball were knee injuries, including 7 ACL injuries. In our study no ACL injuries were reported. None of the knee injuries reported in our study required more than three days away from normal training or competition, so we can assume they were minor injuries.
In our study 12% of overuse injuries were knee injuries, significantly more of these were reported by sports club members. In previous studies it has also been found that Osgood-Schlatter disease, patellofemoral pain, and other unspecified knee pain are common in adolescents participating in sports [
43‐
45]. Leppänen et al. [
46] found in previous study of overuse injuries in basketball and floorball players that 35% of overuse injuries were injuries of the knee, however the age group in this study was 12–20 years. In our study knee overuse injuries were most common in ball game players.
In line with previous research [
35,
47,
48] the most common acute injury type in both groups was sprains followed by contusions or bruises. The most common type of overuse injury was muscle injury in both groups. This injury type includes, for example, chronic exertional compartment syndrome and delayed onset muscle soreness. However, it can be argued whether delayed onset muscle soreness (DOMS) should be classified as an overuse injury. DOMS is primarily caused by eccentric contractions that produce the greatest muscle tension [
49]. Sports club members reported significantly more overuse injuries of the bone tissue than non-members. Included in these injuries may be, for example, Osgood-Schlatter disease. Stress fractures were not statistically significantly more common in sports club members. A potential cause for this may be that this type of injury may have caused withdrawal from sports club activities. Also only a small minority of sports club participants participated in intensive elite level training. Many earlier studies have found that participation in gymnastics, dance, and other high-impact activity carries a higher risk for stress fractures, especially in girls [
50,
51].
Injury circumstances and the need for prevention
The injury risk varies between different sports and other leisure time activities [
52]. In accordance with earlier research [
3,
35,
53] organized training was found to be the most common setting for acute injuries in sports club members, followed by matches or competitions. However, the injury risk is not always higher in organized sports. Monroe et al. [
13] found that players taking part in organized soccer had a higher prevalence of injury compared to players of non-organized soccer. On the contrary, in basketball non-organized players had a higher prevalence of injury compared to players of organized basketball [
13].
McQuillan and Campbell [
34] reported that sports caused 32% of the injuries in 12–17-year-old adolescents. In non-members, most acute injuries occurred during leisure time. Abernethy and MacAuley [
54] reported that 51% of accidental injuries occur during school sports, which was much more than in our study. In addition to formal physical education class, school sports included organized and casual sports performed at school. Although sports club members may have better balance because of their more versatile training methods, this does not cause a difference in the amount of injuries in school sports. This may be due to sports club members participating more intensively in school sports compared to non-members. The most frequently reported cause for an acute injury was falling or stumbling in both groups. Sports club members were more likely than non-members to sustain acute injuries in tackling situations. This can be explained by the high participation rate in team and contact sports [
55].
Earlier studies have reported that greater sports participation is associated with an increased risk of injury both in adolescents and adults [
19,
56]. In accordance with the results of our study, Richmond et al. [
57] found that adolescents had a greater risk of a sports-related injury with increasing hours of play. In our study, training details were inquired only from sports club members, and we found that weekly training for over seven hours was associated with
a higher amount of acute injuries in boys and girls. However, the amount of overuse injuries was significantly increased only among boys when training over seven hours per week. It has also been found that low levels of habitual physical activity increase the injury risk in children when exposure time is taken into account [
58]. As reported earlier [
4], the number of competitions is associated with the yearly incidence of injury, particularly in team games. In our study, boys reported having taken part in twice as many competitions during the preceding year than girls did.
We also surveyed the leisure-time physical activity from both groups. Only 30 min of weekly physical activity was found to significantly increase the likeliness of both the acute and overuse injuries. Injuries were further increased with increasing leisure-time physical activity in those completing 2 h or more of physical activity per week. This was seen more clearly in sports club members. This result is in agreement with previous research showing that more competitive forms of physical activity are associated with higher injury risk. [
59,
60].
Effective preventive measures targeted at risk factors for both acute and overuse injuries are needed in both organized and non-organized sports in order to prevent injuries. Poor knee alignment and control during jump-landing may increase the risk of injury. However, these risk factors does not seem to change in young athletes by time by simply playing the sport. Improvement of knee stability needs specific neuromuscular exercises [
39].
Physical activity programs that develop strength and proprioception have been shown to be effective in preventing acute and overuse sports injuries [
61,
62]. Including this type of training as a part of physical education would also aid adolescents not participating in sports club activities to meet physical activity guidelines without injuries [
63,
64].