**3. Results**

A total of 2294 questionnaires were submitted. Any response was excluded if there was no response for the question of presence or absence of injury. Data from 2269 questionnaires were included in the final analyses. From these responses, 750 athletes (33.1%) specialized in laterally dominant sports (tennis, table tennis, soccer, badminton, fencing, long jump, shot put, high jump, baseball, and softball), and 1519 athletes (66.9%) specialized in nonlaterally dominant sports (swimming, running, cycling, skating, basketball, taekwondo, rope skipping, dance, hockey, volleyball, traditional martial art, judo, kickboxing, karate, roller skating, gymnastics, and boxing). A total of 576 (25.4%) athletes sustained noncontact lower-limb injury causing time loss (≥1 day) from participation in sport activities during the preceding 12 months. The ankle (12.1%), thigh (10.8%), and knee (10.6%) were most commonly reported as the location of injury (Figure 1). Ligament sprain (15.7%) and muscle strain (8.5%) were the most commonly reported non-contact lower-limb injuries (Figure 2).

**Figure 1.** Injury breakdown by location.

**Figure 2.** Injury breakdown by type.

The injured group showed significantly greater age (mean rank: 1417.58 vs. 1009.82, *n* = 2227, *p* = 0.000) and length of training (mean rank: 1192.72 vs. 971.22, *n* = 2059, *p* = 0.000) compared to the non-injured participants (Table 1). Results of the Chi-square tests (Table 2) showed that the injury rate increased with increasing training intensity (χ<sup>2</sup> (2, 2265) = 151.794, *p* = 0.000, Cramér's V = 0.259) and training frequency

(χ<sup>2</sup> (4, 2267) = 183.817, *p* = 0.000, Cramér's V = 0.285); the injury rate in laterally dominant sports was significantly greater compared to the non-laterally dominant sports (χ<sup>2</sup> (1, 2269) = 15.673, *p* = 0.000, Cramér's V = 0.083).

**Table 1.** Mann–Whitney U tests for the comparison of age and length of training between non-injured and injured pediatric-age athletes.


**Table 2.** Chi-square tests comparing the proportion of non-injured vs. injured pediatric-age athletes for each predictor variable.


LD, laterally dominant sport; NLD, non-laterally dominant sport.

In the multivariate logistic regression model (Table 3), risk of non-contact lower-limb injury increased with increasing age (adjusted OR, 1.21 for an increase of 1 year; 95% CI, 1.16–1.26; *p* = 0.000), training intensity (adjusted OR, 1.77 for an increase of 1 level; 95% CI, 1.43–2.19; *p* = 0.000), and training frequency (adjusted OR, 1.36 for an increase of 1 training day per week; 95% CI, 1.25–1.48; *p* = 0.000). Athletes specialized in laterally dominant sports showed a greater risk of non-contact lower-limb injury compared to those specialized in non-laterally dominant sports (adjusted OR, 1.38; 95% CI, 1.10–1.75; *p* = 0.006). Rightleg preference indicated lower risk of non-contact lower-limb injury compared to left-leg preference (adjusted OR, 0.71; 95% CI, 0.53–0.95; *p* = 0.023).


**Table 3.** Multivariate logistic regression analysis for predicting non-contact lower-limb injury in pediatric-age athletes.

LD, laterally dominant; NLD, non-laterally dominant; OR, odds ratio; CI, confidence interval.

#### **4. Discussion**

#### *4.1. Injury Analyses*

This is the first survey study to our knowledge focusing on non-contact lower-limb injury in pediatric-age athletes. Our results showed that 25.4% of the athletes in our respondent sample sustained a non-contact lower-limb injury (≥1-day time loss from sport activities) in a 12-month period. It is difficult to make age-matched comparisons between our results and previous findings because of the lack of research focusing on non-contact lower-limb injury in children and adolescents. Brumitt et al. [20] reported the same injury rate (25.4%) when examining non-contact lower-back and lower-limb injury (≥1-day time loss from sport activities) in 169 male collegiate basketball players in one season. However, the rate of non-contact lower-limb injury varies greatly in other studies: Stiffler et al. [21] reported that 19.4% of 147 collegiate athletes sustained non-contact injuries in the knee or ankle in one academic year; while, Izovska et al. [22] reported that 33.6% of 227 professional soccer athletes sustained non-contact lower-limb injuries in one season. This range in injury rate may be influenced by the definition of injury used in the research and differences in participant characteristics across studies.

Our results showed that ligament sprain (15.7%) and muscle strain (8.5%) were the most frequently occurring injuries. This result is consistent with previous findings generated from 9th to 12th grade students [5] and 12 to 15-year-old students [2] in sport activities. Further, our results showed that ankle (12.1%), thigh (10.8%), and knee (10.6%) were the most frequently injured locations. Similarly, ankle and knee were also reported as the most frequently injured locations in adolescent (aged 14.67 ± 2.08 y) soccer players during training and competition [8], 5 to 17-year-old children and adolescents in sports activities [9], and 12 to 15-year-old students in sports activities [2]. The high rate of injury in these locations may be related to the anatomy of the knee and ankle [23], and the imbalance in force absorption of the quadriceps and hamstrings in sports activities [24]. The preponderance of injuries to the ankle and knee implies particular emphasis in injury prevention and sport training education in this area.

#### *4.2. Effects of Lateral Dominance in Sport on Non-Contact Lower-Limb Injury*

This is the first study to compare the rate of sport injury in athletes specialized in laterally dominant vs. non-laterally dominant sports. Results of the Chi-square test showed that the rate of non-contact lower-limb injury was significantly greater in athletes specialized in laterally dominant sports (30.5%) vs. non-laterally dominant sports (22.8%). The multivariate logistic regression model showed supportive results, wherein athletes specialized in laterally dominant sports were 1.38 times more likely to sustain a non-contact lower-limb injury compared to athletes specialized in non-laterally dominant sports after controlling for the effects of other factors. Cumulatively, these results suggest that pediatricage athletes specialized in laterally dominant sports may need close monitoring for noncontact lower-limb injury by the coaches, athletic trainers, medical staff, and parents. We speculate that the long-term use of a laterally dominant moving pattern may result in greater inter-limb asymmetry in athletes specialized in laterally dominant sports, leading to greater odds of non-contact lower-limb injury. Future research is warranted to examine this postulation further.

To date, there is a lack of research in terms of classifying laterally dominant and non-laterally dominant sports in the literature. The present study suggests a way to classify laterally and non-laterally dominant sports on the basis of the pattern of movement in lower extremities in a sport. Sport that requires a large amount of movement characterized by the two sides performing/functioning differently was classified as a laterally dominant (or asymmetric) sport in the present study. For example, in the lunge, which is frequently performed in fencing, tennis, and badminton, the dominant leg performs as the leading leg and the non-dominant leg performs as the supporting leg [14]. In contrast, a non-laterally dominant (or symmetric) sport was classified as a sport where both legs are expected to be equally involved, such as running, swimming, and cycling [15]. In addition, a sport that requires a large amount of single-leg jumps (e.g., basketball and volleyball) or singleleg support/drive (e.g., kickboxing, taekwondo) on both sides was also classified as a non-laterally dominant (or symmetric) sport in the present study, despite the fact that the dominant leg is usually more involved than the non-dominant leg in practical action [15]. The method suggested in the present study could be used by future research with a need to classify laterally dominant and non-laterally dominant sports.

#### *4.3. Other Risk Factors for Non-Contact Lower-Limb Injury*

Our results indicated that the risk of non-contact lower-limb injury increased with age in pediatric-age athletes. A number of studies have demonstrated similar findings. Cuff, Loud, and O'Riordan [5] reported that the risk of overuse injuries increased with age in 9th to 12th grade students. Bijur, Trumble, Harel, Overpeck, Jones, and Scheidt [9] reported that the rate of sports injury increased with age in 5 to 17-year-old children. In addition, Michaud, Renaud, and Narring [3] reported that the rate of sports injury increased with age in 9 to 16-year-old students. The heightened risk of sports injury with increasing age may stem from the increased level of competition and time participating in sport as a function of age [13]. Taken together, these findings suggest that pediatric-age athletes may need close monitoring for injury, especially as they get older. However, findings in the literature were not always consistent. Some studies demonstrated that age was not associated with the risk of sport injury in junior high school students aged between 12 and 15 years [1], or in adolescent male soccer athletes aged 14.7 ± 2.1 years [8]. The inconsistency of findings may be attributed to differences in age stages and definition of injury across studies. We suggest future studies include participants with a wide range of ages (e.g., from 6 to 17 years) when evaluating the relationship between age and sport injury in pediatric-age athletes.

Currently, there is a lack of research concerning the effects of training frequency and intensity on sports injury. Our results indicate that the risk of non-contact lower-limb injury increased with increasing training frequency (1, 2, 3, 4, 5 or more sessions/week) in pediatric-age athletes. An increase of one training session per week increased the risk of non-contact lower-limb injury by 1.36 times. This finding suggests that coaches in youth sports training may need to reduce training frequency to prevent injury in pediatric-age athletes when required, although the effects of training duration in one training session have not been considered in the present study. Another consideration is that the training frequency of athletes who play on multiple teams (for the same sport) may not be scheduled by one coach. It is recommended that pediatric-age athletes play on only one team at a time to help decrease training frequency and the risk of injury. Our results also indicate that an increase of one level (low, moderate, high) in training intensity increases the odds of non-contact lower-limb injury by 1.77 times when controlling for the effects of other factors. It should be noted that there was no clear boundary of each intensity level (low, moderate, high) in the present study, and the training intensity was self-evaluated by the participants, which may have led to the underreporting of injury cases. Nevertheless, the present study provides preliminary evidence on the effects of training frequency and intensity on non-contact lower-limb injury in pediatric-age athletes.

With respect to the effects of length of training on sports injury, results are inconclusive in the present study. Although the injured group showed a greater length of training than the non-injured group, results of the multivariate logistic regression model showed no association between length of training and non-contact lower-limb injury after controlling for the effects of other factors. Available evidence on the influence of length of training on sports injury is scarce. Bastos, Vanderlei, Vanderlei, Júnior, and Pastre [8] reported that male soccer athletes (14.7 ± 2.1 years) with a training duration greater than 5 years sustained sports injury more frequently compared to those with a shorter training duration. The greater length of training demonstrates greater exposure time to training and competition, which may contribute to a greater risk of injury [25]. Further, with the increase in length of training, games may become more competitive, which may also increase the risk of injury [26].

Regarding the effects of sex (male vs. female) on the risk of sport injury, a systematic review focusing on children and adolescents has reported that boys are generally at greater risk of sports injury compared to girls because of their larger body mass, which may cause increased forces in jumping, sprinting, and pivoting in boys [13]. However, girls showed a greater risk of sports injury compared with boys in specific sports including soccer, basketball, and baseball, which may be related to the physiological and anatomical characteristics of girls [13]. Focusing on non-contact lower-limb injury, our results showed that there was no difference in the risk of injury between boys and girls. This might be attributed to the variance in the sporting backgrounds of the participants in the present study. Overall, practitioners should pay attention to the differences between sports and consider the potential effects of sex (male vs. female) on the risk of injury in pediatric-age athletes.

Another finding is that left-leg preference indicates a greater risk of non-contact lower-limb injury compared to right-leg preference, suggesting that pediatric-age athletes with left-leg preference may need close monitoring for non-contact lower-limb injury. This finding is consistent with previous research examining risk factors for injury in 12 to 18-yearold [27] and 7 to 12-year-old soccer athletes [28]. The reasons for these findings are unclear. It has been suggested that these findings may be associated with the environmental biases in a right-handed world and differences in function related to neurologic development [29]. This is an area for further research.

#### *4.4. Limitations*

We acknowledge the limitations of the present study. Non-contact lower-limb injuries were self-reported by parents/guardians in the present study, which may lead to underreporting of injury cases. It may also cause recall bias as the parents need to remember events up to 12 months before, as well as classify injuries by themselves instead of medically trained staff. Further, it has been suggested that the effects of previous injury [1,2] and exposure time to sports [3,30] should be considered when evaluating injury risk; however, these two factors were not included in the present study. Therefore, our findings did not take into account the effects of exposure time and previous injury.

#### **5. Conclusions**

Pediatric-age athletes who specialize in laterally dominant sports may demonstrate a greater risk of non-contact lower-limb injury compared to those specialized in non-laterally dominant sports. Left-leg preference, increase in age, training intensity, and training frequency were also associated with a greater risk of non-contact lower-limb injury in pediatric-age athletes. These findings should be utilized with caution as exposure time and previous injuries were not included. However, this study provides useful findings in evaluating the risk of non-contact lower-limb injury in pediatric-age athletes, and the effects of lateral dominance in sport (laterally vs. non-laterally dominant sport) on injury. Future research should include more comprehensive predictor variables to further examine risk factors of non-contact lower-limb injury in pediatric-age athletes. Future research should also explore whether the greater odds of non-contact lower-limb injury in pediatric-age athletes specialized in laterally dominant vs. non-laterally dominant sports is a result of greater inter-limb asymmetry.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/jcm10143171/s1, Supplementary File: Questionnaire for Training Information and Non-Contact Lower-Limb In-jury in Pediatric-age Athletes.

**Author Contributions:** Conceptualization, Y.G. and D.E.R.W.; Methodology, Y.G., S.S.D.B., J.T., Q.J. and D.E.R.W.; Validation, S.S.D.B., J.T., Q.J., Y.L. and D.E.R.W.; Formal analysis, Y.G. and N.W.; Investigation, Y.G., Y.L. and N.W.; Resources, Y.G., Y.L. and D.E.R.W.; Data curation, Y.G. and D.E.R.W.; Writing—original draft preparation, Y.G.; Writing—review and editing, Y.G., S.S.D.B., J.T., Q.J., N.W. and D.E.R.W.; supervision, S.S.D.B. and D.E.R.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the University of British Columbia's Clinical Research Ethics Board (H18-02252) and the Shandong Sport University's Human Ethics Committee for research involving human participants according to the standards established by the Declaration of Helsinki.

**Informed Consent Statement:** Informed consent was received upon completion and submission of the survey.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
