Predicting the Risk of Injuries Through Assessments of Asymmetric Lower Limb Functional Performance: A Prospective Study of 415 Youth Taekwondo Athletes

Abstract

Background:

The impact of interlimb asymmetries on sport injuries is unclear because of inconsistent findings, and there is a lack of research on youth athletes and the sport of taekwondo.

Purpose:

To examine the effects of functional interlimb asymmetries on noncontact lower limb injuries in youth athletes.

Study Design:

Cohort study; Level of evidence, 2.

Methods:

A total of 415 taekwondo athletes (318 boys and 97 girls) aged 6 to 17 years underwent baseline testing to determine interlimb asymmetries through the single-leg countermovement jump (CMJ), hop, and triple hop tests as well as the Star Excursion Balance Test. The athletes were then evaluated for 12 months to observe the occurrence of noncontact lower limb injuries.

Results:

During the study, 98 athletes (70 boys and 28 girls) sustained at least 1 noncontact lower limb injury. Athletes with higher interlimb asymmetries in single-leg CMJ height showed a significantly increased risk of noncontact lower limb injuries (boys: odds ratio [OR], 1.053 [95% CI, 1.027-1.080], P < .001; girls: OR, 1.070 [95% CI, 1.016-1.128], P = .011). Asymmetry in single-leg CMJ height of ≥15.28% was found to be the cutoff point for predicting noncontact lower limb injuries in boys (OR, 4.652 [95% CI, 2.577-8.398]; P < .001).

Conclusion:

This study highlights the utility of interlimb asymmetries in unilateral jump performance as a tool for assessing the risk of noncontact lower limb injuries in youth taekwondo athletes of both sexes. A proper evaluation of interlimb asymmetries may improve prevention strategies for youth athletes.

Keywords

interlimb asymmetry functional performance sport injury

Interlimb asymmetry in lower limb functional performance has been suggested as a risk factor for sport injuries.³⁹ This asymmetry can place both legs at a higher risk of injuries in sport, as the strong leg may sustain excessive stress due to increased dependence and high loading, while the weak leg may struggle to withstand average stress.¹¹ The effects of interlimb asymmetries in jump performance and dynamic balance on the risk of noncontact lower limb injuries have been extensively studied.^7,28,39,41 Warren et al⁴¹ found that increased interlimb asymmetries in a single-leg hop, triple hop, and triple crossover hop distances demonstrate a greater risk of noncontact lower limb injuries in female collegiate athletes. Further, interlimb asymmetry in jump performance has been associated with an increased risk of certain severe injuries, including anterior cruciate ligament reruptures.³² For dynamic balance, a ≥4-cm interlimb asymmetry in posterolateral reach distance on the Y Balance Test has been linked to a higher risk of patellofemoral pain in male military recruits.³⁰ However, conflicting results from other studies^6,7,28 make it challenging to determine the relationship between interlimb asymmetries and sport injuries. A recent systematic review of 28 prospective cohort studies concluded that it was difficult to make a clear statement about the relationship between sport injuries and interlimb asymmetries in lower limb power and dynamic balance because of inconsistent findings.²²

A significant limitation in the literature is the paucity of research on athletes younger than 18 years, and the impact of sex (male vs female) on this relationship is unclear for this population. Compared with adults, youth athletes are more vulnerable to sport injuries because of skeletal immaturity, which is signified by the presence of an open growth plate.¹⁵ Read et al³⁴ reported that increased interlimb asymmetries in peak vertical ground-reaction force during a single-leg countermovement jump (CMJ) demonstrate a greater risk of noncontact lower limb injuries in male youth soccer players; however, no evidence is available for female youth athletes. The association between interlimb asymmetries in anterior reach distance on the Star Excursion Balance Test (SEBT) and lower limb injuries has also been reported in high school basketball players,³³ but the influence of sex (male vs female) on this association is not clear.

Despite numerous studies investigating the link between interlimb asymmetries and injuries in various sports, little research has been conducted in the sport of taekwondo. This is surprising, given that taekwondo kicks are performed unilaterally, which can lead to interlimb asymmetry in the lower extremities during long-term training and competition.²⁰ This asymmetry may be related to the prevalence of noncontact lower limb injuries, which are common types of injuries in taekwondo.²⁵ Given the significance of this issue, it is important to examine the impact of interlimb asymmetries in muscle power and dynamic balance on noncontact lower limb injuries in taekwondo athletes.

Taekwondo kicks are performed unilaterally and are characterized by quick muscle actions that engage the stretch-shortening cycle.⁴⁰ This makes the single-leg CMJ test a suitable assessment tool for interlimb asymmetry in lower limb power for taekwondo athletes. The SEBT assesses dynamic postural control while standing on 1 leg,¹⁸ similar to the demands of taekwondo kicks, which require rotating the body with great dynamic postural control from the supporting leg.²⁶ Evaluating the risk of noncontact lower limb injuries in youth taekwondo athletes through interlimb asymmetries in the performance of these widely used and easily accessible tests would be of value.

The purpose of this study was to examine the impact of interlimb asymmetries, as assessed through unilateral jump performance and the SEBT, on the risk of noncontact lower limb injuries in youth taekwondo athletes.

Methods

Study Design and Participants

This study employed a prospective cohort design. A total of 456 youth taekwondo athletes (348 boys and 108 girls), aged 6 to 17 years, were recruited from 8 taekwondo clubs between June and November 2019, and 415 athletes completed both baseline testing and the 12-month follow-up (Figure 1). The estimated sample size was determined using G*Power based on data from a previous study,³³ and a minimum sample size of 270 for each sex was needed. All participants had a minimum of 1 year of specialized taekwondo training experience and consistently engaged in training for at least 1.5 hours a day, 3 days a week. Those with pre-existing injuries or those unable to complete valid trials were excluded, along with participants who were lost to follow-up. The investigation received approval from, and was executed in accordance with, the ethical guidelines set forth by the clinical research ethics board of our institution for research involving human participants as established by the Declaration of Helsinki. Written informed consent was received from the parents or guardians of participants, and written informed assent was obtained from the participants themselves.

Figure 1.

Flowchart of participant inclusion.

Assessment of Baseline Asymmetry

Interlimb asymmetries in lower limb power were assessed using 3 unilateral jump tests: the single-leg CMJ, hop, and triple hop tests. The protocol has been reported in previous studies.^19,20 The purpose of the single-leg CMJ test was to determine the maximum vertical height of a single-leg CMJ for each leg, with participants instructed to keep their hands on their hips to minimize the influence of arm movement. The entire single-leg CMJ was recorded using the My Jump 2 application on an iPhone 6s (Apple) at 240 Hz.²³ The flight time of the jump (t) was determined using the My Jump 2 application by identifying the frame of the takeoff and landing of the jump, which was then transformed into a jump height using an equation reported by Bosco et al⁵: (jump height = t ² × 1.22625). The My Jump 2 application has been found to have high validity (correlation with the force platform: r = 0.995) and reliability (coefficient of variation = 3.4%-3.6%; between-observer intraclass correlation coefficient = 0.999) in assessments of vertical jump height.² For the single-leg hop and triple hop tests (Figure 2), the purpose was to measure the horizontal distance of 1 hop and 3 consecutive hops, respectively. Participants performed 3 valid trials for each leg in each test, and the mean jump height/distance of the 3 valid trials was used for analysis.

Figure 2.

Diagram of the single-leg hop and triple hop tests.

Interlimb asymmetry in dynamic balance was assessed using a modified version of the SEBT, which is a valid and reliable test developed by Gray.¹⁷ The modified SEBT measures the reach distance along 3 directions (anterior, posteromedial, and posterolateral) using the contralateral limb while maintaining a unilateral stance with a solid foundation. The protocol and sketch of the test have been presented in previous studies.^19,20 Participants were instructed to perform 3 valid trials (3 directions in each trial) with each leg. The maximum reach distance (cm) in each direction, measured from the convergence of lines to the point where the most distal part of the foot reached during the 3 valid trials, was used for analysis.

Baseline Testing

Participants completed all assessments in a dedicated testing area at the taekwondo club in 1 visit. First, participants underwent anthropometric measurements including body height (cm), leg length (cm), and body weight (kg). Limb dominance was assessed by letting the participant kick a soccer ball, and the preferred leg for ball kicking was determined to be the dominant leg.¹ Participants then completed all assessments in 1 session. At the start of the session, each participant completed stretching and a 5-minute jog to warm up. Participants were then instructed to practice until they were familiar with the tests. For the SEBT, participants needed to perform at least 6 practice trials in each direction to account for learning effects.²⁴ After the test familiarization period, participants rested for 5 minutes. Participants then completed the tests in the following order: single-leg CMJ test, single-leg hop test, single-leg triple hop test, and SEBT, with a 1-minute rest between each trial.¹⁹ The starting leg was randomly selected to reduce the order effect. The same tester scored all the tests for all participants.

Injury Surveillance

Participants were monitored for noncontact lower limb injuries for 12 months after baseline testing. The coaches and athletic trainers of each club recorded the injuries using a standardized form adapted from previously described injury report forms.^8,14 All injuries were confirmed by the research team and defined as medical issues in the lower extremities that arose from noncontact mechanisms during taekwondo training or competition and that prevented athletes from participating in subsequent training or competition.⁶ Injuries resulting from contact with another player or object, such as the kick target or the punch bag, were excluded. Bruises on the lower extremities caused by contact with the floor were also excluded. If a participant sustained more than 1 injury during the follow-up period, only the first injury was included because of the confounding effects of previous injuries.^34,35 The primary contact person of the study collected injury report forms from the coaches and athletic trainers every 3 months.

Data Analysis

The reach distances on the SEBT were normalized to leg length as a percentage. The composite reach distance was calculated by averaging the reach distances of the 3 directions for each leg. Interlimb asymmetries in jump performance and reach distance (normalized) on the SEBT were quantified using an equation modified from a previous study³⁶: asymmetry = [(stronger limb – weaker limb) × 2/(stronger limb + weaker limb)] × 100. Asymmetries on the SEBT were also quantified using the absolute difference (cm) between the 2 sides in each direction. Maturity offset, expressed as years from peak height velocity, was calculated using a validated equation that incorporated age, body weight, standing height, and sitting height.²⁹

Statistical Analysis

Statistical analyses were conducted separately for boys and girls. Continuous data, reported as the mean and standard deviation, were assessed for normality and homoscedasticity using the Kolmogorov-Smirnov and Levene tests, respectively. The normality of data distribution was further examined using distribution histograms. For nonnormal distribution, the Mann-Whitney U test was used to compare age, training experience, body weight, body mass index (BMI), and each asymmetry between injured and noninjured athletes, with the effect size (ES) reported as the correlation coefficient (r = Z/n). The independent t test was used to compare body height and maturity offset between injured and noninjured athletes.

Univariate binary logistic regression was used to examine the association between each factor and noncontact lower limb injuries. Factors with a P value <.1 were included in further analyses. Factors that showed a significant (P < .05) association with noncontact lower limb injuries according to the Mann-Whitney U and independent t tests were also included in further analyses. Tests for multicollinearity were then conducted for the risk factors identified during the final 2 steps, and variables with a variance inflation factor (VIF) >10 were confirmed. The identified factors with the most clinical significance were included in multivariate binary logistic regression analysis.

For each asymmetry that showed a significant association with noncontact lower limb injuries in multivariate binary logistic regression, receiver operating characteristic (ROC) curves were used to determine the optimal cutoff points for predicting injuries. The optimal cutoff point on the ROC curve was identified as the Youden index of optimal sensitivity and specificity (Youden index = sensitivity + specificity – 1). Subsequent multivariate binary logistic regression was performed to calculate the odds ratio (OR) and 95% confidence interval for injuries by comparing the high-risk group with the low-risk group, as determined by the cutoff point on the ROC curve. All statistical analyses were conducted using SPSS (Version 23; IBM) with the alpha level set a priori at .05. Participants with missing data were excluded from relevant analyses.

Results

A total of 415 athletes, aged 6 to 17 years, completed the 12-month follow-up (91.0% of the initial cohort). Among these, there were 318 boys (mean age, 9.03 ± 2.02 years; mean maturity offset, –3.40 ± 1.45 years; mean training experience, 2.52 ± 1.36 years; mean body height, 1.40 ± 0.12 m; mean body weight, 36.45 ± 12.28 kg; mean BMI, 18.06 ± 3.68 kg/m²) and 97 girls (mean age, 9.32 ± 2.37 years; mean maturity offset, –3.84 ± 1.48 years; mean training experience, 2.36 ± 1.09 years; mean body height, 1.41 ± 0.12 m; mean body weight, 36.70 ± 12.21 kg; mean BMI, 18.06 ± 3.32 kg/m²). Of these, 98 athletes (22.0% boys and 28.9% girls) sustained noncontact lower limb injuries, leading to time loss. The most frequently injured locations were the ankle (7.2%) and knee (7.2%) in boys and the ankle (14.4%), knee (7.2%), and thigh (6.2%) in girls. In boys, 42 injuries (60.0%) occurred on the nondominant leg, and 28 injuries (40.0%) occurred on the dominant leg; in girls, 18 injuries (64.3%) occurred on the nondominant leg, and 10 injuries (35.7%) occurred on the dominant leg. The characteristics of the injured and noninjured athletes are presented in Table 1. There were no significant differences between the injured and noninjured athletes in any of the characteristics in boys or girls.

Table 1

Characteristics of Athletes ^a

	Injured Athletes		Noninjured Athletes
	n	Mean ± SD	n	Mean ± SD	P
Boys
Age, y	70	9.31 ± 2.30	248	8.94 ± 1.93	.219
Training experience, y	70	2.56 ± 1.34	248	2.52 ± 1.36	.807
Body height, cm	69	140.92 ± 12.79	248	139.63 ± 11.80	.430
Body weight, kg	68	38.33 ± 13.68	244	35.96 ± 11.82	.281
BMI, kg/m²	67	18.46 ± 4.29	244	17.95 ± 3.50	.661
Maturity offset, y	67	–3.19 ± 1.58	244	–3.46 ± 1.41	.175
Girls
Age, y	28	9.07 ± 2.49	69	9.42 ± 2.34	.362
Training experience, y	28	2.57 ± 1.20	69	2.28 ± 1.04	.370
Body height, cm	28	138.95 ± 12.82	69	141.74 ± 12.23	.320
Body weight, kg	28	36.49 ± 11.68	69	36.79 ± 12.50	.889
BMI, kg/m²	28	18.45 ± 2.84	69	17.90 ± 3.51	.194
Maturity offset, y	28	–4.07 ± 1.53	69	–3.75 ± 1.46	.333

^a BMI, body mass index.

The single-leg jump tests showed excellent reliability (intraclass correlation coefficient = 0.927-0.980) in both boys and girls. Descriptive statistics (quartiles) for each interlimb asymmetry between injured and noninjured athletes are displayed in Table 2. In boys, injured athletes showed significantly greater interlimb asymmetry in single-leg CMJ height (mean rank, 201.31 vs 147.01, respectively; Z = –4.376; P < .001; ES = 0.246) and triple hop distance (mean rank, 171.71 vs 144.28, respectively; Z = –2.293; P = .022; ES = 0.132) than noninjured athletes. In girls, noninjured athletes showed significantly greater interlimb asymmetry in absolute (mean rank, 51.98 vs 36.39, respectively; Z = –2.525; P = .012; ES = 0.260) and normalized (mean rank, 51.71 vs 37.06, respectively; Z = –2.358; P = .018; ES = 0.243) anterior reach distance on the SEBT compared to injured athletes.

Table 2

Interlimb Asymmetries ^a

	Boys					Girls
	Injured		Noninjured			Injured		Noninjured
	n	Median (IQR)	n	Median (IQR)	P	n	Median (IQR)	n	Median (IQR)	P
Jump, %
CMJ height	70	17.07 (6.60-26.81)	247	8.66 (4.59-14.73)	<.001	28	15.01 (1.87-26.33)	69	7.87 (5.06-12.73)	.093
Hop distance	70	5.15 (2.69-10.63)	243	4.40 (2.28-7.86)	.119	28	5.44 (2.98-8.63)	68	4.61 (2.91-7.03)	.458
Triple hop distance	68	5.39 (2.39-10.75)	232	3.96 (1.47-7.75)	.022	27	4.83 (1.94-8.84)	66	3.61 (1.27-7.16)	.104
Absolute reach distance, ^b cm
Anterior	65	3.00 (1.00-5.00)	223	3.00 (2.00-5.00)	.688	27	2.00 (1.00-4.00)	67	4.00 (2.00-6.00)	.012
Posteromedial	65	5.00 (2.00-9.00)	223	4.00 (2.00-7.00)	.396	27	3.00 (1.00-5.00)	67	4.00 (1.00-7.00)	.283
Posterolateral	65	5.00 (2.00-9.00)	223	5.00 (2.00-9.00	.845	27	3.00 (1.00-6.00	67	6.00 (3.00-9.00)	.151
Composite	65	2.33 (1.33-5.67)	223	2.67 (1.33-4.33)	.427	27	2.00 (0.67-3.33)	67	2.33 (1.33-4.00)	.277
Normalized reach distance, %
Anterior	65	3.97 (1.54-7.56)	223	4.20 (2.17-7.41)	.555	27	3.23 (1.44-5.50)	67	5.33 (2.99-9.23)	.018
Posteromedial	65	9.01 (3.39-15.78)	223	7.55 (3.73-13.33)	.461	27	4.55 (2.25-7.84)	67	7.06 (2.74-11.76)	.179
Posterolateral	65	9.52 (4.49-17.52)	223	10.17 (4.38-18.18)	.953	27	5.31 (2.35-15.38)	67	11.57 (4.76-18.49)	.082
Composite	65	4.59 (2.19-9.58)	223	4.36 (2.15-7.25)	.488	27	4.17 (1.30-5.65)	67	4.20 (2.22-6.98)	.203

^a Boldface P values indicate a statistically significant difference between the injured and noninjured groups by sex (Mann-Whitney U test: P < .05). CMJ, countermovement jump; IQR, interquartile range.

^b On the Star Excursion Balance Test.

Univariate logistic regression analysis indicated that, for boys, interlimb asymmetries in single-leg CMJ height (OR, 1.061 [95% CI, 1.037-1.087]; P < .001), hop distance (OR, 1.038 [95% CI, 0.995-1.083]; P = .085), and triple hop distance (OR, 1.053 [95% CI, 1.014-1.093]; P = .007) were potential risk factors (P < .1) for a noncontact lower limb injury. For girls, potential risk factors (P < .1) for a noncontact lower limb injury included interlimb asymmetries in single-leg CMJ height (OR, 1.068 [95% CI, 1.019-1.119]; P = .006), triple hop distance (OR, 1.092 [95% CI, 0.990-1.204]; P = .080), and absolute posterolateral reach distance on the SEBT (OR, 0.912 [95% CI, 0.819-1.015]; P = .091). The ORs are for a 1% increase in asymmetry.

Multivariate logistic regression analysis (Table 3) revealed that, for both boys (OR, 1.053 [95% CI, 1.027-1.080]; P < .001) and girls (OR, 1.070 [95% CI, 1.016-1.128]; P = .011), interlimb asymmetry in single-leg CMJ height was the only significant risk factor for noncontact lower limb injuries, after including potential risk factors identified during the final 2 steps (ORs for 1% increase in asymmetry). No other factors demonstrated a significant relationship with noncontact lower limb injuries (P > .05). For girls, the absolute asymmetry in anterior reach distance on the SEBT was excluded from the final model based on the results of multicollinearity tests and a comparison of –2 log likelihood between models, and the normalized asymmetry in anterior reach distance (%) was included in the final model.

The multivariate logistic regression models (Table 3) showed the following highly satisfactory collinearity statistics: interlimb asymmetry in single-leg CMJ height (VIF = 1.141), hop distance (VIF = 1.177), and triple hop distance (VIF = 1.237) in boys and interlimb asymmetry in single-leg CMJ height (VIF = 1.054), triple hop distance (VIF = 1.059), absolute posterolateral reach distance (VIF = 1.034), and normalized anterior reach distance (VIF = 1.039) in girls. This indicates that the models were almost free of interdependency among the included factors, presenting nonredundant information about the criterion variable. Additionally, the Hosmer-Lemeshow test yielded nonsignificant results (boys: χ² = 11.063, df = 8, P = .198; girls: χ² = 4.832, df = 8, P = .775), indicating the models’ adequacy in fitting the data for both boys and girls. The omnibus tests of the model coefficients showed highly significant results (boys: χ² = 24.399, df = 3, P < .001; girls: χ² = 14.829, df = 4, P = .005), suggesting a significant departure of the 2 models from the null hypothesis models.

Table 3

Risk Factors in Both Male and Female Youth Athletes ^a

	OR (95% CI)	P
Boys (n = 299)
CMJ height asymmetry	1.053 (1.027-1.080)	<.001
Hop distance asymmetry	1.006 (0.955-1.060)	.810
Triple hop distance asymmetry	1.023 (0.979-1.068)	.305
Girls (n = 90)
CMJ height asymmetry	1.070 (1.016-1.128)	.011
Triple hop distance asymmetry	1.071 (0.958-1.198)	.225
Posterolateral reach distance asymmetry	0.929 (0.827-1.044)	.215
Anterior reach distance asymmetry	0.922 (0.826-1.029)	.148

^a Boldface P values indicate statistical significance (P < .05). CMJ, countermovement jump; OR, odds ratio.

ROC curve analysis of interlimb asymmetry in single-leg CMJ height determined a 15.28% cutoff point for predicting noncontact lower limb injuries in boys (area under the curve = 0.66 [95% CI, 0.58-0.75]; P < .001), with a sensitivity of 60.3% and specificity of 77.5%. Subsequent multivariate binary logistic regression (Table 4) indicated that interlimb asymmetry of ≥15.28% in the single-leg CMJ height was significantly associated with a higher risk of noncontact lower limb injuries in boys (OR, 4.652 [95% CI, 2.577-8.398]; P < .001). However, in girls, ROC curve analysis failed to identify a cutoff point that would maximize sensitivity and specificity.

Table 4

Risk Factors in Male Youth Athletes ^a

	OR (95% CI)	P
CMJ height asymmetry (≥15.28% vs <15.28%)	4.652 (2.577-8.398)	<.001
Hop distance asymmetry	1.003 (0.954-1.055)	.902
Triple hop distance asymmetry	1.031 (0.988-1.076)	.162

^a CMJ height asymmetry was the dichotomous variable. Boldface P value indicates statistical significance (P < .05). CMJ, countermovement jump; OR, odds ratio.

Discussion

The findings of this study revealed that an increase in interlimb asymmetry in single-leg CMJ height was associated with a greater risk of noncontact lower limb injuries in both male and female athletes. Analysis of interlimb asymmetry profiles indicated that the performance of a single-leg jump was frequently asymmetrical in youth taekwondo athletes, particularly in those who sustained noncontact lower limb injuries during the 12-month follow-up period (Table 2). Furthermore, interlimb asymmetry was more pronounced in the single-leg CMJ height than in the single-leg hop and triple hop distances possibly because of the differences in testing protocols. During the single-leg hop and triple hop tests, movement of the arms was unrestricted to ensure the safety of the youth athletes. This may have compensated for the weaker leg’s lack of muscle power and resulted in a lower level of interlimb asymmetry compared to the single-leg CMJ test in which arm movement was prohibited. The results of injury occurrence indicated that more injuries occurred in the nondominant leg among both boys and girls, which might be related to more frequent use of the nondominant leg as the pivoting leg in taekwondo kicks.²¹

To date, few studies have investigated the link between asymmetry in jump performance and sport injuries in youth athletes. Read et al³⁴ found that increased interlimb asymmetry in peak vertical ground-reaction force during single-leg CMJ landing was associated with a higher risk of noncontact lower limb injuries in elite under-11 to under-12 (OR, 0.90; OR <1 because of the following equation: [low – high]/high × 100 [%]) and under-15 to under-16 (OR, 0.91) male soccer athletes at 10-month follow-up. In the present study, multivariate logistic regression analysis identified interlimb asymmetry in single-leg CMJ height as the only risk factor for noncontact lower limb injuries in male and female youth taekwondo athletes. The OR for a 1% increase in asymmetry in single-leg CMJ height was 1.053 for boys and 1.070 for girls, which indicates that a 15% increase in asymmetry in single-leg CMJ height would raise the risk of noncontact lower limb injuries by 2.17 and 2.76 times in boys and girls, respectively. Moreover, in boys, injured athletes showed significantly greater interlimb asymmetry in single-leg CMJ height than noninjured athletes (median, 17.07% vs 8.66%, respectively; P < .001). These results suggest that interlimb asymmetry in the performance of a single-leg CMJ can be used to evaluate the risk of noncontact lower limb injuries in youth athletes, but further investigation is required for sports beyond taekwondo and soccer.

This study is the first to prospectively identify a cutoff value for predicting injuries in youth athletes based on interlimb asymmetry in jump performance. ROC curve analysis for interlimb asymmetry in single-leg CMJ height determined 15.28% as the cutoff value for predicting noncontact lower limb injuries in male youth taekwondo athletes. Subsequent logistic regression analysis revealed that male youth athletes with interlimb asymmetry of ≥15.28% in the single-leg CMJ height had a 4.65 times higher risk of sustaining a noncontact lower limb injury compared to the low-risk group (interlimb asymmetry <15.28%) (Table 4), which supports the ROC curve results. By using this cutoff value, male youth taekwondo athletes at risk can be monitored more closely, with the potential for preventive interventions. However, we could not establish a cutoff value for female youth taekwondo athletes because of the limited sample size. Our analysis only included 97 girls, which might have affected the results.

It is worth noting that a cutoff value of 15% interlimb asymmetry has already been established for injury prediction using lower limb strength tests.^12,13 For example, professional male soccer players with ≥15% interlimb asymmetry in eccentric isokinetic hamstring and quadriceps strength were found to have a higher risk of sustaining hamstring (OR, 3.80) and quadriceps (OR, 5.01) muscle strains, respectively.¹² Further, professional male soccer players with ≥15% interlimb asymmetry in eccentric isokinetic ankle flexion strength were at greater risk (OR, 8.88) of noncontact ankle sprains.¹³ Additionally, interlimb asymmetry cutoff values have been utilized to evaluate rehabilitation progress in severe injuries. For example, interlimb asymmetry <10% in lower limb strength and power has been established as a standard for athletes returning to sport after anterior cruciate ligament reconstruction.^27,37 However, the cutoff value should be used with caution for injury prediction because the equations used to calculate interlimb asymmetry vary across studies. The 15% cutoff value may not represent the same amount of asymmetry across different studies. Moreover, differences in testing protocols (such as isometric/isokinetic strength tests vs unilateral jump tests) and sample characteristics (such as age and sex) may result in different outcomes. Practitioners must consider these factors when applying the reported cutoff value in practical settings.

Other Intrinsic Risk Factors for Noncontact Lower Limb Injury

The importance of interlimb asymmetry in dynamic balance (or neuromuscular control) for injury prediction remains inconclusive. Inconsistent findings based on studies with adult athletes may be attributed to variations in injury definitions (eg, contact vs noncontact injury, requirement of time loss, lower limb vs entire body vs specific-part injuries) and participant characteristics (age, sex, sport). To date, few studies have examined the relationship between this asymmetry and sport injuries in youth athletes. In the current study, although female youth athletes demonstrated a significant difference in asymmetry in anterior reach distance on the SEBT between the injured and noninjured groups, logistic regression analyses revealed no association between each SEBT asymmetry and noncontact lower limb injuries in either girls or boys. Similar findings were reported by Read et al,³⁵ who found no association between interlimb asymmetry (absolute difference) in anterior reach distance on the Y Balance Test and noncontact lower limb injuries in male youth soccer athletes. These findings suggest that interlimb asymmetries on the SEBT may not be suitable for predicting noncontact lower limb injuries in youth taekwondo or soccer athletes. However, given the importance of dynamic balance in sport injuries and the limited research on youth athletes, this factor needs further investigation in various sports with youth athletes of different ages to reach a definite conclusion.

The results of the present study reveal that there was no significant association between noncontact lower limb injuries and other intrinsic factors including age, training experience, maturity offset, body weight and height, BMI, and leg preference in youth taekwondo athletes. However, it would be premature to dismiss these factors when using interlimb asymmetries to predict noncontact lower limb injuries. Indeed, previous research has reported a link between sport injuries and intrinsic factors such as age,²¹ training experience,²¹ and leg preference^10,21 in youth athletes, even though some studies have reported contradictory findings.^3,9 The discrepancy between findings may be attributed to variations in participant ages and injury definitions across studies. Particularly, few studies have specifically focused on noncontact lower limb injuries. Before ruling out these factors as intrinsic risk factors for a noncontact lower limb injury in youth athletes, their potential effects should be considered. Therefore, when predicting sport injuries using interlimb asymmetries, it is advisable to consider the composite effects of multiple risk factors rather than relying on a single factor.

Implications

The findings of this study suggest that practitioners should consider reducing interlimb asymmetries in lower limb power for injury prevention in youth athletes. However, the effects of training interventions on interlimb asymmetries in athletes are still controversial because of potential study bias, as indicated by a systematic review and meta-analysis.⁴ The commonly used methods for reducing asymmetry include both unilateral and bilateral strength and power training, such as body weight and plyometric exercise,³⁸ eccentric strength training,¹⁶ and core and lower limb strength training.³¹ Different training protocols have small-to-moderate effects on interlimb asymmetries on various lower limb functional tests after a minimum 6-week training intervention, and small-to-large effects in reducing asymmetry have been demonstrated in the training groups compared to the control groups.⁴ However, most studies to date have focused on investigating the role of interlimb asymmetries in injury prediction and the methods for reducing asymmetries, and there is a lack of research on whether correcting asymmetries in lower limb power can prevent sport injuries.

Limitations

We acknowledge the limitations of the present study. First, we were unable to stratify participants based on age or puberty stage because the sample size required for logistic regression analyses was large. The subgroup of female athletes was underpowered, with only 97 girls completing the 12-month follow-up. Further, we were unable to report the injury occurrence in stronger and weaker legs because the stronger leg might not always be on the same side across different tests. Another limitation was the potential variation caused by exposure time, an extrinsic factor that falls outside the scope of the present study and is difficult to control in practice. However, the training conditions were homogeneous and had been maximally controlled, as athletes came from the same district area, participated in the same taekwondo tournaments, and had a similar training frequency (≥1.5 h/day, ≥3 d/wk). Additionally, 3 participants were excluded from the study because of pre-existing injuries before baseline testing, as shown in Figure 1. This could potentially result in an underreported injury rate. Lastly, we acknowledge that the present findings need to be validated in sports other than taekwondo.

Conclusion

The current findings have important implications for youth taekwondo. Interlimb asymmetries in unilateral jump performance can serve as a useful measure to evaluate the risk of noncontact lower limb injuries in male and female youth taekwondo athletes. These findings provide a foundation for future investigation on whether correcting asymmetry in lower limb power can prevent injuries in youth taekwondo. Other factors, including interlimb asymmetries on the SEBT, age, maturity offset, training experience, body height and weight, BMI, and leg preference, were not significantly associated with noncontact lower limb injuries in youth taekwondo athletes. Further research should examine the relationship between interlimb asymmetries in jump performance and noncontact lower limb injuries for each age or pubertal stage of youth athletes.

Footnotes

Final revision submitted April 7, 2023; accepted April 14, 2023.

The authors have declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from the University of British Columbia (No. H18-02252).

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