1. Introduction
Breakage of the anterior cruciate ligament (ACL) is one of the most serious injuries in view of its long-term consequences, due to the high risk of repeat injury [
1], the long time required to return to competition and the degenerative changes and reduction in knee function [
2]. The most common situations in which ACL injuries occur in handball are contactless actions involving jumping and landing or changes of direction [
3]. Although the origin of injury is multifactorial, the first strategy to reduce risk must be to identify modifiable risk factors [
4].
Deficits in muscle strength, power, coordination and muscle activation give rise to abnormal biomechanical movement patterns and to neuromuscular disorders which potentially increase the risk of ACL injury [
4]. Hewett et al. [
5], Myer et al. [
6] and Fort-Vanmeerhaeghe et al. [
7] classified them under six main categories: (1) leg dominance, which is dominance of the frontal compared with the sagittal plane as a dynamic control strategy of the lower limb in dynamic actions; (2) quadriceps dominance, which refers to dominance of muscle strength and activation of the extensor muscles compared with the flexor muscles of the knee [
8]; (3) difference in muscle recruitment patterns, strength and coordination between the lower limbs, also called leg dominance; (4) trunk dominance, which is defined as the inability to control the position of the trunk in space in dynamic actions, which can lead to lateral movements of the trunk and thus increase knee abduction loads; (5) neuromuscular fatigue; and (6) reduction of the anticipatory or feedforward mechanisms.
These neuromuscular and biomechanical imbalances need to be examined and identified individually in female players in the early stages of growth, since this is when the ACL injury rate is highest [
9]. The specific literature indicates anatomical, hormonal, biomechanical and neuromuscular control changes related to puberty as the main intrinsic risk factors explaining this higher incidence [
10]. As they reach the last stages of pubertal maturation, their neuromuscular control deteriorates, giving rise to landings with less flexion of the knees and hips [
8], leading to greater ground reaction forces [
11]. They also present relative force deficits in the lower limbs, above all in the hamstrings [
12] and lower activation of these muscles in relation to the quadriceps in landing actions, which potentially subject the ACL to a greater load in comparison with less mature girls [
13].
An immature skeletal and muscular system, combined with the specific requirements of handball, can lead to specific unilateral adaptations of the sport, which can represent an intrinsic risk factor for ACL injury [
14]. The asymmetry (leg dominance) between lower limbs has been a common line of research in recent years, covering three different aspects: (a) identify individuals who may be at risk of injury [
15]; (b) monitor the function of the limbs during the rehabilitation process [
16]; and (c) determine its impact on sport performance [
17]. The types of assessment vary in complexity, from the simple height and distance of the jump to the three-dimensional analysis of the movement of the whole body, and, consequently, have a variety of cost and time implications [
14]. The single leg countermovement jump (SL-CMJ) is one of those most frequently used in view of its validity and reliability [
18]. Fort-Vanmeerhaeghe et al. [
15] demonstrated that the interlimb differences in SL-CMJ could be a possible lower limb injury risk factor in team sports undertaken by young teens. On the contrary, the quantitative value of the asymmetry magnitude does not provide information on the quality of the movement. Assessing the landing technique of the lower limbs during a plyometric activity can provide information on neuromuscular and biomechanical deficits related to asymmetry [
19]. The Tuck Jump Assessment (TJA) is an easy and cost-effective tool which allows errors to be identified in the landing technique of continuous jumps for 10′’ [
19]. There are ten defects of technique which can be assessed qualitatively; three of them refer to asymmetry [
6]. Assessing the level of motor competence during repeated jumping tasks, in which the female player is requested to respond to the disturbances and to the forces of the movement with presence of fatigue, may be more valid, from an ecological viewpoint, to identify ACL injury risk factors [
20].
Fort-Vanmeerhaeghe et al. [
7], Arundale et al. [
21] and Benet-Vigo et al. [
22] provide modified TJA values but consider the population by maturation age, sex or type of sport. Moreover, to date, no study has related the detection of asymmetries by grouping defects in the landing technique in TJA to the asymmetry magnitude assessed quantitatively using the SL-CMJ. Therefore, the main aim of this study was to analyse and compare, by chronological age, jumping and landing deficits in young female handball players using the TJA, and the secondary objective was to relate the qualitative asymmetry values detected using the TJA to the quantitative asymmetry values detected, starting with the SL-CMJ.
3. Results
The Kappa value according to the two raters in the TJA test was good to excellent for all items (
Table 3). The ICC values for the total score of the TJA were excellent for all groups (between 0.92 and 0.95). The consistency between attempts for each age group in the SL-CMJ
L and SL-CMJ
R test was between 5.82% and 12.88%. All the ICC values were considered to be between good and excellent (between 0.82 and 0.96). Only the SL-CMJ
L test for the U16 category showed poor values (0.64) (
Table 4).
The female players from the U12 category obtained the highest scores in the TJA (12.11 ± 1.97) (
Table 5) and significant differences were only observed between the U12 and U14 categories (
p = 0.017; ES = 0.374). Three of the ten criteria of the TJA showed significant differences between categories, these being: lower extremity valgus at landing (criterion 1) (χ
2 = 17.31;
p = 0.002; V = 0.38), foot contact timing not equal (criterion 6) (χ
2 = 10.47;
p = 0.033; V = 0.22) and landing contact noise (criterion 7) (χ
2 = 8.73;
p = 0.013; V = 0.38) (
Table 5). Eleven players had “high to extreme” values (z-score 13.41) and sixteen were “small to moderate” (z-score = 11.84) (
Figure 2).
The highest asymmetry magnitudes in the SL-CMJ test (14.88 ± 4.77) were observed in the U16 category (
Table 6). Significant differences were not found between the asymmetry values of the SL-CMJ by category.
There was a directly proportional and statistically significant association in the U12 category between the asymmetry of the SL-CMJ test and the sum of items 3, 5 and 6 of the TJA test (r = 0.43; p = 0.027). The female players who obtained higher scores in the sum of the three criteria (3, 5 and 6) were also those who presented bigger interlimb asymmetry magnitudes in the unilateral vertical jump.
4. Discussion
The main aim of this study was to analyse and compare, according to chronological age, the jumping and landing patterns in young female handball players using the TJA. The main findings indicated that the players from the U12 category were those who presented higher values and, therefore, greater neuromuscular and biomechanical deficits in jumping and landing technique. The three criteria in which significant differences were observed between categories were: lower extremity valgus at landing (criterion 1), foot contact timing not equal (criterion 6) and landing contact noise (criterion 7). Second, the qualitative and quantitative asymmetry values detected were related using the TJA and the SL-CMJ, respectively. The female players from the U12 category who obtained higher scores in the sum of the three qualitative criteria of the TJA (qualitative value) which assessed asymmetry (3, 5 and 6) were also those who presented bigger interlimb asymmetry magnitudes in the SL-CMJ (quantitative value).
The mean of the overall score of the participants in the modified version of the TJA was 11.44 ± 1.97. These values were higher than in the studies by Benet-Vigo et al. [
23] and Fort-Vanmeerhaeghe et al. [
7] with girls of the same age; 8.82 ± 2.31 and 9.86 ± 2.01, respectively. This could be attributed to the nature of the sample of the present study, female handball players with less experience who did not undertake training sessions devoted exclusively to strength training. The highest total scores of the TJA were recorded in the U12 category, which was also the only category in which players of pre-PHV (n = 2/27) and circa-PHV (n = 10/27) maturation age were identified (
Table 1). In the U14 and U16 categories, the mean of the total scores was lower than in the U12 category (
Table 5) and all the players were already of post-PHV maturation ages. Taking into account the category in which the female players competed, significant differences were only found in the total value of the TJA in the U12 category in relation to the U14 category (
p = 0.017; ES = 0.374). In the studies by Read et al. [
37] and Fort-Vanmeerhaeghe et al. [
7], the total score of the TJA went down as the chronological and maturation age of the participants went up. The improvement in the TJA score with age could be explained by neuromuscular and structural changes which occur during growth and maturation or by the fact that the players with more experience adapted positively to a greater training history [
37].
In the original version of the TJA [
19], the 10 criteria are assessed in a dichotomous manner and it is suggested that a total score above six should be the threshold to identify female players with a greater risk of injury and, therefore, the target of specific prevention training. In the present study, in which the modified version of the TJA was used [
6], the participants with the highest scores in relation to the mean were identified by classifying them into two groups using the z-score calculation [
23,
36]. Eleven players out of the total obtained scores from “high to extreme” (z ≥ 1) and fifteen players obtained scores from “small to moderate” (z = 0.20 to 0.99) (
Figure 2). The majority of the players with scores from “high to extreme” (8/11) and from “small to moderate” (8/15) were in the U12 category (
Figure 2). The use and interpretation of the z-score allow identification of the players whose injury prevention strategies should be optimised [
38] since, as demonstrated, the maturation phase and the chronological age of the female players can influence the jump landing performance [
7].
The U12 category was the only one in which all participants performed the landing with accentuated valgus (the knees were touching) during the TJA (
Table 6). In this same criterion (1), significant differences were found between the three categories (χ
2 = 17.31;
p = 0.002; V = 0.38). This neuromuscular deficit is attributed to a lower limb strength deficit which prevents the correct absorption of the forces generated in the jumping and landing actions and has a greater impact in girls and at early maturation ages [
39,
40]. Hewett et al. [
39] indicate that the girls who present a greater valgus movement at landing have a high risk of ACL injury. These results confirm the need for neuromuscular training appropriate for the age in order to reduce the strength and motor control deficits with the aim of reducing female players’ injury risk [
10,
41].
Criteria 6 (foot contact timing not equal) and 7 (excessive landing contact noise) also showed significant differences between the three categories (χ
2 = 10.47;
p = 0.033; V = 0.22 and (χ
2 = 8.73;
p = 0.013; V = 0.38, respectively). Criterion 6 is grouped under the leg dominance neuromuscular risk factor [
6], which is associated with lower limb asymmetries in the jumping and landing model. Over 70% of the sample in the U12 and U14 categories presented an asymmetric jumping arrival, in which one foot landed before the other. This neuromuscular deficit can be attributed to the nature of the sport, since its practice contributes to asymmetric development of the limbs [
42]. In regards to criterion 7, no player was capable of performing the jumping arrivals without noise and only making contact with the tip of the foot, indicating that the study sample did not use sufficiently effective movement strategies to mitigate and dissipate landing forces [
37].
The TJA contains 10 jumping and landing technique criteria rateable from 0 to 2. The criteria are added, obtaining an overall score. The bigger the resulting value, the more deficits the female player assessed will have in the jumping and landing model. Lininger et al. [
43] and Arundale et al. [
21] recommend not using the accumulated final value of the TJA as the only score, but rather identifying each risk factor described by each category of grouped criteria [
6] (
Figure 1). The biggest asymmetry score (3.71 ± 1.11) was detected in the U16 category, without there being significant differences between categories. In the case of the SL-CMJ, where the asymmetry was determined with a percentage value referring to the difference in jump between the two legs [
44], the highest asymmetry values were also in the U16 category (14.88 ± 4.77). In the U16 category, despite having the highest asymmetry values in both the qualitative and the quantitative assessment, there was not a significant correlation between them; that is to say that the players who had greater asymmetry values in the SL-CMJ were not those who had higher asymmetry scores in the TJA. There was a significant correlation in the U12 category (r = 0.42;
p = 0.027). The female players who presented high asymmetry values in the SL-CMJ also obtained a higher score in the asymmetry criterion of the TJA. These results, significant in early categories, suggest the need to include the qualitative assessment to identify which motor strategies the player uses to carry out the skill being developed, in this case jumping and landing. Populations who exhibit asymmetries between lower limbs have been associated with a higher incidence of injury [
44], and ACL injuries during jumping tasks are more likely to occur during in the landing phase [
13], so in training there should not only be the opportunity to improve the performance of the limb which shows a clear and consistent deficit compared to the other [
42], but emphasis should also be placed on the technique; on how it is executed.
It should, however, be taken into account that the asymmetry is specific to the task [
17,
45,
46], and therefore more than one test would be necessary to determine an asymmetry profile for a player and thus increase the sensitivity of the assessment.
To date, there have not been any studies which correlate lower limb asymmetry assessed by means of a qualitative test, which gives information on how it is executed, and the difference in interlimb performance expressed as a percentage (magnitude). There are two studies conducted with a sample of similar characteristics (young girls) which related motor skills in jumping ability to performance. Sommerfield et al. [
29] used the drop vertical jump (DVJ) as a tool to assess the landing model and found a negative association with the relative isometric force indicator (r = −0.35,
p < 0.01); that is to say that the girls who presented a deficit in the landing model were those who had less relative isometric force. On the other hand, Pullen et al. [
47] used the TJA as a qualitative tool to assess the jumping and landing model, but did not find any significant relation between motor skills (overhead squat, push-up, lunge, front support brace and shoulder Touch and Tuck Jump) and horizontal jump performance with both legs.
Although this study compared a bilateral (TJA) with a unilateral (SL-CMJ) task, probably involving different motor strategies [
48], the results indicate the need to prioritise training of the motor model in basic categories in order to create a good coordination strategy. The stabilisation of the landing to optimise muscle activation and thus ensure an adequate technique and alignment of the jumps (gentle landing with knees aligned) should be the first step before increasing the intensity and variability, for example, going from a bilateral to a unilateral task, involving different planes and axes, including disturbances with internal or external stimuli, combined with expected and unexpected actions to improve the feedforward capacity, increase the intensity of the muscle stretching and contraction cycle, combine elastic and reactive actions and progressively introduce the state of fatigue [
49,
50,
51,
52,
53].