Discriminative Validity of the Lower and Upper Quarter Y Balance Test Performance in Healthy Youth

Background: The Lower (YBT-LQ) and Upper (YBT-UQ) Quarter Y Balance Test have been widely used for the assessment of dynamic balance and shoulder mobility/stability, respectively. However, investigations on the validity of the two tests in youth are lacking. Therefore, we performed two studies to determine discriminative validity of the YBT-LQ (study 1) and the YBT-UQ (study 2) in healthy youth. Methods: Sixty-nine male soccer players (age: 14.4 ± 1.9 yrs) and 69 age-matched untrained male subjects (14.3 ± 1.6 yrs) participated in study 1 and 37 young swimmers (age: 12.3 ± 2.1 yrs) as well as 37 age-/sex-matched individuals (age: 12.5 ± 2.0 yrs) took part in study 2. Normalized maximal reach distances per reach direction and the composite score of the YBT-LQ/UQ were used as outcome measures. One-way analysis of variance and the receiver operator characteristic curve analysis (i.e., calculating the area under the curve [AUC]) were conducted to assess discriminative validity. Results: Youth athletes showed signicantly better YBT-LQ (study 1: p < 0.001, d = 0.86-1.21) and YBT-UQ (study 2: p < 0.001, d = 0.88-1.48) test performances compared to age- and sex-matched untrained subjects. Further, AUC-values indicated a chance of ≥ 74% (YBT-LQ) and ≥ 71% (YBT-UQ) to discriminate between youth athletes and controls. Conclusions: In healthy youth, the YBT-LQ and the YBT-UQ seem to be valid test instruments for the discriminative assessment of dynamic balance and shoulder mobility/stability, respectively.


Background
The Y-Balance-Test (YBT) represents a eld-based test [1,2] assessing i) dynamic balance performance when applied to lower extremities (YBT-LQ) [3] and ii) shoulder mobility/stability when applied to upper extremities (YBT-UQ) [4]. Both testing procedures have widely been used in previous literature to detect performance differences in different cohorts [3,[5][6][7]. Concerning the YBT-LQ, Butler et al. [3] investigated male high school, collegiate, and professional soccer players and detected signi cantly lower reach distances in the posteromedial and posterolateral directions for the high school players than the two other groups of player. Further, Bullock et al. [5] studied middle school, high school, college, and professional basketball players and observed that high school players performed signi cantly better (i.e., anterior reach direction) compared to middle school and college players. Regarding the YBT-UQ, Bullock et al. [6] tested high school and collegiate swimmers and found signi cantly better values for the medial reach direction in favour of the latter group of swimmers. In another study, Krysak et al. [7] compared middle school, high school, college, and professional golfer players and reported greater reach distances in the medial, inferolateral, and superolateral directions for the professional golfers compared to the three other groups. Results of the aforementioned studies shed light onto performance differences based on the athletes' level of competition and experience. However, not only their level of competition but also the age differed between cohorts. For example, in the study conducted by Butler and colleagues [3], age differed between 15.6 years in high school soccer players and 26.2 years in professional soccer players.
Further, participants in Krysak and co-workers [7] were aged between 12.2 years (middle school golfers) und 31.8 years (professional golfers). Thus, participants' age might have in uenced performance differences in these studies and it remains unclear whether the YBT-LQ/UQ is discriminatively valid. As a consequence, studies comparing age-and sex-matched persons with various levels of competition are needed.
Most notably, examining discriminative validity of the YBT-LQ/UQ in children and adolescents is important since these age groups are used for talent selection and scouting [8]. More precisely, the investigation of age-/sex-matched youth in relation to their training status is useful to discriminate highperformer versus low-performer using the YBT-LQ/UQ. Thus, the aim of the present study was to determine discriminative validity of the YBT-LQ (study 1) and the YBT-UQ (study 2) by comparing age-and sex-matched trained versus untrained youth. With reference to the relevant literature [3,[5][6][7], we expected better performances in both tests for trained compared to untrained youth and we hypothesized good discriminative validity for both tests.

Participants
Participants' characteristics are summarized in Table 1. In study 1, 69 male soccer players from a local sports club and 69 age-matched untrained male subjects performed the YBT-LQ. In study 2, 37 female and male swimmers from a local sports club and 37 age-/sex-matched untrained individuals conducted the YBT-UQ. Participants' assent and parents' written informed consent were obtained prior to the start of the study. The Human Ethics Committee at the University of Duisburg-Essen, Faculty of Educational Sciences approved the study protocol.

Testing procedures
Discriminative validity of the YBT-LQ and the YBT-UQ was assessed in study 1 and study 2 respectively. In both studies, we used a standardized general warm-up comprising ve minutes of running at a moderate speed and a test-speci c warm-up consisting of three submaximal reaches per arm/leg and reach direction. All participants received standardized verbal instructions and a visual demonstration regarding the testing procedure that included assessment of anthropometric variables (i.e., body mass, body height, arm length, leg length) followed by performance assessment in the YBT-LQ (study 1) or YBT-UQ (study 2).

Assessment of anthropometric variables
Body mass (kg) was measured in light clothing and without shoes to the nearest 100 g with an electronic scale (seca 803, Basel, Switzerland). Further, body height (cm) was determined without shoes to the nearest 0.5 cm with a stadiometer (seca 217, Basel, Switzerland). Body mass index was calculated using body mass divided by height squared (kg/m 2 ). Length (cm) of the right and left arm was determined with a cloth tape measure from the seventh cervical spinous process to the distal tip of the middle nger with the shoulder being in a 90° abduction [9]. Further, left and right leg length (cm) were assessed by measuring the distance from the anterior superior iliac spine to the most distal aspect of the medial malleolus using a cloth tape with the participant lying supine [10].

Assessment of Lower Quarter Y Balance Test performance
YBT-LQ performance was assessed by means of the YBT Kit (Functional Movement Systems®, Chatham, USA). The test kit consists of a centralized platform to which three pipes were attached representing the anterior (AT), posteromedial (PM), and posterolateral (PL) reach directions ( Figure 1A). Each pipe is marked in 1.0-cm increments for measurement purposes and equipped with a moveable reach indicator. The participants were asked to move the reach indicator as far as possible into the AT direction with the right leg while standing on the centralized platform with their left leg followed by standing on the right leg and reaching with the left leg. This protocol was then replicated for the PM and PL directions. Each participant performed three practice trials followed by three data-collection trials per leg and reach direction. A one-minute rest was provided between trials. The absolute maximal reach distance (cm) per leg and reach direction was used for further analysis. Reliability of the YBT-LQ has been shown to be predominately "excellent" in healthy youth [11].

Assessment of Upper Quarter Y Balance Test performance
The YBT Kit was also used for the assessment of YBT-UQ performance, with the three pipes representing the medial (MD), inferolateral (IL), and superolateral (SL) reach directions ( Figure 1B). Participants were instructed to move the reach indicator with the right arm as far as possible in the MD, IL, and SL directions while maintaining a weight bearing one-arm push-up position with their left arm on the centralized platform. This protocol was then replicated for the left arm. Three practice trials were conducted followed by three data-collection trials. The rest between trials comprised one minute. The best values (i.e., absolute maximal reach distance in cm) per arm and reach direction was used for further analysis. The reliability of the YBT-UQ ranged from "moderate-to-good" to "excellent" in healthy youth [12].

Data and statistical analyses
For both tests, normalized maximal reach distances (%) per reach direction and leg/arm were calculated by dividing the absolute maximal reach distance (cm) by leg/arm length (cm) and then multiplying by 100. In addition, the normalized (%) composite score (CS) per leg/arm was computed as the sum of the absolute maximal reach distance (cm) per reach direction divided by three times leg/arm length and then multiplied by 100.
Further, the mean value was calculated as a measure of central tendency and the standard deviation (SD) as a dispersion measure. Discriminative validity was analyzed using the one-way analysis of variance (ANOVA). Statistically signi cant differences were identi ed at p < 0.05. Furthermore, effect size (Cohen's d) was calculated and classi ed as "small" (0 ≤ d ≤ 0.49), "moderate" (0.50 ≤ d ≤ 0.79), and "large" (d ≥ 0.80) [13]. Moreover, we conducted a receiver operator characteristic (ROC) curve analysis and calculated the area under the receiver operator characteristic (AUC) curve. The AUC measures the entire twodimensional area underneath the entire ROC curve. In this regard, Deyo and Centor [14] stated that an AUC-value of 0.50 indicates "no" and an AUC-value of 1.0 indicates "perfect" discriminative validity. All statistical analyses were performed using Statistical Package for Social Sciences version 24.0 (SPSS Inc., Chicago, IL, USA).

Results
Discriminative validity of Lower Quarter Y Balance Test performance (study 1) Statistical data on the discriminative validity for YBT-LQ performance between young male soccer players and age-matched male untrained subjects are displayed in Table 2 Table 3 shows the statistics on the discriminative validity for YBT-UQ performance between young female and male swimmers and age-/sex-matched untrained subjects. Irrespective of the reach arm, analyses showed signi cant, large-sized differences in favour of swimmers for the MD (p < 0.001, d = 0.92-1.09), IL (p < 0.001, d = 0.88-0.99), and SL (p < 0.001, d = 1.11-1.48) directions as well as for the CS (p < 0.001, d = 1.17-1.28). In addition, AUC-values ranged between 0.73 to 0.85 and 0.71 to 0.79 for the right and left arm reach, respectively. In other words, there is a chance of 71-85% that the YBT-UQ is possible to distinguish between swimmers and age-/sex-matched untrained subjects.

Discussion
To our knowledge, the present studies investigated discriminative validity of YBT-LQ/UQ performance between healthy trained and untrained youth for the rst time. Main results can be summarized as follows: (1) trained youth (i.e., soccer players and swimmers) showed large-sized signi cantly better YBT-LQ/UQ performance compared with age-/sex-matched untrained controls; (2) ROC analyses revealed a chance of ≥74% (YBT-LQ) and ≥71% (YBT-UQ) to discriminate youth athletes from untrained youth.
In line with our hypothesis stating better performances in both tests for trained compared to untrained youth, one-way ANOVA revealed signi cantly larger YBT-LQ/UQ reach distances in trained participants (i.e., soccer players and swimmers) compared to age-/sex-matched controls. This result corresponds with ndings from studies [3, 5-7, 15, 16] that investigated groups of athletes with varying levels of competition (e.g., high school vs. collegiate vs. professional players) and reported better YBT-LQ/UQ performance for those with a higher than for those with a lower competition level. However, besides the differences in competition level, the included persons also differed in age (i.e., adults versus adolescents), which might have in uenced the results. In the present study, we included age-/sex-matched controls and results nonetheless showed better YBT-LQ/UQ-performance in trained compared to untrained youth. This nding is in line with a study by Engquist et al. [17] that investigated young adults (mean age: 20 ± 1.6 years) and found larger YBT-LQ reach distances in trained (i.e., female Division I student-athletes) compared to non-trained but same-aged (i.e., general female college students) individuals. Our ndings and the results by Engquist and colleagues indicate that differences in YBT-LQ/UQ performance in ageand sex-matched individuals are based on training status and competition level. Additionally, and also in accordance with our hypothesis stating good discriminative validity for the YBT-LQ/UQ in healthy youth, our ROC analysis yielded a chance between 74 to 81% (YBT-LQ) and 71 to 85% (YBT-UQ) to discriminate youth athletes from age-/sex-matched untrained youth. Thus, YBT-LQ (i.e., dynamic balance) and YBT-UQ (i.e., shoulder mobility/stability) demands seem to be associated with respective athletic requirements in soccer and swimming.
What might be the reason for performance differences in YBT-LQ/UQ between trained and age-/sexmatched untrained youth? One might argue that trained compared to non-trained youth possess a higher amount of long-lasting, continuous and intense training experience [18]. Another reason might be the genetic pro le of trained individuals [19,20]. For example, Murtagh et al. [19] investigated the relationship of multiple single nucleotide polymorphisms with physical performance measures in elite male youth soccer players and control participants. The authors observed differences in the genetic pro le (e.g., higher genotype frequency distribution in soccer players) and showed that physical performance was associated with some measures of the genetic pro le. Taken together, both preconditions cause speci c adaptations [21,22] that allow for higher performances in sport-speci c as well as in physical tness measures.
From a practical perspective, our ndings of signi cantly better YBT-LQ/UQ values in trained compared to untrained youth and the good discriminative validity indicate that both tests can be used to distinguish between young athletes and age-/sex-matched controls based on dynamic balance (YBT-LQ) and shoulder mobility/stability (YBT-UQ) data. Consequently, both testing procedures can be used to discriminate persons with higher compared to lower levels of performance. This allows for the possibility to offer speci cally tailored sport programs to support growing-ups according to their individual performance level, e.g., tness promoting programs for low t individuals and young athlete training regimens for high t subjects.