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 five minutes of running at a moderate speed and a test-specific 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/m2). 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 finger 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 significant differences were identified at p < 0.05. Furthermore, effect size (Cohen’s d) was calculated and classified 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 two-dimensional 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).