Physical Therapy Korea

Various functional tests are used in the medical, sports, and training fields to assess scapular stability and mobility [1] because they are easy to perform, inexpensive, and capable of providing essential information on functional performance [1,2]. Functional tests can measure basic elements of functional movement (muscle strength, mobility, and stability) or specific physical movements [3,4] and can also be used as criteria for return after rehabilitation or may identify people at risk of injury [3]. The upper quarter Y-balance test (UQYBT) and the closed kinetic chain upper limb stability test are commonly used to evaluate the stability and mobility of the upper extremity with good reliability in closed kinetic chain position [1,5-8]. UQYBT allows quantitative evaluation of physical ability by reaching the arm in different directions with one hand while supporting the body with the other hand [7]. During the evaluation, the participant stabilizes the body weight while reaching the maximum distance in each direction (medial, superolateral, and inferolateral) [8].

Performing UQYBT needs sufficient mobility and stability of the scapular and core stability to maintain body alignment in the narrow base of support [5]. If the case of insufficient scapular stability or mobility and core stability, blocks may not be pushed far when performing UQYBT. The anterior serape refers to the transmission path of force in front of the body while moving it [9]. These paths consist of diagonal lines from one side of the hip flexor muscle to the other side of the serratus anterior muscle [9]. Moreover, the iliopsoas muscle is the main hip flexor and is considered a core muscle because it attaches to the lumbar spine [10,11]. The push-up test is also used to evaluate core muscle [12], and the hip flexors play a significant role in maintaining body alignment during push-ups [13]. Therefore, it is necessary to consider the hip flexor muscle along with other muscles when performing UQYBT in the push-up posture.

Previous studies showed correlations between the scapular muscle (scapular protractor and lower trapezius [LT]) strength measured using a hand-held dynamometer (HHD) and the UQYBT score [14] and did not confirm the correlation between the UQYBT score and hip flexor strength, which is important as the core muscle and plays an important role in transferring power from the upper limb to the lower limb. Therefore, in addition to scapular muscle strength, it is necessary to determine the correlation between UQYBT score and hip flexor strength, which has not been investigated to date, with an objective muscle strength measurement sensor. The purpose of this study to investigate the relationship between the UQYBT score and strengths of the scapular (scapular protractor and LT), trunk, and hip flexor muscles using a smart KEMA strength sensor (KOREATECH Inc.) for healthy male participants. If the correlation between hip flexor and UQYBT is revealed, not only scapular muscle and trunk muscle but also hip flexor should be considered when using UQYBT for rehabilitation programs or physical function evaluation. The study hypothesizes that there would be a positive correlation between the UQYBT score and strengths of the scapular (scapular protractor and LT), trunk, and hip flexor muscles.

1. Participants

The sample size was 37 with an alpha risk of 0.05, a power (beta risk) of 0.9, and a correlation coefficient of 0.5 using the G*Power program (ver. 3.1; Franz Faul, Kiel University) based on a previous study [14]. For our study, 37 healthy male participants (age = 27.54 ± 6.20 years; weight = 76.89 ± 12.36 kg; height = 174.76 ± 6.45 cm) were recruited from the community (Table 1). Exclusion criteria were: (1) persons without a normal range of motion for the shoulder joint; (2) those with shoulder pain, trauma, dislocation, or rotator cuff injury in the previous 6 months; and (3) those with a neurological condition, spine deformation, or root symptoms [14]. The Yonsei University Mirae campus Institutional Review Board approved the study (IRB no. 1041849-202209-BM-160-02), and all participants provided written informed consent.

Table 1 . Participants’ characteristics.

Variable	Male (N = 37)
Age (y)	27.54 ± 6.20
Height (cm)	174.76 ± 6.45
Weight (kg)	76.89 ± 12.36

Values are presented as mean ± standard deviation..

2. Instrumentation

In this study, the strength of the scapular protractor, LT, hip flexors, trunk flexor, and trunk extensor was measured in kg utilizing a smart KEMA strength sensor. Measured data were sent through Bluetooth to a tablet device (Korea’s Galaxy Tab A6 10.1; Samsung, Inc.), which were analyzed using the smart KEMA software (KOREATECH, Inc.). A previous study measuring shoulder strength reported that the smart KEMA strength sensor indicated good to high intra-examiner reliability (range = 0.85 [95% confidence interval, CI = 0.54–0.95] to 0.98 [95% CI = 0.94–0.99]) [15]. Another study that measured the hip flexor muscle strength reported that test-retest reliability was higher in a smart KEMA strength sensor (0.86; 0.81–0.89 with 95% CI) than in HHD (0.71; 0.65–0.76 with 95% CI) [16]. Our study calculated the average muscle strength value of the two trials. Moreover, participants were allowed a 1-minute rest time between all measurements to minimize muscle fatigue. Each muscle was contracted with the maximum strength for 5 seconds, and the mean value of the middle 3 seconds was used for data analysis. For muscle strength normalization, the muscle strength was divided by the participant’s weight and multiplied by 100.

3. Procedures

To measure the isometric strength of the scapular protractor, the participant lies on the table while looking at the ceiling. Then, they were asked to flex their shoulders at 90° and held the handle of the smart KEMA strength sensor vertically connected to the floor. The initial tension of the strap was set at 2 kg. The participant executed a scapular protraction using the maximum strength toward the ceiling. To prevent contraction of the pectoralis major muscle, the participant was required not to adduct or internal rotation the arm and do not rotate trunk during the scapular protraction motion (Figure 1A). For LT, the participants were asked to abduct their arms by 130°, and the thumb remained facing the ceiling in prone position. The strap connected to the smart KEMA strength sensor was located at the distal humerus to prevent elbow flexion of the participant during measurement. The initial tension of the strap was set at 2 kg. Participants raised their arms toward the ceiling with their arms abducted and their thumbs facing the ceiling. Then, they were instructed to move only the arm as much as possible without trunk extension or rotation (Figure 1B). For hip flexor muscles on both sides, participants flexed the hips and knees at 90° in supine position, and a strap connected to the smart KEMA strength sensor was placed 5 cm above the knee [16,17]. The initial tension of the strap was set at 2 kg. The participant was instructed to pull the knee toward the chest using maximum strength while maintaining the posture (Figure 1C). For trunk flexor, the participant lies on the table in a crook-lying position with their feet fixed to the table using an orthopedic belt to stabilize the lower extremity during trunk flexion. Then, the belt connected to the smart KEMA strength sensor was placed at the center of the participant’s sternum. The participant’s arms were crossed in front of the chest. The participant was instructed to feel the resistance of the belt and perform trunk flexion with maximum strength (Figure 1D). For trunk extensor, participant lies face down with the sternum at the end of the table. The feet and hips were fixed using a strap, and one side of the smart KEMA strength sensor was positioned to the floor and the other side was placed at the participant’s trunk. The participants placed their hands on their foreheads with their hands overlapped and were instructed to perform trunk extension motion using maximum strength (Figure 1E).

Figure 1. Measurement of isometric muscle strength. (A) Scapular protractor, (B) lower trapezius, (C) hip flexor, (D) trunk flexor, and (E) trunk extensor.

The measuring tape was attached to the floor in a Y shape [7]. To mark the point where the supporting hand is located using the dominant-side hand and to fix the measurement tape, a midline was made at the intersection of the measurement tape using another tape. The participant assumed a push-up posture with his feet at shoulder width. The little finger of the dominant-side hand was placed on the midline (Figure 2A). Using the fingertips, the blocks placed in the medial, superolateral, and inferolateral directions were pushed as far as possible with an unsupported hand (Figure 2B). Then, two repetitions were performed in each direction, and the reached distance was measured in cm. The distance pushed in each direction was normalized by dividing the length of the participant’s arm and then multiplied by 100. The composite score calculated using each direction’s average value was added, divided by three times the length of the upper limb, and then multiplied by 100. It was considered that UQYBT was not accurately performed in the following cases: 1) When participants are unable to maintain stability with one arm or touch the floor with the unsupported hand; 2) When the fingertips cannot maintain contact with the block during push the block; and 3) when the foot is lifted off the ground [7,8]. The length of the upper limbs was assessed with the shoulder abduction of 90°, the elbow extension, and the wrist, and hand in neutral. It was measured from the spinous process of C7 to the end of the middle finger using a tape measure [14].

Figure 2. Upper quarter Y-balance test (UQYBT). (A) Modified UQYBT start position. (B) Modified UQYBT end position (each direction of medial, superolateral, and inferolateral).

4. Statistical Analysis

Statistical analysis was conducted utilizing the IBM SPSS for Windows ver. 26.0 software (IBM Co.). To assess the data normality, the Kolmogorov–Smirnov test was used. Pearson’s correlation was used to assess the correlation between the isometric strength of the muscles (scapular protractor, LT, hip flexors, trunk flexor, and trunk extensor) and the score for each direction and the composite score of UQYBT. The British Medical Journal’s classifications recommended the level of association was graded as follows: 0–0.19 extremely weak, 0.2–0.39 weak, 0.40–0.59 moderate, 0.6–0.79 as strong, and 0.8–1.0 very strong [1,18].

Table 2 indicates the mean ± standard deviation of the scores for each direction and composite scores of UQYBT and the strength of the supporting-side scapular protractor, supporting-side LT, non-supporting-side hip flexor, and supporting-side hip flexor, trunk flexor, and trunk extensor. Table 3 presents the correlation between UQYBT and isometric strength of the scapular (scapular protractor and LT), trunk, and hip flexor muscles. The superolateral direction of UQYBT was moderately to strongly related to the isometric strength of both hip flexors, supporting-side scapular protractor, supporting-side LT, and trunk extensor. Moreover, the inferolateral direction of UQYBT was moderately related to the isometric strength of the supporting-side scapular protractor, supporting-side LT, and both hip flexors. Furthermore, the medial direction of UQYBT was moderately to strongly related to the isometric strength of the supporting-side scapular protractor, supporting-side LT, and both hip flexors. Lastly, the composite score of UQYBT was weakly to moderately related to the isometric strength of the trunk extensor, supporting side of the scapular protractor, and LT, and strongly related to both sides of hip flexors.

Table 2 . Isometric muscle strength and each direction score and a composite score of UQYBT.

Variable	Male (N = 37)
Muscle strength (%bw)
Supporting-side scapular protractor	21.04 ± 8.72
Supporting-side lower trapezius	8.97 ± 3.66
Supporting-side hip flexor	19.78 ± 6.91
Non-supporting-side hip flexor	18.19 ± 6.83
Trunk flexor	16.75 ± 6.43
Trunk extensor	33.65 ± 11.69
UQYBT score (%AL)
Medial	108.00 ± 9.80
Superolateral	58.94 ± 13.17
Inferolateral	70.38 ± 12.69
Composite score	79.26 ± 9.97

Values are presented as mean ± standard deviation. UQYBT, upper quarter Y-balance test; bw, body weight; AL, arm length..

Table 3 . Correlation between isometric strength and upper quarter Y-balance test.

	Supporting-side scapular protractor	Supporting-side lower trapezius	Non-supporting-side hip flexor	Supporting-side hip flexor	Trunk flexor	Trunk extensor
Medial direction	0.522**	0.541**	0.561**	0.605**	0.232	0.261
Superolateral direction	0.412*	0.436**	0.641**	0.669**	0.379*	0.443**
Inferolateral direction	0.429**	0.511**	0.521**	0.481**	0.171	0.250
Composite score	0.521**	0.567**	0.664**	0.678**	0.302	0.368*

Values are presented as r value. *p < 0.05, **p < 0.01..

This study was performed to assess the relationship between UQYBT and isometric strengths of the scapular protractor, LT, trunk flexor, trunk extensor, and hip flexors. The composite score of UQYBT was strongly related to the isometric strength of both sides of hip flexor strength, moderately with the isometric strength of the supporting-side scapular protractor and LT. Outstandingly, the superolateral direction of UQYBT score was moderately related to the isometric strengths of the trunk flexor and trunk extensor.

In this study, the hip flexor strength was moderately to strongly related to the UQYBT score. Especially, with inferolateral direction, the correlation with the non-supporting-side hip flexor strength may be higher correlation than that with the supporting-side hip flexor strength due to the connection with the anterior oblique sling force like the anterior serape, a type of muscle connection that runs from one hip flexor to the internal oblique and the contralateral external oblique to the shoulder muscle [9]. Moreover, the anterior serape refers to a form of force transmission from one hip flexor to the abdomen diagonally to the opposite shoulder muscle [9]. The proximal regions of the shoulder and hip muscles are stabilized by generating a stiffened core in a spiral shape, resulting in greater arm and leg movements across the body [9]. McGill et al. [19] confirmed that the deep muscles such as the psoas are activated when performing push-up movement. Sahrmann [20] mentioned that the psoas muscle is activated during push-ups, and Juker et al. [21] confirmed that the psoas muscle, which acts as a hip flexor, showed a maximum voluntary contraction value of up to 25% during push-ups. Therefore, since UQYBT is also based on the push-up posture, our study may have shown a moderate to strong correlation between the isometric strength of both sides of the hip flexors and the UQYBT score. Therefore, the muscle strength of the hip flexors should also be considered when using UQYBT.

Our results showed a correlation between the isometric strength of the supporting-side scapular protractor, supporting-side LT, and UQYBT score. UQYBT is based on the push-up position; however, a motion matching the one-arm push-up position is created, in which the body is supported by one hand while pushing the block with an unsupported hand. Previous studies on muscle activation in the one-arm push-up posture on a stable support surface revealed that the mean normalized root mean square values for the scapular protractor, such as the serratus anterior muscle, was statistically higher than those for the biceps brachii, anterior portion of the deltoid, and trapezius upper fibers [22]. Consistent with the findings of previous studies, our study also supported the entire body with one hand and pushed the block with the other hand, a form similar to a one-arm push-up, showing a correlation between the isometric strength of the supporting-side scapular protractor such as the serratus anterior muscle and the UQYBT score. Also, Mendez-Rebolledo et al. [14] confirmed that the inferolateral direction of UQYBT correlated to LT (r = 0.845). In addition, a study of overhead athletes playing volleyball or handball also confirmed the correlation between LT strength and inferolateral direction of the UQYBT score (r = 0.53) [23]. Previous studies have confirmed the correlation between the scapular muscles such as serratus anterior and trapezius when performing close kinetic chain (CKC) tests such as the side-bridge test [24]. Therefore, as our study also revealed, sufficient periscapular muscle strength such as scapular protractor and LT is required when performing CKC tests such as UQYBT.

In this study, the isometric strength of the trunk flexor and trunk extensor was correlated to the superolateral direction of the UQYBT score, a form of twisting the body. Maeo et al. [25] confirmed greater electromyography of the upper extremity and abdominal muscles when performing push-ups on unstable support surfaces such as slings. When conducting anterior chain exercises such as push-up workouts, in which the arm is extended forward and touches the floor, the trunk flexor activation is increased by 110% [26]. Shah et al. [27] and Kavcic et al. [28] confirmed that the trunk extensor such as the longissimus muscle is activated while raising the arms and legs in the quadruped posture. This activation is believed to be caused by increased muscle activity to sustain the body in the narrowed space [27,28]. Therefore, the correlation between the isometric strengths of the trunk flexor, trunk extensor, and superolateral direction of the UQYBT score is particularly high, because it is similar to the motion of an extended arm and twisted trunk with the superolateral direction of UQYBT in the narrowed basement.

Mendez-Rebolledo et al. [14] examined the correlation between muscle strengths and UQYBT in volleyball players, and the results revealed a very strong correlation between LT strength and inferolateral direction of UQYBT (r = 0.845), whereas our study found a moderate correlation (r = 0.521) between the isometric strength of the supporting-side LT and inferolateral direction of the UQYBT score. This difference may be associated with two reasons. First, the muscle strength measurement equipment was different. Previous studies used an HHD, which may have the possibility of measuring the participant’s muscle strength differently depending on the examiner’s pressure. Further, a recent systematic review that examined the reliability of upper-limb muscle strength measurement using HHD demonstrated that only 48% showed good intra-rater reliability [29]. Thus, physical therapists should not depend on HHD when measuring the muscle strength of the upper extremities [29]. Second, since previous studies have been conducted on volleyball athletes, a difference in participants’ characteristics is observed because they frequently engage in sports activities such as throwing motions [14]. Athletes who do a lot of throwing motions, such as baseball players, need periscapular muscle strength to maintain proper scapular upward rotation [30]. A previous study reported a moderate to good positive correlation between LT strength and scapular upward rotation at 90° (r2 = 0.56) and 120° (r2 = 0.53) [30]. Moreover, the LT plays an important role in controlling the scapular elevation and protraction of overhead throwers while performing a throwing motion [31,32], and exercise to strengthen the LT is routinely performed [31]. Therefore, compared to previous studies that included athletes, the probability of experiencing training is lower in our study.

This study has several limitations. First, because only male participants engaged in this study, it is difficult to explain the correlation between UQYBT and muscle strength in females. Second, the results of this study cannot be applied to patients with pathological factors because only healthy participants were included. Third, most of the participants were young; thus, the research results cannot be generalized to other age groups.

The isometric strength of both sides of hip flexors was associated with medial, superolateral, and inferolateral directions of the UQYBT test in healthy male participants. The hip flexors are essential for maintaining push-up posture and performing UQYBT. Based on the study results, the strength of not only the scapular and trunk muscles but also the hip flexor muscles should be considered when using UQYBT as part of the rehabilitation program or a measurement method to assess sports performance in the future.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: JJ, OK. Data curation: JJ. Formal analysis: JJ, JK. Investigation: JJ. Methodology: JJ, JK, OK. Project administration: JJ. Supervision: JK, OK. Validation: JJ. Writing - original draft: JJ. Writing - review & editing: JK, OK.