BILATERAL DIFFERENCES IN THE UPPER QUARTER FUNCTION OF HIGH SCHOOL AGED BASEBALL AND SOFTBALL PLAYERS

Abstract

Purpose/Background:

The Upper Quarter Y Balance Test (YBT‐UQ) was developed as a way to identify upper extremity and trunk mobility in the open kinetic chain in the reaching limb as well as midrange limitations and asymmetries of upper extremity and core stability in the closed kinetic chain on the stabilizing limb. Performance on the YBT‐UQ is similar between genders and between limbs; however, this has not been examined in athletes who participate in sports that result in upper extremity asymmetries. The primary purpose of this study is to determine if differences exist between the throwing vs. non‐throwing sides in high‐school baseball and softball athletes on the YBT‐UQ.

Methods:

In order to complete this forty‐eight male high school baseball players and seventeen female high school softball players were tested on the YBT‐UQ. Reach distances were normalized to arm length (% AL). Comparisons were made between the throwing (T) and non‐throwing (NT) arm for each direction as well as the composite score.

Results:

No significant differences were observed between the T and NT arm for the medial (NT: 98.4 ± 8.6 %AL, T: 99.1 ± 8.6 %AL, p=0.42), inferolateral (NT: 90.8 ± 11.8 %AL, T: 90.3 ± 11.5 %AL, p =0.61), superolateral (NT: 70.6 ± 10.9 %AL, T: 70.4 ± 11.1 % AL, p=0.91) reaches, or the composite score (NT: 87.2 ± 8.9 % AL, T: 86.6 ± 8.1 %AL, p=0.72). Similarly, no differences were observed between the male baseball and female softball players (p=0.30‐0.90).

Conclusions:

Based on these findings, it was concluded that there was no difference in performance on the YBT‐UQ between throwing and non‐throwing limbs in high school baseball and softball players.

Level of Evidence:

3

INTRODUCTION

Shoulder injuries have been estimated to occur at a rate of 2.27 per 10000 to athlete exposures across high school sports.¹ High school baseball and softball athletes exhibited some of the highest injury rates (4.5 per 1000 athlete exposures) with the majority of these injuries being overuse in nature.^1,2 In high school baseball players, the throwing shoulder is the most common site of injury with a rate of 17%.³ Pitching accounts for 13% of the injuries in this group.³ These injuries have been associated with elevated pitch counts and limited range of motion in the shoulder complex.^4‐6 During the rehabilitation of these injuries it is suggested that the injured tissues are progressively loaded while integrating in local joint specific rehabilitation components with body region movements, core stabilizing exercises/activities, and eventually, functional patterns.⁷ The role of anatomical variation of the humerus on normal arthrokinematic changes and it's relationship to shoulder biomechanics has been extensively reported upon in the literature, however there is less reported on basic closed kinetic chain tests of the upper extremity that may have relevance in progressing a patient during rehabilitation who participates in throwing sports.^8‐21

There are currently few tests that assess closed chain upper quarter function, as opposed to sports specific tests, and only one prior study has examined throwing athletes. The primary non‐sport specific tests of basic closed chain upper extremity function are the Closed Kinetic Chain Upper Extremity Stability Test (CKCUEST), the One‐arm Hop Test, and the Upper Quarter Y Balance test (YBT‐UQ).^22‐24 The CKCUEST scores how often an individual can tap the floor past the contralateral hand while maintaining an upright pushup position with their feet and stabilizing hand in contact with the ground while keeping their hands 36 inches apart. In comparison, the One‐arm Hop test measures a more powerful movement by recording how long it takes an individual to hop with their hand onto a 10.2 cm step 5 times from a 3‐point plank position (contralateral hand placed behind back). Finally, the YBT‐UQ examines how far an individual can reach with one hand in the medial, inferolateral and superolateral directions while maintaining a 3‐point plank position on the opposite hand. Isolating unilateral closed chain function may be beneficial in identifying unilateral upper quarter performance limitations in order to optimize intervention strategies during rehabilitation of athletes who participate in movement activities where left and right upper extremities serve different roles. Based on expected performance requirements alone, it is likely that a continuum of testing exists that would suggest examining basic stability with the CKCUEST prior to testing a relatively low speed closed kinetic chain task (YBT‐UQ) before finally examining a more powerful closed kinetic chain task (One Arm Hop). Utilization of the YBT‐UQ may have a broader application in the adolescent and youth setting due to the reduced need of upper quarter power to perform the test. Research on all of the aforementioned tests has suggested that performance does not differentiate between sides; however, the subjects during the study did not participate in sports that result in the large upper extremity asymmetries associated with throwing activities.^22,24,25 Reliability on all of the aforementioned tests has been established, however, the inherent validity of these tests is still being established. The primary benefit of these data is to provide an understanding of normal asymmetry on tests of upper quarter function, however, none of the aforementioned studies have examined a cohort that participates in activities that promote upper quarter asymmetry, such as baseball and softball, thus research in this area is beneficial. It may be that these glenohumeral adaptations associated with participating in baseball and softball may bias performance on a unilateral basic test of shoulder complex closed chain function.

Previous research on upper extremity symmetry supports the concept of side‐to‐side differences, however, to date little is known about bilateral differences in scores of basic closed‐chain shoulder function in baseball and softball athletes. As a result, the primary purpose of this study is to determine if differences exist between the throwing vs. non‐throwing sides in high‐school baseball and softball athletes on the YBT‐UQ. A second purpose of this study is to examine performance differences between male baseball and female softball athletes scores of the YBT‐UQ. Based on previous research it is not expected that differences will exist between throwing and non‐throwing sides or across genders.^24,25

METHODS

Sample size estimates for the current study were developed by using an α = 0.05, β = 0.20 and a meaningful difference of 10% using previously published work.²⁴ Sample size estimates across the multiple reach directions were calculated manually and revealed that 28‐42 subjects would be needed to adequately power the study. As a result the research team aimed to obtain data from 50 baseball and softball players in order to count for potential dropout. Athletes at two high schools completed the testing as part of their standard pre‐participation physical testing (n = 48, 15 pitchers, age: 15.8 +/‐ 1.2 years) and softball (n = 17, 4 pitchers, age: 15.2 +/‐ 1.1 years). Any athlete who was currently painful (has pain but was participating in normal training), exhibited pain during the testing, or was currently injured (currently not participating do to injury) was excluded from the study. All other athletes who were currently participating in full team activities were included in the study. The data were entered into a centralized database from which de‐identified data were extracted and analyzed. The research protocol was approved by the institutional review board prior to data analysis. Forty‐eight high school baseball players and seventeen high school softball players comprised the final sample, upon whom testing was performed during pre‐season physicals. The average age of the baseball players was 15.8 +/‐ 1.2 years and the average age of the softball players was 15.1 +/‐ 1.1 years. The average upper limb length of the athletes was 90.8 +/‐ 4.0 cm for the baseball players and 83.7 +/‐ 4.2 cm for the softball players.

Procedures

The YBT‐UQ test was utilized in order to examine upper quarter closed chain function in the high school baseball and softball players. The YBT‐UQ has previously been established as a reliable functional test of the upper quarter and it has been determined that gender or bilateral differences do not exist in an active adult population.^24,25 Previous research on the YBT‐UQ has suggested that performance on the test exhibits moderate correlation with established shoulder and core stability measures.²⁵ Reliability measures for the research team were established across the testers in order to maximize testing validity (Inter‐rater ICC: 0.99‐1.00, Inter‐session ICC: 0.92‐0.95). Prior to the testing, the upper quarter limb length of each athlete was measured with the athlete standing with their feet together and their shoulder in 90 degrees of abduction in the frontal plane per protocol. In this position, a cloth tape measure was used to determine the distance (cm) from the spinous process of the 7^th cervical vertebrae to the tip of the right middle finger. The YBT‐UQ examines the ability of an individual to perform a unilateral activity while maintaining a three‐point plank position (one hand and two feet in contact with the ground) with the feet shoulder width apart. During the test, the athlete reaches in the three reach directions (medial, inferolateral, and superolateral [cm]) in a systematic order. Each trial of the YBT‐UQ consisted of the athlete reaching in the three reach directions (cm) then subsequently returning to the starting position in a controlled manner (Figure 1). In order for the trial to be acceptable the following criteria had to be maintained: 1) three points of contact had to be maintained between the floor and the hand and feet, 2) the athlete could not use momentum to move the reach box (i.e. push the box), 3) the athlete could not let the reach hand touch the ground during the trial, 4) the athlete could not use the top of the reach box or the testing equipment to help stabilize their body. In order to orient the athlete to the testing procedure, two practice trials were completed on the right side followed by two practice trials on the left side. The tested side was named based on which hand was providing support during the trial. After the practice trials were completed three performance trials were completed for each side, right followed by left. All of the athletes were asked if pain was present during the practice and performance trials. A Y Balance Test Kit (Move2Perform, Evansville, IN) and the YBT‐UQ protocol was used during the testing sessions.²⁴

Figure 1.

Performance on the Upper Quarter Y Balance test in the three reach directions (a. medial, b. inferolateral, c. superolateral).

The primary variables of interest for the study were the maximum reach in the medial, superolateral, and inferolateral directions for the throwing and the non‐throwing sides as well as the symmetry between the throwing and non‐throwing sides for each independent reach direction. In order to complete this analysis the maximum score for each reach direction was extracted to represent the end range of each individual's performance. The average maximum normalized reach across the three directions was calculated for each side in order to record a composite score for each subject.

Statistical Analyses

The data collected from the study were statistically analyzed with SPSS (version 17.0, Chicago, IL). Comparisons on the YBT‐UQ between the throwing and non‐throwing sides were completed using a dependent samples t‐test. This statistical model was utilized over an ANOVA to examine specific difference for each of the independent reach directions. Meanwhile, the gender differences were analyzed using an independent samples t‐test. All statistically significant differences were identified at p<0.05. Effect size indices (ESI: absolute value of [Mean Softball – Mean Baseball]/ Pooled SD) were also calculated for all comparisons due to a lower number of softball players in comparison with the baseball players. Any ESI over +/‐ 0.7 would be considered large and any lack of statistical significance with an ESI of this size may be attributed to limitations in sample size.

RESULTS

No differences were found for any of the reach directions or the composite score between the throwing and non‐throwing sides. (p=0.42‐0.91, ESI: 0.01‐0.08, Table 1, Figure 2) Performance on the test was greatest for the medial reach, followed by the inferolateral reach, and superolateral reach when examining the normalized reach scores. The composite score was also not statistically significant (p=0.72) with values being 87.2 +/‐ 8.9 % limb length (LL) for the non‐throwing side and 86.6 +/‐ 8.1 % LL for the throwing side.

Figure 2.

Differences in performance on the Upper Quarter Y Balance Test between the throwing and non‐throwing hands (* designates p<0.05, %LL = % limb length).

No difference in performance was observed between the genders for any of the reach directions or the composite reach (p=0.30‐0.90, ESI: 0.09‐0.16, Table 1, Figure 3). Both male and female subjects exhibited higher scores in the medial and inferolateral directions when compared with the superolateral. The composite reach for males and females was 87.1 +/‐ 8.6 % LL and 86.3 +/‐ 8.4 % LL respectively, which was not a statistically significant (p=0.63) difference either.

Figure 3.

Differences in performance on the Upper Quarter Y Balance Test between the female and male subjects (* designates p<0.05, %LL = % limb length).

DISCUSSION

Upper quarter injuries are common in athletes, particularly in sports involving a high level of repetitive overhead activity.^1,3 However, few closed kinetic chain tests have been identified for the upper quarter to identify athletes with deficits of performance in this region, which can be of particular relevance when progressing an athlete through rehabilitation of an injury.⁷,^22‐24 The YBT‐UQ has previously been identified as being a reliable test to assess basic closed kinetic chain ability of the upper quarter in a 3‐point plank position.²⁴ The purpose of the current study was to identify if performance on the test was affected by which side was utilized to stabilize the body during the performance. The results of the current study suggest that no difference exists between the throwing and non‐throwing sides when performing the YBT‐UQ. In addition, there does not appear to be an inherent gender difference attributed to performance on the YBT‐UQ test.

Few studies have examined upper extremity measures of closed chain performance. Falsone and colleagues²² reported that no difference existed on the One Arm Hop test between dominant and non‐dominant sides when tested in college athletes. Similar findings were recently reported on the YBT‐UQ.^24,25 Gorman and researchers²⁴ examined a group of active male (n = 51) and female (n=45) adults and observed no statistically significant differences between the men and women as well as no statistically differences between left and right sides. Similar work was conducted by Westrick and colleagues²⁵ who examined a cohort of male (n = 24) and female (n = 6) soldiers using the YBT‐UQ and observed no statistically differences between genders although the study had a limited female sample size. To date, however, no other studies have examined performance on this test in high school throwing athletes. The results of the present study suggest that inherent differences between sides do not exist during a basic closed kinetic chain task. Since performance on the YBT‐UQ did not occur at end range joint range of motion it is hypothesized that end range adaptations in joint function would not have a significant effect on performance of this test. Larger asymmetries in local glenohumeral function has been observed in more experienced athletes (college and professional) and as a result it is relevant to conduct additional testing in these populations. In addition, it may be that athletes who exhibit different levels of spinal mobility (i.e. gymnasts, divers, or wrestlers) that are required for successful skill specific tasks may perform differently on the YBT‐UQ. The sport‐based difference in performance has previously been observed on the star excursion balance test and thus may warrant investigation on the YBT‐UQ.

The current study is one of few that has examined whether gender differences exist in upper quarter functional testing. There are currently no published research studies examining the effect of gender on the CKCUEST or one arm hop test. Two prior studies on the YBT‐UQ revealed no differences between genders in a group of active adults, which is similar to the findings of the current study^24,25. It is important to reiterate that this is likely due to the fact that the reach distances are normalized. One study examining higher‐level performance measures did reveal that females perform lower than males in tasks that required a higher level of upper extremity strength and power.²⁶ The findings of this study suggest that performance deficits in females may not become apparent until achieving a certain level of loading. However, additional work needs to be conducted in this area particularly when loads and performances are normalized to anthropometric properties.

Since it has been suggested that performance varies on the lower quarter version of this test (Star Excursion Balance Test and Lower Quarter Y Balance Test) based on competition level, gender and sport ^27,28, it would be reasonable to expect that performance on the YBT‐UQ would demonstrate similar variance. Future studies should examine if there are gender differences for other sports and at other competition levels as well as if performance is different across different competition levels and sports alone.

Several limitations exist with the current study and its application. Primarily, the cohort is small, particularly the softball group, and limited in heterogeneity since all of the data were collected across two sites and as a result the external validity of the study is limited. The small sample size in the study may have increased the potential of a type II error, however based on the effect size index calculation any potential difference due to limited sample size alone would be small in nature and may have limited clinical relevance. Additional research with larger samples and different populations should examine these relationships in athletes who participate in sports where overhead function is asymmetrical. In addition, athletes of higher performance level(s) should be tested and compared to better understand how athletic strength, skill and experience influences the results of this type of testing. It may be that athletes with more ingrained motor programs would demonstrate larger asymmetries in these basic closed kinetic chain upper quarter patterns compared to high school aged athletes. Finally, it should be acknowledged that the YBT‐UQ tests closed kinetic chain function in mid range while simultaneously examining open kinetic chain function across a range of motion and thus serves primarily to examine the basic movement ability as opposed to the skill specific task. Inclusion of additional local measures of glenohumeral joint function, e.g. internal rotation range of motion and total arc of motion, to the current study would have been of benefit in order to understand how local glenohumeral function effects basic upper quarter closed kinetic chain performance.

Upper quarter functional testing research has many facets that still need to be examined. Currently the results of the test can serve as an indicator of average upper quarter function when looking for reference measures during the rehabilitation in high school athletes recovering from an upper quarter injury. It is important to understand how performance on the YBT‐UQ varies in other sports that involve a high frequency of overhead activity and to see if the same trends hold true with respect to gender and side‐to‐side differences. While, the average side to side difference reported in this study was not statistically different, 15% of the athletes did exhibit a difference of >10% which may suggest that inherent symmetry on this test cannot be assumed. It is also relevant to examine how these relationships change in athletes as years of experience increase. Performance on the test is likely based on a combination of joint mobility, motor control and proprioception and it would be helpful to understand if any of these factors dominate the overall performance on this test. It is likely that performance on the test is not only multi‐factorial for a given joint but it is also expected to be multi‐segmental. In the interim it may be beneficial to utilize the YBT‐UQ as a continuous measure of upper quarter closed kinetic chain stabilization, to follow CKCUEST testing and proceed One Arm Hop testing, which would be beneficial to normalize in order to maximize performance of the peripheral segments in achieving the high velocity associated with throwing.

CONCLUSION

In conclusion, performance on the YBT‐UQ does not appear to be affected by which hand is used to stabilize the body when testing overhead throwing high school athletes. In addition, there are no inherent differences between genders in high school aged athletes albeit additional research should be conducted in this area due to the lower number of softball athletes examined in this study. Current clinical application of the YBT‐UQ could consist of utilizing the test to determine if The results of the current research suggest that no statistically significant bilateral differences in upper quarter closed kinetic chain function exist in high school baseball and softball players. Initially this would suggest that baseball and softball players during rehabilitation should not exhibit significant asymmetries on the YBT‐UQ, regardless of a preferential throwing side.