Journal of Bodywork & Movement Therapies 28 (2021) 332e340

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Journal of Bodywork & Movement Therapies
journal homepage: www.elsevier.com/jbmt

Scapular dyskinesis type is associated with glenohumeral joint andscapular kinematic alteration during pitching motion in baseballplayers
Atsushi Ueda a, b, *, Aoi Matsumura c, Takafumi Shinkuma d, Takeshi Oki d,Yasuo Nakamura eaGraduate School of Health and Sports Science, Doshisha University, Japan
Department of Rehabilitation, Hankai Hospital, Japan
TAKE PHYSICAL CONDITIONING Inc., Japand
Department of Orthopaedic Surgery, Hankai Hospital, Japane
Department of Health and Sports Science, Doshisha University, Japanbca r t i c l e i n f o

a b s t r a c t

Article history:
Received 12 February 2021
Received in revised form
10 June 2021
Accepted 17 July 2021

Introduction: Scapular dyskinesis (SD) is associated with an increased risk of throwing-related shoulder
injury onset, resulting in abnormalities in glenohumeral joint (GH) and scapular motions during pitching.
The effects of SD on shoulder motion during pitching remain unknown. This study aimed to investigate
kinematic alterations in GH and scapular motions during pitching in baseball players with type I SD.
Methods: Sixty-seven university and independent-league baseball players with and without SD were
included. Pitching motion was measured using an optical three-dimensional motion capture system, anda SD test was conducted to evaluate SD. SD was classiﬁed into types IeIV. The inter-rater reliability of SD
assessment was calculated using kappa coefﬁcients. Three-dimensional GH and scapular kinematics
during pitching motion were analyzed.
Results: The percentage of agreement representing the inter-rater reliability of SD assessment was 77.6%(52/67; kappa coefﬁcient: 0.72). Overall, 24 and 27 participants were categorized into abnormal (type ISD) and normal group (type IV SD), respectively, with normal scapular motion; one individual with type
III SD was excluded. The abnormal group exhibited a signiﬁcantly increased GH external rotation angle(9 ) and decreased scapular posterior tilt angle (6 ) during the maximum external rotation period
compared with the normal group.
Conclusions: Baseball players in the abnormal group showed increased GH motion and decreased
scapular motion during pitching. The SD test for the evaluation of type I SD can help predict excessive GH
external rotation and decreased scapular posterior tilt during pitching.
© 2021 Elsevier Ltd. All rights reserved.

Keywords:
Scapular dyskinesis
Pitching motion
Injury prevention
Baseball

1. Introduction
In recent years, preventive measures against throwing injuries
in baseball players have been described (Sakata et al., 2019).
Nonetheless, the incidence of throwing injury continues to increase(Chalmers et al., 2019), with pitching arm injuries accounting for
approximately 50% of all injuries in baseball players (Conte et al.
2016) and throwing-related shoulder injuries accounting for* Corresponding author. 1-3 Tatara Miyakodani, Kyotanabe-shi, Kyoto-fu, 6100394, Japan, Tel.: þ8172-233-6745; fax: þ8172-233-6928.
E-mail address: u.atsushi1119@gmail.com (A. Ueda).
https://doi.org/10.1016/j.jbmt.2021.07.039
1360-8592/© 2021 Elsevier Ltd. All rights reserved.

approximately 35% of all pitching arm injuries (n ¼ 1609) (Fares
et al., 2020). Moreover, throwing injury onset has reportedly been
associated with shoulder dysfunction and pitching biomechanics(Bullock et al., 2021; Salamh et al., 2020). Consequently, clinicians
have been investigating the risk factors for shoulder dysfunction
from various perspectives and analyzing the biomechanical parameters during pitching motion to effectively prevent throwingrelated shoulder injuries in baseball players (Beckett et al., 2014;
Ellenbecker & Cools 2010; Laudner et al., 2006; Shitara et al., 2017).
Scapular dyskinesis (SD) is a potential risk factor for shoulder
injuries, which include superior labral anterior-posterior (SLAP)
lesions, rotator cuff tears, and shoulder impingement syndrome

A. Ueda, A. Matsumura, T. Shinkuma et al.

Journal of Bodywork & Movement Therapies 28 (2021) 332e340

glenohumeral joint and scapular motion during pitching in baseballplayers with type I SD.

(Burkhart et al., 2003). Kibler et al. deﬁned SD as a condition
characterized by scapular malposition and abnormal motion(Kibler 1998). An SD incidence of >54% and >50% was reported inoverhead athletes (Burn et al., 2016) and high school baseballplayers (Myers et al., 2013), respectively. Moreover, athletes withSD have a 43% higher risk of developing shoulder pain than those
without SD (Hickey et al., 2018), suggesting a relationship between
SD and shoulder pain onset. Considering these data, clinicians needto evaluate SD and take measures to manage the health of overheadathletes with shoulder injuries.
The scapular dyskinesis test (SDT) is a reliable method for SD
evaluation (Huang et al., 2015;Kibler et al. 2002). In the SDT, SD is
assessed by observing abnormal scapular movements when theshoulders are raised and lowered (Huang et al., 2015; Kibler et al.
2002). According to Kibler et al. SD can be categorized into thefollowing types during the SDT: type I, prominence of the inferior
scapular angle; type II, prominence of the medial scapular border;
type III, excessive upward rotation and elevation of the scapula; andtype IV, no abnormality (Kibler et al., 2002). Each SD type isreportedly caused by some form of scapular dysfunction, including
inﬂexibility of the scapular muscles, muscle weakness, and posterior shoulder tightness (Ellenbecker and Cools 2010;Kibler et al.
2002).
With respect to the relationship between SD types and scapularmotion, Kibler et al. reported that individuals with SD types I, II, andIII presented with increased scapular anterior tilt, internal rotation,and upward rotation, respectively, upon raising and lowering theirshoulders (Kibler et al., 2002). In contrast, Huang et al. revealed that
individuals with mixed types I and II SD had increased scapularinternal rotation and anterior tilt during the SDT compared to thosewith other types (Huang et al., 2015). These ﬁndings have quantitatively clariﬁed the relationship between SD types and scapularmotion during the SDT.
Regarding the relationship between SD and throwing motion,
Kibler et al. and Burkhart et al. revealed that abnormal scapularmotion in SD is associated with abnormal glenohumeral joint
motion involving shoulder symptoms during pitching (Burkhart
et al., 2003; Kibler et al., 2013). Nevertheless, the relationship between SD types and shoulder biomechanics during pitching motion
has not yet been quantitatively clariﬁed.
The maximum external rotation (MER) period refers to the
crucial phase in pitching motion involving shoulder injury onset
due to an anterior shearing force of 380 N on the shoulder joint(Fleisig et al., 1995). Shoulder MER during pitching motion consistsof not only external rotation of the glenohumeral joint but also
scapular posterior tilt and thoracic extension (Miyashita et al.,2010). A decrease in scapular posterior tilt during pitching motion results from excessive shoulder external rotation (Mihata et al.,2010; Miyashita et al., 2010; Suzuki et al., 2019). Burkhart et al.
reported that baseball players with SD had excessive glenohumeral
joint motion during pitching (Burkhart et al., 2003). Excessive
shoulder external rotation during the MER period in SD consequently leads to SLAP injury, rotator cuff injury, and internal
impingement (Burkhart et al. 2000, 2003; Fleisig et al., 1995; Kuhn
et al., 2003; Mihata et al. 2008, 2010). Thus, we hypothesized that
individuals with type I SD (reduced scapular posterior tilt) have alarger range of motion (ROM) in the glenohumeral joint and asmaller ROM for scapular posterior tilt at MER than those without
SD (type IV). As the relationship between type I SD and pitchingmechanics of the shoulder joint has not yet been described, weinvestigated the inﬂuence of SD types on shoulder motion during
pitching to better understand the pathology during pitching motion, resulting in further applicable measures to prevent shoulder
injuries. Hence, this study aimed to clarify the incidence of SD types
in baseball players and investigate kinematic alterations in

2. Methods
2.1. Design
A cross-sectional design was employed to investigate differences in the glenohumeral joint and scapular kinematics during
pitching motion in baseball players with type I SD and those
without SD.
2.2. Participants
In all, 67 baseball players (61 university players and six
independent-league players) were included in this study. The study
period was from 2019 to 2021. The a-priori power analysis (a＝0.05,
1-b ¼ 0.80) of data from a previous study (Kim et al., 2020) was
performed using G*Power (Version 3.1.9.4 Kiel University, Germany). We estimated that a sample size of 23 participants pergroup would be adequate to determine signiﬁcant differences between the groups based on large effect size (Cohen's d > 0.8). Thosewith shoulder pain and any pain related to throwing within thepast three months were excluded from the study. This study was
conducted in accordance with the Declaration of Helsinki and wasapproved by the ethical review board of our institution (approval
number: 18019). Written informed consent was obtained from eachparticipant.
2.3. Demographic data of study participants
Demographic data, including age, height, weight, body massindex (BMI), and competition history, were collected from all study
participants via an interview using a questionnaire (Table 1).
Furthermore, shoulder internal/external rotation ROM wasmeasured on both sides with the participant in the supine position
and the shoulder and elbow abducted at 90 and ﬂexed at 90 ,respectively (Wilk et al., 2011). Ball velocity was measured using a
speed gun (Sports Radar Ltd. SRA3000, FL, USA).
2.4. SD evaluation
Considering its high measurement reliability, the SDT was used
to evaluate SD (Huang et al., 2015; Kibler et al. 2002). Brieﬂy, theparticipants grasped 2ekg weights on both sides and simultaneously performed maximum shoulder ﬂexion and abduction
along with maximum elbow extension of both sides for threecycles. Moreover, repetitions were performed at the same speed
for 3 s for elevation and 3 s for descent. The scapular motion
during the SDT was recorded using a digital video camera (FDRAX60, Sony Group Corp., Tokyo, Japan) from the thoracic dorsalside and was evaluated by video observation and direct palpationon the scapular joint (Huang et al., 2015; Kibler et al. 2002). Twophysiotherapists assessed SD in each participant, and only participants who were deemed to have SD by both physiotherapists
completed the study. The inter-rater reliability of SD assessment
between the physiotherapists was calculated using kappa coefﬁcients. The discriminant criterion for SD was based on theclassiﬁcation methods of Kibler et al. and Huang et al. Each participant's SD type was classiﬁed as follows according to scapularkinematics: type I, prominence of the inferior scapular angle; typeII, prominence of the medial scapular border; type III, excessive/
inadequate upward rotation and elevation of the scapula; a combination of types IeIII; and type IV, normal scapular motion (Fig. 1)(Huang et al., 2015; Kibler et al. 2002). To determine the effect of333

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Journal of Bodywork & Movement Therapies 28 (2021) 332e340

Table 1
Demographic data of the abnormal and normal group.
Abnormal groupa
Age, years
20 ± 1Height, cm
175 ± 5Weight, kg
75 ± 8BMI
24 ± 2Competition history, years
12 ± 1
Shoulder internal rotation range of motion, deg
Throwing side
34 ± 12
Non-throwing side
49 ± 13
Shoulder external rotation range of motion, deg
Throwing side
112 ± 9Non-throwing side
101 ± 8Ball velocity, m/s
31 ± 2

Normal groupa

95% CI for difference

P valueb

Effect sizec

20 ± 1176 ± 575 ± 724 ± 212 ± 3

0.4 to 1
2 to 4
4 to 5
1 to 1
2 to 1

0.40
0.64
0.88
0.87
0.70

0
0.20
0
0
0

36 ± 1351 ± 14

5 to 9
6 to 10

0.56
0.59

0.16
0.15

111 ± 9102 ± 731 ± 2

7 to 4
4 to 5
1 to 1

0.63
0.79
0.92

0.12
0.13
0

Abbreviations: BMI, body mass index; CI, conﬁdence interval.
aMean ± standard deviation.
b
Values less than 0.05 were considered statistically signiﬁcant.
cCohen's d effect size.

Fig. 1. Classiﬁcation of scapular dyskinesis. (A) Prominence of the inferior scapular angle: type I. (B) Prominence of the medial scapular border: type II. (C) Excessive/inadequate
upward rotation or elevation of the scapula: type III. (D) Normal scapular motion: type IV.

2.5. Pitching data collection

the posterior scapular tilt during the pitching motion according toSD types in baseball players, the inclusion criteria of this study
were as follows: “abnormal group (with type I SD)”, which
comprised subjects with isolated type I, type I þ II, type I þ III, andtype I þ II þ III SD, and “normal group”, which comprised cases oftype IV SD only (without SD; as a control group). Before the study,
the posterior scapular tilt angle during the shoulder motion was
signiﬁcantly lower in the type I SD than in the other types (Huang
et al., 2015). Non-type I SDs, such as isolated type II and III, were
excluded from this study.

Pitching motion was measured using an optical threedimensional motion capture system (MAC3D system; Motion
Analysis Corp., MA, USA) at a sampling frequency of 240 Hz. All
participants practiced pitching for approximately 10 min before themeasurements were taken. All participants threw a maximal effort
foureseam fastball on ﬂat ground (Oliver & Weimar 2015). In
previous studies, scapular motion was measured during pitching
using an electromagnetic tracking device attached to the acromion(Konda et al., 2015; Meyer et al., 2008; Oliver & Weimar 2015). For
334

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Journal of Bodywork & Movement Therapies 28 (2021) 332e340

motion was deﬁned as follows: the moment when the hand markerof the throwing side was at the lowest point (STA); stride footcontact (SFC) period; shoulder MER period; and ball release (BR)(Fig. 3) (Fleisig et al., 1995; Jobe and Bradley 1988). The interval of
the analysis period was normalized from 0% at STA to 100% at BR.
GH, ST, HT, and trunk joint angles were calculated during the SFC
and MER periods. Moreover, the peak value of the ST joint angle
was calculated from SFC to MER periods, and analysis was performed using MATLABR2019a (MathWorks, MA, USA).

scapular motion measurement during pitching, we employed theacromion marker cluster (AMC) method as a substitute for theelectromagnetic tracking device (Matsumura et al., 2019). Videobased motion analysis, such as the AMC method, is superior to
the electromagnetic tracking device as it provides a threedimensional image, is non-invasive, and causes less movement
restriction because it is cableless (Chu et al., 2012; Meyer et al.,2008). The AMC method has a measurement error of 2.96 e4.48
according to Brochard et al., (2011) and employs a marker clustercomprising 4-mm markers of four points on the acromion (Fig. 2).
To reduce skin artifacts with shoulder joint motion during pitching,bilateral marker clusters made of lightweight material (3 g) wereattached to the acromion using tapes. This study used the AMC
method to measure the scapular position in multiple shoulder
postures using a locator (Fig. 2) to correct the scapular position
prior to pitching motion measurement (Matsumura et al., 2019;
McClure et al., 2001). Furthermore, the locator had a 4emm marker
on each triangular top, which pointed at the acromial posterior
angle, spinous triangle, and inferior angle (Matsumura et al., 2019;
McClure et al., 2001). In this study, scapular motion was estimatedduring pitching using the scapular posture in the scapular natural
position at 0 , elevation at 120 , and elevation plane at 30 . Pitchingmotion measurement was conducted with 60 reﬂective markers
attached to the whole body (Wu et al. 2002, 2005). Moreover, toanalyze the posture of the upper arm and thorax during pitchingmotion, markers were attached to the medial and lateral humeralepicondyles, 7th cervical and thoracic spinous processes, jugularnotch, and xiphoid process (Wu et al., 2005).
Local coordinate systems for the scapula, humerus, and thorax
based on the recommendations of the International Society ofBiomechanics (ISB) were used (Wu et al., 2005). The shoulder joint
center was identiﬁed as the center of the humeral head estimatedfrom the acromion on the throwing side using the Fleisig method(Fleisig et al., 1995) as follows: (1) scapular orientation with respect
to the thorax (ST): internal rotation/external rotation, posterior tilt/
forward tilt, and downward rotation/upward rotation; (2) humeralorientation with respect to the scapula (GH): horizontal abduction/
horizontal adduction, external rotation/internal rotation, andabduction/adduction; (3) humeral orientation with respect to thethorax (HT): horizontal abduction/horizontal adduction, externalrotation/internal rotation, and abduction/adduction; and (4)thoracic orientation with respect to the ground (trunk): nonthrowing-side tilt/throwing-side tilt, non-throwing-side rotation/
throwing-side rotation, and extension/ﬂexion, which were calculated using Euler angles (Wu et al., 2005). Similarly, rotationalsequence was determined using the ISB method (Wu et al., 2005).

2.7. Statistical analyses
Age, height, weight, BMI, bilateral shoulder internal/externalrotation ROM, ball speed, and shoulder kinematic data (HT, GH, andST joints) during pitching motion from SFC to MER periods werecompared between the abnormal and normal (with and withoutSD) groups using the unpaired tetest. Additionally, SD (with andwithout) was considered the dependent variable, whereas age,height, weight, BMI, bilateral shoulder internal/external rotation
ROM, ball speed, and shoulder kinematic data were considered the
independent variables during pitching motion. The level of significance was set at 5%, and statistical analyses were performed using
IBM SPSS Statistics version 25 (IBM Corp., Tokyo, Japan). Additionally, the effect size (Cohen's d) was calculated to clarify the
degree of difference in each parameter between the two groups.
3. Results
The percentage of agreement representing the inter-rater reliability of our SD assessment was 77.6% (52/67; kappa coefﬁcient:0.72). In all, 51 participants were included in this study and classiﬁed into types IeIV. The incidence of SD, including types IeIII, was
49% (25/51). In the total study cohort, 24 participants (47%) had
type I SD; 12 participants (24%) had isolated type I SD, 8 (16%) had
types I and II SD, 3 (6%) had types I and III SD, and 1 (2%) had types I,
II, and III SD (Table 2). Twenty-seven individuals (53%) had type IV
SD with normal scapular motion, and no one was diagnosed withisolated type II SD. Only 1 participant had isolated type III SD(Table 2) and was excluded from the study.
No signiﬁcant differences in age, height, weight, BMI, competition history, and shoulder internal/external rotation ROM wereobserved between the abnormal and normal groups. Additionally,ball speed was not signiﬁcantly different between the two groups(Table 1).
The characteristics of GH and ST motion during pitching in theabnormal and normal groups are presented in Tables 3 and 4 andFig. 4eA, B, and C, respectively. The pitching cycle (%) for eachpitching period was as follows: 56% in the abnormal group and 61%
in the normal group during the SFC period, and 91% in the abnormalgroup and 91% in the normal group during the MER period (Figs. 3
and 4eC).

2.6. Data analysis
The fastest pitching measured in this study was selected foranalysis (Oliver & Weimar 2015). The analysis period of pitching

Fig. 2. Location of the acromion marker cluster and measurement of scapular posture using the locator.
335

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Journal of Bodywork & Movement Therapies 28 (2021) 332e340

Fig. 3. Phases of a pitching cycle. The pitching cycle in the abnormal and normal groups was as follows: SFC: 56% in the abnormal and 61% in the normal group; MER: 91% in theabnormal and 91% in the normal group. The STA and BR periods in the abnormal and normal groups were 0% and 100%, respectively. STA, hand marker of the throwing side at thelowest point; SFC, stride foot contact; MER, maximum external rotation; BR, ball release.

GH external rotation during the MER period was signiﬁcantly
higher in the abnormal group than in the normal group (P < 0.01,
d ¼ 0.75; Table 3). No signiﬁcant differences in other GH parameters during the SFC and MER periods were identiﬁed between thetwo groups (Table 3).
ST posterior tilt during the MER period was signiﬁcantly lower
in the abnormal group than in the normal group (P ¼ 0.02, d ¼ 0.71;
Table 3). No signiﬁcant differences in other ST parameters during
the SFC and MER periods were noted between the two groups(Table 3).

Table 2
Incidence of scapular dyskinesis (n ¼ 51).
Number (% total)
Abnormal group (with type I)
Isolated type I
Type I þ II
Type I þ III
Type I þ II þ III
Normal group (with type IV)

Detail (% total)

24 (47)12 (24)8 (16)3 (6)1 (2)
27 (53)Only 1 participant was diagnosed with isolated type III SD and was excluded.

Table 3
Scapulothoracic joint, glenohumeral joint, humerothoracic joint, and trunk angles during pitching motion in the two groups. a.
SFC

MER
Normalgroup

95% CI for
difference

EffectP
valueb sizec

Scapulothoracic joint, deg
Upward rotation
19 ± 10
Internal rotation
15 ± 11
Posterior tilt
5 ± 1221 ± 815 ± 109 ± 105 to 5
8 to 4
3 to 10

0.93
0.48
0.26

0.22
0
0.36

Glenohumeral joint, deg
Horizontal abduction 20 ± 20
Abduction
67 ± 12External rotation
28 ± 28

22 ± 1465 ± 1229 ± 26

9 to 6
7 to 6
15 to 14

0.64
0.86
0.95

Humerothoracic joint, deg
Horizontal abduction 34 ± 11
Abduction
84 ± 11External rotation
41 ± 2933 ± 1382 ± 944 ± 30

8 to 6
8 to 4
15 to 19

6 ± 8

4 ± 6

7 ± 11
1 ± 10Abnormalgroup

Trunk, deg
Non-throwing-side
tilt
Non-throwing-siderotation
Extension

Abnormalgroup

Normalgroup

95% CI for
differenceP
Effectvalueb sizec

Scapulothoracic joint, deg
Upward rotation
26 ± 10
Internal rotation
2 ± 15
Posterior tilt
19 ± 925 ± 72±925 ± 8

3 to 7
9 to 4
1 to 10

0.37
0.45
0.02b

0.12
0.17
0.04

Glenohumeral joint, deg
Horizontal abduction 0 ± 11
Abduction
74 ± 10External rotation
142 ± 113±674 ± 6133 ± 13

6 to 3
5 to 5
15 to 2

0.50
0.07
0.97
0
<0.01b 0.75

0.76
0.48
0.79

0.08
0.20
0.10

Humerothoracic joint, deg
Horizontal abduction 6 ± 7
Abduction
101 ± 8External rotation
160 ± 97 ± 799 ± 5157 ± 9

4 to 4
5 to 2
9 to 1

0.84
0.46
0.14

0.14
0.15
0.33

7 to 1

0.13

0.28

24 ± 826 ± 66 to 2

0.35

0.28

5 ± 12

4 to 10

0.34

0.17

100 ± 8100 ± 8

5 to 50.99

0

3 ± 94 to 7

0.60

0.21

14 ± 716 ± 95 to 4

0.71

0.25

Trunk, deg
Non-throwing-side
tilt
Non-throwing-siderotation
Extension

0.12
0
0.71

Abbreviations: CI, conﬁdence interval; MER, maximum external rotation; SFC, stride foot contact.
aMean ± standard deviation.
Values less than 0.05 were considered statistically signiﬁcant.
cCohen's d effect size.
b

Table 4
Peak value of scapulothoracic joint angle during pitching motion in the two groups. a.
Peak value (SFC-MER)

Scapulothoracic joint, deg
Upward rotation
Internal rotation
Posterior tilt

Abnormal group

Normal group

95% CI for difference

P valueb

Effect sizec

29 ± 818 ± 1124 ± 926 ± 8
21 ± 1028 ± 92 to 7
8 to 4
0.1 to 9

0.24
0.44
0.06

0.38
0.29
0.44

Abbreviations: CI, conﬁdence interval; MER, maximum external rotation; SFC, stride foot contact.
Mean ± standard deviation.
b
Values less than 0.05 were considered statistically signiﬁcant.
cCohen's d effect size.
a336

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Journal of Bodywork & Movement Therapies 28 (2021) 332e340

Fig. 4. (A, B and C) d Temporal changes in the scapulothoracic and glenohumeral joint angles during pitching motion in the two groups (solid line: abnormal group, dashed line:
normal group). The pitching cycle during each period shows the average value between the four types of scapular dyskinesis. STA, hand marker of the throwing side at the lowest
point; SFC, stride foot contact; MER, maximum external rotation; BR, ball release; GH, humeral orientation with respect to the scapula: abduction (þ)/adduction (), horizontal
abduction (þ)/horizontal adduction (), external rotation (þ)/internal rotation (). ST, scapular orientation with respect to the thorax: downward rotation (þ)/upward rotation (),internal rotation (þ)/external rotation (), posterior tilt (þ)/anterior tilt ().

et al., 2013). Thus, the incidence of SD in our study was roughly
similar to the incidence reported by previous studies.
Among the four SD types, type I was the most common, accounting for 47% (24/51) of SD types, contrary to the results obtained by Kawasaki et al. who used the fouretype classiﬁcation in
103 rugby players and reported the incidence of different types of
SD to be 5.8% (6/103) for type I, 3.9% (4/103) for type II, 22.3% (23/103) for type III, and 68% (70/103) for type IV and that the numberof type III cases was greater than the number of cases of SD types I
and II (Kawasaki et al., 2012). Hence, the difference in SD characteristics between our baseball players and rugby players may be
attributed to differences in SD types among baseball players andother competitive athletes. Nonetheless, studies on the incidence of
SD in baseball players using the four-type classiﬁcation are limited.
Thus, further larger-sample studies are required to conﬁrm ourﬁndings.
In this study, the ball speed was 31 m/s in the abnormal groupand 31 m/s in the normal group (Table 1). Previous studies reported
a ball speed of approximately 35e36 m/s at university and professional levels (Fleisig et al., 2016; Kageyama et al., 2015; Konda
et al., 2015). The kinematic parameters of pitching motion indicated a slower ball speed in this study than in previous studies(Fleisig et al., 2016; Kageyama et al., 2015; Konda et al., 2015).
Therefore, the baseball players in this study might have lower
performance levels than those in previous studies. Furthermore,
our study participants threw the ball on ﬂat ground (throwing
without a pitching mound), which might have further affected theball speed.
Regarding the shoulder motion from SFC to MER periods, the GH
angle in this study showed horizontal abduction, abduction, andexternal rotation in both abnormal and normal groups (Fig. 4),
whereas the ST angle increased upward rotation, internal rotation,

The peak value of ST posterior tilt from SFC to MER periods wastended to be lower in the abnormal group than in the normal group(P ¼ 0.06, d ¼ 0.44; Table 4). Moreover, the timing of the peak angleof ST posterior tilt (abnormal group: 85%, normal group: 88%) was
prior to the MER period（abnormal group: 91%, normal group: 91%)
(Fig. 4-C). No signiﬁcant differences in the kinematic parameters ofpeak values of ST motion from SFC to MER periods were observed
between the two groups (Table 4). Furthermore, there were no
signiﬁcant differences in HT posture parameters and trunk motion
parameters during the SFC and MER periods between the twogroups (Table 3).
4. Discussion
This study investigated the characteristics of GH and ST motion
during pitching in baseball players in the abnormal (with type I SD)
and normal groups (with type IV SD). Our results indicated thatduring the MER period, the abnormal group had signiﬁcantly
increased GH external rotation (9 ) and decreased ST posterior tilt
(6 ) compared to the normal group. Our results reveal that theabnormal group exhibited increased GH motion and decreased ST
motion compared to the normal group during the MER period.
The ﬁndings of this study were similar to those reported byKibler et al. and Kawasaki et al. (Kawasaki et al., 2012; Kibler et al.
2002). Besides, the high inter-rater reliability of SD assessment inthis study showed that the SDT was sufﬁciently reliable. The incidence of SD, including types IeIII, was 49% (25/51) in our study. A
systematic review conducted by Burn et al. revealed that the incidence of SD was approximately 55% in overhead athletes, including
baseball, handball players and swimmers (Burn et al., 2016).
Additionally, Myers et al. reported that the incidence of SD was 46%(122/246) among American high school baseball players (Myers
337

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Journal of Bodywork & Movement Therapies 28 (2021) 332e340

Table 5
Glenohumeral joint and scapular kinematic data at MER during pitching motion reported in the literature.
a
Glenohumeral joint

Scapular

External rotation
Horizontal abduction
Abduction
Posterior tilt
Internal rotation
Upward rotation

Miyashita et al.

Konda et al.

Oliver and Weimar

The present study (abnormal)

The present study (normal)

106
e
e
24
e
e

114
6
85
11
15
23

e
e
e
14
22
37

142
0
74
19
2
26

133
3
74
25
2
25

b)Trunk Kinematic Data at MER During Pitching Motion Reported in the Literature

Trunk

Extension
Non-throwing-side rotation
Non-throwing-side tilt

Miyashita et al.

Oyama et al.

Kageyama et al.

Barﬁeld et al.

The present study (abnormal)

The present study (normal)

9
e
e

e
96
22

e
80
e

e
e
13

14
100
24

16
100
26

Abbreviations: MER, maximum external rotation.

type IV SD. Thus, each ratio of GH external rotation and ST posteriortilt, including shoulder external rotation (Miyashita et al., 2010),
could differ between the abnormal and normal groups.
The abnormal group exhibited an increase in GH externalrotation and a decrease in ST posterior tilt. Previous studies havereported on the effect of excessive GH external rotation on theonset of internal impingement and SLAP tear (Kuhn et al., 2003;
Mihata et al. 2004, 2012) due to the posterior-superior shift of thehumeral head (Mihata et al., 2015) and excessive extension of superior labral and bicepsesuperior labral complexes (Kuhn et al.,
2003; Pradhan et al., 2001). Moreover, the decrease in scapular
posterior tilt at MER during pitching motion involves the development of throwing-related shoulder injuries due to excessive GH
external rotation (Burkhart et al., 2003; Miyashita et al., 2010;
Suzuki et al., 2019). We observed greater GH external rotation andsmaller ST posterior tilt angle in the abnormal group than in thenormal group, which may result in an increased risk ofthrowingerelated injuries, such as internal impingement and SLAP
tear, in the abnormal group. Hence, suppression of an excessive
increase in GH external rotation during pitching motion, which canbe attributed to an increase in scapular posterior tilt, may prevent
throwingerelated shoulder injuries.
SD with prominence of the inferior scapular angle reportedly
affects the function of muscles around the scapula and results inpectoralis minor muscle tightness (Burkhart et al., 2003), muscle
weakness, fatigue, and abnormalities in the ﬁring patterns of theserratus anterior muscle (Ellenbecker and Cools 2010; Huang et al.,2015; Martin and Fish 2008) during shoulder motion. Tightness inthe pectoralis minor muscle and dysfunction of the serratus anterior muscle has been suggested as a cause of decrease in ST posterior tilt at MER (Burkhart et al., 2003; Ellenbecker & Cools 2010;
Huang et al., 2015). Additionally, pectoralis minor stretching andscapular stabilization exercises involving the serratus anterior
muscle have been demonstrated to be useful for an increase in ST
posterior tilt (Başkurt et al., 2011; Morais and Cruz, 2016). Thus, it is
crucial to improve the ﬂexibility of the pectoralis minor muscle andthe strength of the serratus anterior muscle by speciﬁc exercises for
scapular rehabilitation, including stretching and stabilization exercises, in baseball players with reduced scapular posterior tilt at
MER (Burkhart et al., 2003; Ellenbecker & Cools 2010; Longo et al.,2020).

and posterior tilt in both abnormal and normal groups (Fig. 4).
Oliver and Weimar and Meyer et al. reported that the ST motion
from SFC and MER periods changed to upward rotation, internalrotation, and posterior tilt (Meyer et al., 2008; Oliver & Weimar2015). Previous studies reported that the shoulder motions at
MER were as follows: GH abduction at approximately 85 (Konda
et al., 2015) and ST upward rotation at 25 e37, (Konda et al.,2015; Oliver & Weimar 2015), GH horizontal adduction at 6
(Konda et al., 2015) and ST internal rotation at 15 e22 (Konda et al.,2015; Oliver & Weimar 2015); and GH external rotation at
106 e114 (Konda et al., 2015; Miyashita et al., 2010) and ST posterior tilt at 11 e24 (Tables 3 and 5). Thus, in this study, ST upward
rotation, GH horizontal adduction, and ST posterior tilt showed asimilar trend to the results of previous studies (Tables 3 and 5).
In our study, we observed that the values of GH external rotation(approximately 133 e142 ) were high, whereas those of GH
abduction (74 ) and ST internal rotation (2 ) were low compared tothe previously reported values (Tables 3 and 5). This might bebecause of the variations in the data for GH and ST motions during
pitching in previous studies (Konda et al., 2015; Miyashita et al.,2010; Oliver & Weimar 2015) or due to various factors, such as
differences in competition level and pitching form among countries, which may inﬂuence GH and ST motions during pitching(Escamilla et al., 2002; Fleisig et al., 1999).
Previous studies showed that the trunk posture at MER led tocontralateral rotation (Kageyama et al., 2015; Oyama et al., 2013),
contralateral tilt (Barﬁeld et al., 2018; Oyama et al., 2013), andextension (Miyashita et al., 2010). The measured trunk motion
during pitching in our study suggested the possibility of measuring
it with a similar tendency as in previous studies (Tables 3 and 5).
Finley and Lee and Suzuki et al. reported that trunk posture affectedscapular motion during shoulder joint motion. (Finley and Lee
2003; Suzuki et al., 2019). Nevertheless, the trunk posture relative to scapular motion was not signiﬁcantly different between thetwo groups. Therefore, the difference in the effects of trunk posture(rotation, lateral tilt, and extension) between noneSD individuals
and players with SD might be minimal in this study.
Similarly, ball velocity showed no signiﬁcant difference between
the two groups. Wang et al. reported that the external rotation
angle of the shoulder joint at MER during pitching (measured at152 in the HT joint angle) was positively correlated with ball velocity (Wang et al., 1995). Similarly, in this study, the externalrotation angle of HT at MER was approximately 160 and 157 in theabnormal and normal groups, respectively; however, the difference
between the two groups was not signiﬁcant. Therefore, ball velocity
may not result in any difference in pitching performance between
the two groups. However, the GH external rotation angle and ST
posterior tilt angle at MER signiﬁcantly increased and decreased,
respectively, in participants with type I SD compared to those with

4.1. Limitations
This study has some limitations. First, the method for measuring
scapular motion during pitching in this study might have measurement errors in the gliding of the AMC on the skin over thescapula (Konda et al., 2018). Previous studies on measurement errors have indicated that the AMC method is accurate for up to 120338

A. Ueda, A. Matsumura, T. Shinkuma et al.

Journal of Bodywork & Movement Therapies 28 (2021) 332e340

of shoulder elevation (Chu et al., 2012). The elevation angle of theshoulder joint during the pitching motion measured in this study
was less than 120 (Table 3). According to a previous study, themeasurement errors of scapular motion may be lower in the highspeed acceleration phase (from MER to BR) during pitching motion
compared to those in other phases (Konda et al., 2015). In this study,
we analyzed shoulder motion during pitching prior to the acceleration phase. Thus, we consider that the data on shoulder joint
angles without the acceleration phase were reliable. Second, thekinematic characteristics of shoulder motion during pitching inparticipants with type II and III SD could not be clariﬁed because
the data of participants with type I and types II and III SD was
mixed. In the future, it is necessary to evaluate the effect of types II
and III SD on the pitching motion of baseball players. Third, it was
not possible to determine whether the results were directly
attributable to the cause of throwing injury rather than the crosssectional study design. Therefore, longitudinal studies exploring
the correlation of excessive increase in GH external rotation anddecrease in ST posterior tilt during pitching with throwing injury
onset are required in the future.

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4.2. Clinical relevance The use of the SDT for the evaluation of type I SD can be a usefuland simple screening method to predict excessive GH externalrotation and decreased scapular posterior tilt during pitching.
 To reduce the excessive GH external rotation during pitching inbaseball players with prominence of the inferior scapular angle,
it is crucial to improve the scapular function.
 Our results provide useful insights about the shoulder exercise
programs for overhead athletes, such as baseball, handball, andtennis players, with healthy and injured shoulders.
5. Conclusion
This study showed that participants in the abnormal group (typeI SD) exhibited a signiﬁcantly increased GH external rotation angle
and decreased ST posterior tilt angle at MER compared to those in
the normal group. Our results indicated that baseball players who
had SD with prominence of the inferior scapular angle might be at
an increased risk of shoulder injury onset during pitching motion.
Declaration of interest
The authors declare that they have no known competing
ﬁnancial interests or personal relationships that could haveappeared to inﬂuence the work reported in this paper.
CRediT authorship contribution statement
Atsushi Ueda: Conceptualization, Writing introduction andmethods, results, discussion. Takafumi Shinkuma: Discussion.
Takeshi Oki: Methodology. Yasuo Nakamura: Data analyses andsupervision.
Acknowledgments
This work was supported by the Athlete Care Association.
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