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Abstract

BACKGROUND:

Posterior shoulder tightness (PST) has been identified as an important factor that causes scapular dyskinesis. Therefore, rehabilitation programs should focus on an intervention involving a posterior shoulder stretch combined with a scapular stabilization exercise (PSSE). However, few studies have investigated the effects of PSSE on rotator cuff (RC) muscle strength in overhead athletes with scapular dyskinesis.

OBJECTIVE:

To determine the effects of PSSE on RC muscle strength, dynamic control ratio (DCR), range of motion (ROM), and pain.

METHODS:

Twenty-four adolescent baseball players with scapular dyskinesis were randomly allocated to one of two groups which trained for 6-weeks. Group I performed PSSE while group II performed same exercises but without stretching (SSE). The isokinetic peak moment/body weight (PM/BW) of concentric and eccentric external rotation (ERc and ERe) and concentric and eccentric internal rotation (IRc and IRe), the DCR (ERe/IRc and IRe/ERc), ROM, and pain were measured in pre- and post-intervention.

RESULTS:

A time-by-group interaction effect was observed for the concentric and eccentric ER, the DCR, and the ROM only the PSSE group.

CONCLUSIONS:

PSSE intervention is a more effective program than SSE for improving RC muscle strength and balance, particularly in terms of its ER component and the associated DCR.

Keywords

Scapular dyskinesis rotator cuff posterior shoulder stretch isokinetic strength

Figure 1.

Flow chart of the study.

1. Introduction

Glenohumeral (GH) joint injuries are common in adolescent baseball players, more than 50% of whom experience shoulder pain [33]. Such injuries are caused by cumulative microtrauma due to repetitive high-velocity dynamic overhead arm motion during throwing [24]. The repetitive throwing motion alters scapular kinematics, defined as scapular dyskinesis, and is common in overhead athletes [17, 18]. Scapular dyskinesis has been associated with rotator cuff (RC) muscle strength, as the scapula provides a stable base of support and an optimal length-tension relationship for the RC muscle. Many factors have been implicated as the cause of scapular dyskinesis, including posterior shoulder tightness (PST), which is thought to be an important factor [5, 19].

A moderate to strong relationship has been reported between PST and scapular dyskinesis in baseball players [19]. PST also leads to GH internal rotation deficit (GIRD) [4, 5, 38], resulting in an imbalance of the shoulder muscles [13, 16]. Recently, an isokinetic study reported reduced isokinetic strength of the shoulder flexor and extensor in high school baseball players with a PST [21]. Guney et al. [13] found a smaller range of motion (ROM) of shoulder internal rotation (IR), decreased strength in external rotation (ER), and a lower ER to IR strength ratio in amateur tennis players with pain. Therefore, rehabilitation programs should focus on posterior shoulder stretch combined with scapular stabilization exercise (PSSE) intervention.

Previous studies have investigated the effects of PSSE on RC muscle strength in patients with scapular dyskinesis. Merolla et al. [28] recently reported increased supraspinatus and infraspinatus isometric strength and reduced pain scores in overhead athletes with scapular dyskinesis. Başkurt et al. [2] reported that stretching with strengthening exercises was effective in improving RC muscle strength and quality of life in scapular dyskinesis patients. Based on this rationale, we believe that PSSE could be effective in improving RC muscle strength in adolescent baseball players with scapular dyskinesis.

However, only a few studies have investigated PSSE effects on RC muscle strength in overhead athletes with scapular dyskinesis, using isometric strength as the outcome measure following PSSE [2, 28, 29]. However, we argue that a measurement of the dynamic strength is strongly indicated to determine the dynamic function of the RC muscle during throwing. Isokinetic dynamometry has been used in objective assessment of shoulder muscle strength and risk of shoulder injury [10]. Fair to good reliability and reproducibility have been reported in previous studies [10].

The purpose of the present study was therefore to determine the effects of PSSE on RC muscle strength, the dynamic control ratio (DCR) of the acting muscles, the GH joint rotational ROM, and the pain in adolescent baseball players with scapular dyskinesis.

2. Methods

2.1 Participants

Thirty-five potential participants volunteered to take part in the study. Following the screening tests, eleven participants were excluded because they did not meet the inclusion criteria. Eventually 24 adolescent male baseball players participated in the study (Fig. 1). Sample size calculation was performed using the free statistical package G*Power 3. The calculation was based on our pilot test. Eleven participants (six participants in the PSSE group and five participants in the SSE group) took part in the pilot test, which was used to determine the number of participants needed for this study (mean difference, 9.90%; standard deviation, 4.52%; effect size, 1.28). The results of the pilot test indicated that at least nine participants per group would be required to detect a difference in concentric external rotation (ERc) peak moment/body weight (PM/BW) between groups using a one-tailed test with a power of 0.80% and an $\alpha$ level of 0.05.

Inclusion criteria included scapular dyskinesis, pain caused by or increasing during baseball games or training, and more than 15 ${}^{\circ}$ GIRD compared with the non-throwing shoulder. Exclusion criteria were as follows: history of shoulder surgery, fracture in the shoulder or upper extremity, excessive instability of the GH joint, and neurological signs in the upper limbs. Prior to participation, all participants read and signed a consent form approved by the Inje University Ethics Committee for Human Investigations.

Following the block randomization method, a study assistant who was not involved in the baseline evaluation results and interventions randomly assigned participants to a group. The block randomization method was used to ensure that two groups had an equal number of members. Participants picked a single card without looking from a swathed set of two cards marked with P or S (P card, PSSE group; S card, SSE group). Because the same examiners were involved in both outcome determination and intervention, they were not blind to the group allocations; however, participants were not informed of their group assignment or the aim of the study.

2.2 Procedures and instrumentation

2.2.1 Scapular dyskinesis

The disorder was identified through the following test: Beginning in a standing position, participants were asked to elevate their arms with 3-kg or 1.5-kg dumbbells in their hands, with their thumbs up to the end position over a period of 3 s, and then to lower their arms to the initial position in 3 s. The examiner classified the scapular positions and movement patterns during arm movement [14] (Table 2).

2.2.2 Isokinetic testing

RC muscle strength was measured only dominant shoulder with scapular dyskinesis using a Biodex System 4 dynamometer (Biodex Corp., Shirley, NY, USA). Participants completed 3–5 submaximal contraction trials to become familiar with the procedure and to warm up their muscles. The test was performed in a supine hook-lying position; straps were placed on the shoulder girdle and trunk to prevent compensation. Participants were asked to abduct the shoulder to 90 ${}^{\circ}$ and flex the elbow to 90 ${}^{\circ}$ , with the forearm in the pronated position. Strength was tested through 100 ${}^{\circ}$ ROM, between ER 50 ${}^{\circ}$ and IR 50 ${}^{\circ}$ at 120 ${}^{\circ}$ /s [32, 39]. Gravity correction was not used for this testing position, as both the IR and ER moved with and against gravity as necessary [11]. Concentric strength was tested first, followed by eccentric strength; each test consisted of five maximal effort reciprocal repetitions (Fig. 4).

2.2.3 Rotational RoM

GH joint ROM was measured by two examiners by determining IR and ER ROM using the bubble goniometer [30, 42]. The goniometer axis was positioned over the olecranon process, with the stationary arm aligned perpendicular to the ground and the moveable arm aligned along the ulnar styloid process [42]. IR was permitted until the participant’s coracoid was felt rising into the examiner’s thumb, and then the motion was stopped. ER ROM was measured by passive external rotation until the end range was reached. GIRD was calculated based on the difference in IR ROM between the throwing shoulder and the non-throwing shoulder.

2.2.4 Shoulder pain

Shoulder pain was evaluated using a visual analog scale (VAS) [2].

Figure 2.

Sleeper stretch. A, Start position. B, End position.

Figure 3.

Scapular stabilization exercise. A, Punch exercise. B, Forward flexion (FF) in a side-lying position. C, Prone horizontal abduction (PHA). D, Prone extension (PE).

Figure 4.

Isokinetic strength test of RC muscle. A, Start position. B, External rotation 50 ${}^{\circ}$ . C, Internal rotation 50 ${}^{\circ}$ .

2.3 Intervention

The PSSE group performed the sleeper stretch with four SSE over 6 weeks: a punch exercise, forward flexion (FF) in the side-lying position, prone horizontal abduction (PHA), and prone extension (PE). The SSE group performed four SSE without stretching for 6 weeks. The sleeper stretch was performed to improve the flexibility of the posterior shoulder muscle and soft tissue [16, 20, 23, 26, 27]. Participants began in a side-lying position on the floor with the throwing shoulder below, flexed to 90 ${}^{\circ}$ , and the elbow flexed to 90 ${}^{\circ}$ . The chest was perpendicular to the floor, with the head relaxed and supported by a cushion, and hip flexed. The non-throwing hand grasped the dorsal side of the throwing wrist and gently pushed in to a more internally rotated position (Fig. 2). The intensity of the stretch was 70–90% of the point of discomfort (POD), where 0 $=$ “no stretch discomfort at all” and 100 $=$ “maximum imaginable stretch discomfort” [3]. The sleeper stretch was performed daily, with three repetitions lasting 30 s each, over a period of 6 weeks [20, 26]. The punch exercise was performed to improve the participant’s serratus anterior (SA) muscle strength [22]. FF in the side-lying position was performed to improve the lower trapezius (LT) and middle trapezius (MT) muscles [8, 9]. PHA was performed to improve the LT muscle and for optimal restoration of the UT muscle to LT muscle ratio [8, 9]. PE was performed to improve the MT muscle and for optimal restoration of the UT muscle to MT muscle ratio [36]. Four SSE were performed three times per week, with three sets of 10 repetitions, over 6 weeks (Fig. 3).

2.4 Outcome measures

The isokinetic outcome measures consisted of the PM/BW of the IR, in the concentric and eccentric mode – IRc and IRe respectively and the corresponding values for external rotation: ERc and ERe. The DCR comprised of two variants: DCRi-e $=$ IRe/ERc and DCRe-i $=$ ERe/IRc. The GH joint IR and ER ROM, GIRD, and shoulder pain were recorded pre- and 6 weeks post-intervention.

2.5 Statistical analysis

Statistical analyses were performed with SPSS software (ver. 18.0 for Windows; SPSS Inc., Chicago, IL USA). Two-way repeated-measures analysis of variance (ANOVA) was used to determine the time-by-group interaction effect or the main effect for time. The within-group factor was time (pre-intervention versus post-intervention) and the between-group factor was group (PSSE group vs. SSE group). When significant main effects or time-by-group interaction effects were found, post hoc $t$ -tests were performed. A $p$ -value $<$ 0.05 was considered statistically significant.

Table 1
Demographic characteristics of the participants

Variable	PSSE ( $n=$ 12)		SSE ( $n=$ 12)		p
Age (years)	17.25	$\pm$ 1.35	16.90	$\pm$ 1.52	0.575
Height (cm)	176.01	$\pm$ 3.79	176.74	$\pm$ 7.64	0.771
Body weight (kg)	70.08	$\pm$ 10.14	77.23	$\pm$ 8.75	0.078
Sports experience	6.41	$\pm$ 0.99	5.91	$\pm$ 1.37	0.320
(years)
Pain (VAS)	5.33	$\pm$ 1.87	6.41	$\pm$ 1.92	0.177
Isokinetic PM/BW
ERc, 120 ${}^{\circ}$ /s (%)	40.81	$\pm$ 6.25	35.38	$\pm$ 7.84	0.074
ERe, 120 ${}^{\circ}$ /s (%)	44.00	$\pm$ 4.78	40.62	$\pm$ 3.72	0.067
IRc, 120 ${}^{\circ}$ /s (%)	37.73	$\pm$ 6.34	35.49	$\pm$ 6.31	0.396
IRe, 120 ${}^{\circ}$ /s (%)	42.26	$\pm$ 5.00	39.09	$\pm$ 2.47	0.067
DCR
IRe:ERc	1.06	$\pm$ 0.22	1.16	$\pm$ 0.30	0.354
ERe:IRc	1.19	$\pm$ 0.21	1.17	$\pm$ 0.19	0.797
ROM
IR ( ${}^{\circ}$ )	59.16	$\pm$ 7.81	60.83	$\pm$ 5.89	0.561
ER ( ${}^{\circ}$ )	103.00	$\pm$ 9.64	101.66	$\pm$ 9.07	0.731
GIRD ( ${}^{\circ}$ )	$-$ 17.25	$\pm$ 0.96	$-$ 16.50	$\pm$ 1.24	0.113

Values are mean $\pm$ SD. Abbreviations: PSSE, posterior shoulder stretch combined with scapular stabilization exercise; SSE, scapular stabilization exercise; Isokinetic PM/BW, isokinetic peak moment/body weight; ERc, concentric external rotation; ERe, eccentric external rotation; IRc, concentric internal rotation; IRe, eccentric internal rotation; DCR, dynamic control ratio; IR, internal rotation; ER, external rotation; GIRD, glenohumeral internal rotation deficit; VAS, visual analog scale.

Table 2

Classification of the main patterns of scapular dyskinesis

Pattern	Description
Pattern I	The inferior medial angle of the scapula is displaced posteriorly from the thorax and prominent during dynamic observation and palpation
Pattern II	The entire medial border of the scapula is displaced posteriorly from the posterior thorax and prominent during dynamic observation and palpation
Pattern III	Early scapular elevation or excessive or insufficient scapular upward rotation during dynamic observation and palpation compared to the asymptomatic side
Pattern IV (Normal)	1. No evidence of posterior displacement in the medial border/inferior angle of the scapula and excessive or insufficient scapular movement 2. Minimal motion during the initial 30–60 ${}^{\circ}$ of scapulohumeral elevation, then smooth and continuous upward and downward rotation during humeral elevation and lowering, respectively

Table 3

Isokinetic PM/BW and DCR pre- and post-intervention

Variable	Group	Pre-intervention		Post-intervention		Post – Pre		p
Isokinetic PM/BW
ERc 120 ${}^{\circ}$ /s (%)	PSSE ( $n=$ 12)	40.81	$\pm$ 6.25	46.83	$\pm$ 5.17	6.02	$\pm$ 4.76 (2.99, 9.05)	0.001 ${}^{\text{b}}$
	SSE ( $n=$ 12)	35.38	$\pm$ 7.84	36.98	$\pm$ 8.08	1.60	$\pm$ 5.07 ( $-$ 1.61, 4.82)	0.297
ERe 120 ${}^{\circ}$ /s (%)	PSSE ( $n=$ 12)	44.00	$\pm$ 4.78	49.40	$\pm$ 3.54	5.39	$\pm$ 4.22 (2.71, 8.07)	0.001 ${}^{\text{b}}$
	SSE ( $n=$ 12)	40.62	$\pm$ 3.72	41.07	$\pm$ 5.32	0.44	$\pm$ 4.10 ( $-$ 2.15, 3.05)	0.713
IRc 120 ${}^{\circ}$ /s (%)	PSSE ( $n=$ 12)	37.73	$\pm$ 6.34	36.66	$\pm$ 4.69	$-$ 1.06	$\pm$ 5.08 ( $-$ 4.29, 2.16)	0.432
	SSE ( $n=$ 12)	35.49	$\pm$ 6.31	35.65	$\pm$ 4.21	0.16	$\pm$ 4.88 ( $-$ 2.94, 3.27)	0.910
IRe 120 ${}^{\circ}$ /s (%)	PSSE ( $n=$ 12)	42.26	$\pm$ 5.00	42.47	$\pm$ 4.77	0.20	$\pm$ 3.88 ( $-$ 2.25, 2.67)	0.855
	SSE ( $n=$ 12)	39.09	$\pm$ 2.47	39.56	$\pm$ 2.27	0.47	$\pm$ 2.52 ( $-$ 1.12, 2.07)	0.529
DCR
IRe:ERc	PSSE ( $n=$ 12)	1.06	$\pm$ 0.22	0.91	$\pm$ 0.13	$-$ 0.14	$\pm$ 0.18 ( $-$ 0.26, $-$ 0.02)	0.021 ${}^{\text{c}}$
	SSE ( $n=$ 12)	1.16	$\pm$ 0.30	1.11	$\pm$ 0.23	$-$ 0.005	$\pm$ 0.19 ( $-$ 0.16, 0.05)	0.322
ERe:IRc	PSSE ( $n=$ 12)	1.19	$\pm$ 0.21	1.36	$\pm$ 0.17	0.17	$\pm$ 0.16 (0.06, 0.27)	0.005 ${}^{\text{b}}$
	SSE ( $n=$ 12)	1.17	$\pm$ 0.19	1.16	$\pm$ 0.19	$-$ 0.01	$\pm$ 0.19 ( $-$ 0.02, 0.11)	0.928

Values are mean $\pm$ standard deviation except for Post – Pre, where values are mean difference $\pm$ standard error (95% confidence interval). Abbreviations: PSSE, posterior shoulder stretch combined with scapular stabilization exercise; SSE, Scapular stabilization exercise; Isokinetic PM/BW, isokinetic peak moment/body weight; ERc, concentric external rotation; ERe, eccentric external rotation; IRc, concentric internal rotation; IRe, eccentric internal rotation; DCR, dynamic control ratio. ${}^{\text{b}}$ Indicates significant difference Post – Pre (time by group interaction effect). ${}^{\text{c}}$ Indicates significant difference Post – Pre (main effect: time).

Table 4

G.H joint ROM and pain pre- and post-intervention

Variable	Group	Pre-intervention		Post-intervention		Post – Pre		p
GH joint ROM
IR ( ${}^{\circ}$ )	PSSE ( $n=$ 12)	59.16	$\pm$ 7.81	74.25	$\pm$ 6.25	15.08	$\pm$ 3.57 (12.80, 17.35)	$<$ 0.	001 ${}^{\text{b}}$
	SSE ( $n=$ 12)	60.83	$\pm$ 5.89	61.58	$\pm$ 5.51	0.75	$\pm$ 1.60 ( $-$ 0.26, 1.76)	0.	133
ER ( ${}^{\circ}$ )	PSSE ( $n=$ 12)	103.00	$\pm$ 9.64	91.00	$\pm$ 5.67	$-$ 12.00	$\pm$ 6.94 ( $-$ 16.41, $-$ 7.58)	$<$ 0.	001 ${}^{\text{b}}$
	SSE ( $n=$ 12)	101.66	$\pm$ 9.07	100.41	$\pm$ 4.50	$-$ 1.25	$\pm$ 6.79 ( $-$ 5.56, 3.06)	0.	537
GIRD ( ${}^{\circ}$ )	PSSE ( $n=$ 12)	$-$ 17.25	$\pm$ 0.96	0.16	$\pm$ 2.62	$-$ 17.41	$\pm$ 2.84 ( $-$ 19.23, $-$ 15.61)	$<$ 0.	001 ${}^{\text{b}}$
	SSE ( $n=$ 12)	$-$ 16.50	$\pm$ 1.24	$-$ 15.58	$\pm$ 2.81	0.91	$\pm$ 2.15 ( $-$ 0.45, 2.28)	0.	168
Pain
VAS	PSSE ( $n=$ 12)	5.33	$\pm$ 1.87	2.08	$\pm$ 1.31	$-$ 3.25	$\pm$ 1.60 ( $-$ 4.26, $-$ 2.23)	$<$ 0.	001 ${}^{\text{c}}$
	SSE ( $n=$ 12)	6.41	$\pm$ 1.92	3.58	$\pm$ 1.92	$-$ 2.83	$\pm$ 1.85 ( $-$ 4.00, $-$ 1.65)	$<$ 0.	001 ${}^{\text{c}}$

Values are mean $\pm$ standard deviation except for Post – Pre, where values are mean difference $\pm$ standard error (95% confidence interval). Abbreviations: GH joint ROM, glenohumeral joint range of motion; PSSE, posterior shoulder stretch combined with scapular stabilization exercise; SSE, Scapular stabilization exercise; IR, internal rotation; ER, external rotation; GIRD, glenohumeral internal rotation deficit; VAS, visual analog scale. ${}^{\text{b}}$ Indicates significant difference Post – Pre (time by group interaction effect). ${}^{\text{c}}$ Indicates significant difference Post – Pre (main effect: time).

3. Results

All twenty-four enrolled participants completed the follow-up. All participants denied any adverse ( $n=$ 0) and/or side effects ( $n=$ 0) during the follow-up periods of 6 weeks. The demographic characteristics of the participants are outlined in Table 1. There were no significant inter-group differences regarding any of the demographic parameters in base line.

A significant time-by-group interaction effect was observed for the ERc ( $F=$ 4.84, $p=$ 0.039) and ERe ( $F=$ 8.47, $p=$ 0.008) PM/BW, and ERe to IRc ratio ( $F=$ 5.82, $p=$ 0.025). In the PSSE group, the ERc and ERe PM/BW were significantly higher 6 weeks post-intervention (mean difference $=$ 6.02 $\pm$ 4.76% and mean difference $=$ 5.39 $\pm$ 4.22%, respectively), and the ERe to IRc ratio was significantly higher 6 weeks post-intervention (mean difference $=$ 0.17 $\pm$ 0.16). There was no significant time-by-group interaction effect for the IRe to ERc ratio; however, there was a main effect for time ( $F=$ 7.16, $p=$ 0.014). The IRe to ERc ratio was significantly lower 6 weeks post-intervention (mean difference $=-$ 0.14 $\pm$ 0.18). There was no significant difference in the IRc and IRe PM/BW 6 weeks post-intervention. In the SSE group, there was no significant difference in isokinetic PM/BW or DCR 6 weeks post-intervention (Table 3).

A significant time-by-group interaction effect was observed for IR ROM ( $F=$ 160.30, $p<$ 0.001), ER ROM ( $F=$ 14.69, $p=$ 0.001), and GIRD ( $F=$ 256.99, $p<$ 0.001). In the PSSE group, IR ROM was significantly higher 6 weeks post-intervention (mean difference $=$ 15.08 $\pm$ 3.57 ${}^{\circ}$ ). ER ROM and GIRD were significantly lower 6 weeks post-intervention (mean difference $=-$ 12.00 $\pm$ 6.94 ${}^{\circ}$ and mean difference $=-$ 17.41 $\pm$ 2.84 ${}^{\circ}$ ). In the SSE group, there was no significant difference in ROM 6 weeks post-intervention (Table 4).

There was no significant time-by-group interaction effect for pain ( $F=$ 0.34, $p=$ 0.56); however, there was a main effect for time ( $F=$ 74.10, $p<$ 0.001). Both the PSSE and SSE groups displayed a significantly lower VAS score 6 weeks post-intervention (mean difference $=-$ 3.25 $\pm$ 1.60 and mean difference $=-$ 2.83 $\pm$ 1.85, respectively) (Table 4).

4. Discussion

We investigated the effects of PSSE over 6 weeks on RC muscle strength, DCR, ROM, and pain in adolescent baseball players with scapular dyskinesis. Our results showed significantly decreased pain in both the PSSE and SSE groups. However, RC muscle strength, DCR, and ROM improvements were observed only in the PSSE group.

The PSSE group showed improvements in the ERc and ERe PM/BW; there are several possible reasons for the increased muscle strength. First, the restoration of flexibility affects the length-tension relationship and produces muscle force [31]. Altered muscle length affects the joint kinematics-muscle activation relationship and can produce a certain degree of muscle force [12]. We observed increased IR ROM in the PSSE group, indicating restored flexibility in posterior shoulder muscles and soft tissue [20, 26, 27]. This increases the length-tension relationship and activation of the ER muscle, and may have increased the ERc and ERe PM/BW in this study. Second, increased muscle tension over time may have affected our results. Stretching increases relaxation and reduces muscle tone over time. This leads to increased muscle length and viscoelastic properties, ultimately increasing muscle force production [37]. A 6-week intervention period was applied in this study. There was a greater increase in the ER muscle length and viscoelastic properties post-intervention in the PSSE group, which would have resulted in increased ERc and ERe PM/BW. Previous researchers have reported the long-term effects of stretching on ROM and strength. Worrell et al. [44] reported 8.5% and 13.5% increases in eccentric PM of the hamstring muscle measured at 60 ${}^{\circ}$ /s and 120 ${}^{\circ}$ /s, respectively, and an 11.2% increase in concentric PM after a 3-week hamstring stretch program. Abdel-Aziem and Mohammad [1] demonstrated increased plantar flexor concentric PM from 82 Nm to 90 Nm and from 55 Nm to 61 Nm measured at 30 ${}^{\circ}$ /s and 120 ${}^{\circ}$ /s, respectively, and increased eccentric PM from 86 Nm to 96 Nm and from 93 Nm to 104 Nm measured at 30 ${}^{\circ}$ /s and 120 ${}^{\circ}$ /s, respectively, after a 6-week plantar flexor stretch program. Our results were similar to those found in previous studies; after 6 weeks of PSSE, ERc and ERe PM/BW increased by 6.02% and 5.39%, respectively. Therefore, our results support the long-term effects of stretching on muscle strength. Finally, energy absorption may have affected our results. The speed of the eccentric contraction and muscle length are important factors that determine the amount of energy absorbed by muscles [41]. Increased muscle length can lead to increased absorbed energy during eccentric contraction, which is immediately used in concentric contraction [6, 43]. The increased IR ROM in this study may have led to an increase in the amount of energy absorption during eccentric contraction of the ER muscle, which would have increased both the ERe and ERc PM/BW in the PSSE group.

The increased ER PM/BW observed in our study may have affected the DCR results. Our DCR results showed a significantly increased ERe to IRc ratio and decreased IRe to ERc ratio post-intervention in the PSSE group. Previous authors have reported that a lower ERe to IRc ratio predisposes individuals to throwing shoulder injuries [7, 34, 39]. Therefore, PSSE intervention may inhibit the distractive force in the deceleration phase and prevent such injuries. The reduction in the IRe to ERc ratio can be explained by the increased strength of the ERc PM/BW.

IR ROM significantly increased and ER ROM and GIRD significantly decreased post-intervention in the PSSE group. GIRD has been closely related to the pathologic changes and shoulder dysfunction in younger throwing athletes [15, 16, 35]. Shanley et al. [35] found that high school baseball players with $\geqslant$ 25 ${}^{\circ}$ GIRD are at four times greater risk of an upper extremity injury. It is necessary to appropriately manage IR ROM and GIRD to prevent shoulder injuries in adolescent baseball players. Our results showed a significant increase in IR ROM after 6 weeks in the PSSE group, consistent with other studies. McClure et al. [27] reported a significant increase in IR ROM from 48.2 $\pm$ 8.8 ${}^{\circ}$ to 60.6 $\pm$ 10.4 ${}^{\circ}$ and 46.6 $\pm$ 11.5 ${}^{\circ}$ to 66.6 $\pm$ 15.9 ${}^{\circ}$ after 4 weeks of sleeper stretching and cross body stretching, respectively. Maenhout et al. [26] also reported that IR ROM increased significantly from 33.7 $\pm$ 8.3 ${}^{\circ}$ to 47.5 $\pm$ 7.0 ${}^{\circ}$ and that the acromiohumeral distance increased after a 6-week sleeper stretch program. Consequently, we believe that PSSE intervention can be successful for IR ROM and GIRD management and can reduce the risk of shoulder injuries. Our results showed that ER ROM significantly decreased post-intervention in the PSSE group and that there was no significant difference in the SSE group. Previous authors have suggested that the increase in ER ROM was caused by PST and the repetition of a late cocking motion [5, 40]. PST causes the GH joint contact point to shift to posterior superior, allowing greater ER ROM [5], and the repetition of late cocking motion causes excessive ER ROM [40]. Based on our findings, we hypothesize that PSSE is effective in preventing excessive ER ROM during the late cocking phase and reducing the risk of shoulder injuries.

The VAS score was significantly lower post- intervention in both the PSSE and SSE groups. Our results may be closely related to the dynamic stability of the humeral head provided by the ER muscles during throwing. Throwing-related pain inhibits the ER muscle, leading to ERe weakness, as observed in previous studies [7, 28, 34, 39]. The ER muscle plays an important role in providing the dynamic stability of the humeral head. Therefore, increased ERc and ERe PM/BW after 6 weeks of PSSE intervention would have increased the dynamic stability of the humeral head during throwing and decreased the VAS score in the PSSE group. The SSE program focused on LT, MT, and SA strengthening exercises to restore scapular kinematics. De Mey et al. [9] reported significantly reduced UT activity and increased LT and MT activity, as well as an earlier LT and SA onset than UT after a 6-week SSE program. Cools et al. [8] observed a significantly increased LT/UT and MT/UT ratio during SSE. Lynch et al. [25] also reported that LT, MT, and SA strengthening exercises for 12 weeks significantly increased the LT, MT, and SA strength. Although we did not evaluate scapular kinematics, it appears that scapular stabilizer muscle balance increased after 6 weeks in the SSE group. Thus restoring normal scapulohumeral rhythm may reduce throwing-related pain.

The present study had several limitations. First, we did not evaluate scapular kinematics. Thus, we could not its effect on RC muscle strength. Future studies are hence needed to investigate the effects of PSSE and SSE on scapular kinematics and the relationship between scapular kinematics and dynamic strength in the RC muscle. Second, we did not evaluate RC muscle activity. Therefore, we could not confirm differences in RC muscle strength due to muscle activity changes; therefore, we recommend future studies to fill this gap. Finally, since the intervention effect was compared only in the dominant shoulder with scapular dyskinesis in this study, it is unclear whether there is a difference in intervention effect compared to the non-dominant shoulder.

5. Conclusion

Both PSSE and SSE interventions led to more pain relief. However, only the PSSE group showed improvements in ROM, ERc, ERe PM/BW, and DCR. These results suggest that PSSE intervention is a more effective program for improving RC muscle strength and balance, particularly concentric and eccentric ER muscle strength and the ERe to IRc ratio, in adolescent baseball players with scapular dyskinesis.

Footnotes

Acknowledgments

This work was supported by the 2017 Inje University research grant.

Conflict of interest

All authors have completed the ICMJE uniform disclosure form and no conflicts of interests were reported for this study. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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The effects of posterior shoulder stretch on rotator cuff strength ratio in adolescent baseball players with scapular dyskinesis: A randomized controlled trial

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

2. Methods

2.1 Participants

2.2 Procedures and instrumentation

2.2.1 Scapular dyskinesis

2.2.2 Isokinetic testing

2.2.3 Rotational RoM

2.2.4 Shoulder pain

2.4 Outcome measures

2.5 Statistical analysis

Table 1 Demographic characteristics of the participants

4. Discussion

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Demographic characteristics of the participants