Effects of scapular stabilization exercises on posture and muscle imbalances in women with upper crossed syndrome: A randomized controlled trial

Abstract

BACKGROUND:

Scapular stabilization exercises can improve forward shoulder posture in young women. However, the effects of optimal exercise protocols on neck and shoulder postures, scapular muscle imbalance and thoracic kyphosis are still unclear.

OBJECTIVES:

To investigate the effects of scapular stabilization exercises on neck and shoulder postures, scapular muscle imbalance and thoracic kyphosis in young women with upper crossed syndrome.

METHODS:

Thirty-nine women aged 18–25 years with upper crossed syndrome were recruited. Participants were randomly allocated intotwo groups. The exercise group performed scapular stabilization exercises using elastic bands at 10 repetitions/set, 3 sets/day, and 3 days/week for 4 weeks. The control group did not perform any exercises during the experimental period. The cervical and shoulder angles were evaluated using the Kinovea program. A caliper, handheld dynamometer and flexi ruler were used to evaluate the length of the pectoralis minor, strength of the scapular stabilizer muscles and degree of the mid-thoracic curve, respectively.

RESULTS:

The exercise group showed significant differences ( $p<$ 0.05) in the increase in cervical and shoulder angle, length of the pectoralis minor, and strength of the scapular muscles, except in the degree of mid-thoracic curve compared to the control group.

CONCLUSION:

This study indicated that scapular stabilization exercises can improve forward head and shoulder posture, increase the flexibility of the pectoralis minor and strengthen the scapular muscles.

Keywords

Upper crossed syndrome posture muscle imbalance scapular stabilization exercise

1. Introduction

Upper crossed syndrome (UCS) is a type of upper muscle imbalance defined as tightening of the upper trapezius, sternocleidomastoid, levator scapulae and pectoral muscles along with weakness of the deep cervical flexors, rhomboids, middle and lower trapezius, and serratus anterior muscles. The posture of UCS is characterized by a forward head, forward or rounded shoulders, increased upper thoracic kyphosis, protracted shoulder, anterior tilted and winged scapula [1, 2]. UCS is prevalent among the general population, including 28% of laundry workers aged between 25 and 50 years old [3]. In a survey of 101 Iranian office workers, 61.3%, 48.7% and 78.3% demonstrated forward head, thoracic kyphosis and rounded shoulder postures, respectively [4]. Young adults, especially university students aged between 18 and 25 years old, showed symptoms of UCS as 63.96% and 52.9% demonstrated forward head and forward shoulder postures, respectively, which may be related to UCS [5, 6]. Moreover, people with this syndrome can develop neck and shoulder impairments, such as headaches, chronic neck tension, scapular dyskinesia, shoulder impingement syndrome and median nerve tension, which result in neck and shoulder pain [2]. Although previous studies did not clearly report UCS problems that have affected people, disorders that may be related to UCS were reported to be major causes of work-related absences as well as physical and mental sickness among employees [7]. Evidence from self-reports on musculoskeletal symptoms showed that the most commonly affected areas are the head and neck, accounting for 42% and 28% of respondents in Thailand and the EU, respectively [7, 8]. The habitual posture of falling or slouching as a result of prolonged sitting, studying, and using a computer or laptop has also been associated with UCS [1, 2]. Therefore, UCS management is important in reducing the incidence of impairment and subsequent problems.

Based on UCS posture, forward head and shoulder posture has been commonly assessed using measurements of the cervical and shoulder angle in previous studies [2, 9, 19, 20]. A cervical angle (measured between the horizontal line drawn through the spinous process of C ${}_{7}$ and a line drawn between the spinous process of C ${}_{7}$ and the tragus of the ear) less than 50 ${}^{\circ}$ is considered a forward head posture, and a shoulder angle (measured between the horizontal line drawn through the acromion process and a line drawn between the spinous process of C ${}_{7}$ and the acromion process) less than 52 ${}^{\circ}$ is considered as forward shoulder posture [9, 19, 20]. These conditions can occur individually or in combination. An increase in thoracic kyphosis is also associated with forward head and shoulder posture [2].

Scapular stabilizer muscles, such as the rhomboids, middle and lower trapezius, and serratus anterior muscles, play an important role in keeping the scapula attached to the thorax and controlling the normal movement of the scapula during arm elevation. Weakness of the scapular stabilizer muscles leads to downward rotation, anterior tilted and protracted scapula. The abnormal positioning of the scapula induces shortening of the pectoralis minor muscle [10]. Previous studies revealed that strengthening the scapular stabilizer muscles or scapular stabilization exercises combined with pectoralis minor stretching, which can decrease pain, improve scapular dysfunction, correct neck and shoulder posture in case of neck and shoulder impairment [11-113], stroke patients [14], athletes [15, 16, 17, 18] and dental students [19], and people with forward head and shoulder posture [16, 20, 21, 22, 23].

Hajihosseini and colleagues [20] compared the 6-weeks effects of three types of exercise on the alteration of forward shoulder angle in young females with forward shoulder posture. The exercises included scapular muscle strengthening, pectoral muscle stretching, and a combination of scapular muscle strengthening and pectoral muscle stretching. The study showed a significant decrease in the forward shoulder angle within all groups, but no significant difference was found between groups. This study might suggest that scapular muscle strengthening exercises alone are effective in correcting forward shoulder posture. According to the correction in forward shoulder posture, it can be assumed that the scapular stabilizer muscles have been strengthened. From the theory of reciprocal inhibition, the contraction of the opposite muscle would inhibit the activity of the target muscle, decreasing neural activity and increasing the inhibition of the proprioceptive structure. The continuous contraction of the antagonist muscle is a result of the relaxation of the target muscle [24]. This theory leads to the following question: If the scapular stabilizer muscles are strengthened, will the pectoralis minor muscle increase in flexibility and the normal positioning of the neck, shoulder, and thoracic spine improve? The previous study [20] did not investigate other parameters that can affect scapular stabilization exercises. Therefore, this study aims to investigate the effects of scapular stabilization exercises on the alteration of posture and muscle imbalance.

Previous studies have led to questions regarding the protocol of scapular stabilization exercises. The first question is regarding the optimal form of scapular stabilization exercises in relation to the appropriate application for exercise training, since there are several forms of scapular stabilization exercises that have been used in previous studies [16, 17, 20, 23, 25]. A systematic review of optimal scapular stabilization exercises on the ratio of scapular stabilizer muscles suggests that the exercises decreased the activation of the upper trapezius muscle. However, the exercises increased the activation of the serratus anterior, middle and lower trapezius muscles, which could result in normal scapular movement and proper strengthening of the scapular muscles [25]. Therefore, optimal scapular stabilization exercises should consider the application for strengthening scapular muscles. The second question is regarding the optimal exercise prescription in order to gain muscle strength in a short duration. According to ACSM’s guidelines for exercise testing and prescription [26], it is suggested that the resistance exercise training of 8–12 repetitions per set for 2–4 sets at 2–3 days per week is an effective method for increasing muscle strength. The strengthening of muscle is usually achieved after 4–8 weeks of moderate to high resistance training exercises. If the exercise program is effective, 4 weeks of exercise should result in muscle strength.

Based on previous studies, we hypothesized that the application of optimal scapular stabilization exercises according to the strengthening exercise guidelines would result in improvements on forward head and shoulder posture, scapular muscle imbalance, and thoracic kyphosis over an exercise duration of 4 weeks. The purpose of our study was to investigate the effects of scapular stabilization exercises on the alteration of posture and muscle imbalance through measuring the cervical and shoulder angles, pectoralis minor muscle length, scapular stabilizer muscle strength, and mid-thoracic curve degree in young women with UCS.

2. Materials and methods

2.1 Study design

The study was a simple randomized, controlled, parallel-group trial of exercise training conducted among university students. This study was conducted from August 2019 to January 2020. Participants who demonstrated UCS were recruited. A researcher who was not directly involved in the exercise training session randomized each participant either to the exercise or control group with an allocation ratio of 1:1 using the simple random sampling method. A total of 40 pieces of paper written either number 1 (20 pieces) or 2 (20 pieces) in a wellkept envelope. The participants were asked to draw one white folded paper from the envelop. If the participants selected number 1, they were in the exercise group. Those who selected number 2 were in the control group. The allocation was also kept confidentially from both the exercise provider and the participants. The baseline measurements included the participant’s cervical and shoulder angle, pectoralis minor muscle length, mid-thoracic curve, and scapular muscle strength. All outcome measurements were performed by blinded examiners that were not involved in the exercise training sessions. After 4 weeks of scapular exercise training at 3 sets of 10 repetitions/set for 3 days/week with supervision, a posttest was measured. This study was approved by the Ethics Committee for the Protection of Humans, Walailak University (Reference No. WUEC-19-011-01)

2.2 Participants

A total of 97 young women participants were recruited from Walailak University. Fifty-seven participants did not meet the inclusion criteria. Therefore, 40 participants with UCS were selected according to the criteria of the study (Fig. 1). The inclusion criteria integrated the following: aged between 18 and 25 years; body mass index between 18.5–22.9 kg/m ${}^{2}$ ; (cervical angle $<$ 50 ${}^{\circ}$ and shoulder angle $<$ 52 ${}^{0}$ ); and being free of neck and shoulder pain with full range of motion. The exclusion criteria were pregnancy, history of any medical treatment, fracture, or operations involving the neck, upper extremity and trunk. Simple random allocation was used to divide participants into two groups, consisting of 20 participants in the exercise group and 20 participants in the control group. All participants were informed on the study protocols, agreed to participate, and signed the consent forms prior to the study. Sample size calculation was based on a previous study [17] using the following formula ( $n=(\sigma^{2}_{d}(z_{\alpha}+z_{\beta})^{2}\div{\mu}^{2}_{d})$ ). The statistical values from the lower trapezius muscle strength were $\sigma_{d}=$ 2.8, ${\mu}_{d}=$ 1.88. Therefore, assuming a 5% type 1 error with statistical power of 80% and factoring in a 15% dropout rate, a sample size of 20 participants for each group was approximated.

Figure 1.

Flowchart of the study.

2.3 Posture assessment

2.3.1 Measurement of the cervical (CA) and shoulder angles (SHA)

CA and SHA were assessed from side view photographs. A camera (Sony Alpha SLT-A33, USA) was set at a height of 98 cm above the ground and at a distance of 265 cm from the footprints, which participants were instructed to step on. The footprints were placed 23 cm from the wall. Participants were instructed to step on the footprints and place their feet 3 inches apart. They were also instructed to look ahead with their arms hanging by their bodies. A 10 cm vertical line was made as a reference using a plumb line attached to the wall close to the participant’s position. This vertical line was used to correct the image of a participant. An examiner had the responsibility of photographing and placing reflective markers on a participant’s anatomical points, which included the tragus of the ear, the C7 spinous process and the acromion tubercle, before taking a photo. A participant’s photograph was analyzed for forward head posture (cervical angle $<$ 50 ${}^{\circ}$ ) and forward shoulder posture (shoulder angle $<$ 52 ${}^{\circ}$ ) [9] using the Kinovea computer program (free program) as this program has demonstrated a high reliability (ICC $=$ 0.985–0.997) [27]. Cervical angle (CA) is formed between the intersection of a horizontal line drawn through the spinous process of C ${}_{7}$ and a line between the spinous process of C ${}_{7}$ and the tragus of the ear. Shoulder angle (SHA) is formed between the intersection of a horizontal line drawn through the acromion process and a line between the spinous process of C ${}_{7}$ and the acromion process (Fig. 2). A high intra-rater reliability was obtained (ICC $=$ 0.80).

Figure 2.

The measurement of cervical and shoulder angles.

2.3.2 Measurement of the mid-thoracic curve

The mid-thoracic curve was measured using a flexi ruler. The reliability and validity of the measurements were revealed to be 0.906 the intraclass correlation (ICC) with a sensitivity of 85% and specificity of 97% for a thoracic hyperkyphosis diagnosis [28]. An examiner palpated and scored points of C7 and T12 on a participant’s spine. The flexi ruler was placed on the participant’s spine, starting at C7 to T12 (Fig. 3a). Then the flexi ruler was transferred to a sheet of paper and the ruler’s curve was traced on the sheet. The mid-thoracic curve was calculated according to the procedures of Moezy and colleagues [13]. A high intra-rater reliability was obtained (ICC $=$ 0.87).

2.4 Muscle imbalance assessment

2.4.1 Measurement of the pectoralis minor muscle length

The length of the pectoralis minor muscle was measured using a caliper. Participants were measured in a standing position. An examiner determined two anatomical landmarks, the first was the inferomedial aspect of the coracoid process of the scapula, and the second landmark was lateral to the sternocostal junction of the inferior aspect (caudal end) of the fourth rib. Then, a caliper measured the distance between these two bony reference points (Fig. 3b) [13]. A high intra-rater reliability was obtained (ICC $=$ 0.89).

2.4.2 Measurement of scapular muscle strength

Scapular muscle strength was measured by a hand-held dynamometer (HHD) with the make technique [29], where two consecutive trials for each muscle were performed and the average was used for data analysis (data in kg). A high intra-rater reliability was obtained (ICC $=$ 0.87–0.92). Scapular muscle strength included the following muscles [30]:

Rhomboids muscles: Participants were placed in a prone position with one hand behind their back. An examiner placed the HHD in the middle of the upper arm. Participants were then asked to retract and rotate their scapula downward. The force of the HHD was applied at the midway of the humerus (Fig. 4a).

Figure 3.

The measurement of mid-thoracic curve (a) and pectoralis minor length (b).

Figure 4.

The measurement of scapular muscle strength; rhomboids (a), middle trapezius (b), lower trapezius (c) and serratus anterior (d).

Middle trapezius muscle: Participants were placed in a prone position, with abduction of shoulders at 90 ${}^{\circ}$ and flexion of the elbows at 90 ${}^{\circ}$ . The examiner placed the HHD in the middle between the acromion process and the root of the spine of the scapula. Participants were asked to retract their scapula. The force of the HHD was applied in the lateral direction of the humerus (Fig. 4b).

Lower trapezius muscle: Participants were placed in a prone position with abduction of the shoulders at 140 ${}^{\circ}$ . The examiner placed the HHD in the middle between the acromion process and the root of the spine of the scapula. Participants were asked to adduct and depress their scapula. The force of the HHD was applied in the upper and lateral direction of the humerus (Fig. 4c).

Serratus anterior muscle: Participants were placed in a supine position with flexion of shoulders at 90 ${}^{\circ}$ and flexion of the elbows at 90 ${}^{\circ}$ across the chest. The examiner placed the HHD on the olecranon process of the elbow joint. Participants were asked to protract their scapula. The force of the HHD was applied toward the humerus (Fig. 4d).

2.5 The reliability test of raters

Four evaluators were responsible for assessing posture and muscle imbalance. The first rater evaluated the cervical and shoulder angles. The second rater was responsible for assessing thoracic kyphosis. The third and fourth raters assessed the length of the pectoralis muscle and the strength of the scapular muscles, respectively. The reliability sample consisted of 10 participants in each test. The procedures were repeated one week later, and the results were compared to assess intra-rater reliability.

2.6 The intervention protocol

Participants in the exercise group performed three scapular stabilization exercises at 3 sets of 10 repetitions, holding 10 seconds per one repetition, 3 days per week for 4 weeks under supervision [25, 26]. Scapular stabilization exercise training took place in the exercise station of Walailak University. The exercises included:

Exercise for the middle trapezius and rhomboid muscles: Participants were instructed to stand with their arms hanging beside their bodies. Then, they were instructed to flex their elbows 90 ${}^{\circ}$ , holding an elastic band and performing an external rotation of the shoulder, pulling both scapula together. Subsequently, they returned to a neutral shoulder position with elbow flexion (Fig. 5).

Figure 5.

The exercise for the middle trapezius and rhomboid muscles. Participants flexed both elbows 90 ${}^{{\circ}}$ and performed external rotation of shoulders.

Exercise for the lower trapezius muscle: Participants were instructed to lie down in a prone position while holding an elastic band attached at one end under a mattress. They were then instructed to raise their shoulder 140 ${}^{\circ}$ (Fig. 6).

Figure 6.

The exercise for the lower trapezius muscle. Participants were in a prone position and elevated the shoulder 140 ${}^{\circ}$ .

Exercise for the serratus anterior muscle: Participants were instructed to stand and flex their elbows 90 ${}^{\circ}$ while holding an elastic band fixed behind the participants. Next, they were instructed to move the arm forward with a protracted scapula (Fig. 7).

Before performing exercises, participants were instructed to choose an elastic band with a moderate level of resistance, which was measured using the OMNI-Resistance Exercise Scale (OMNI-RES). Participants tested the resistance of the elastic band five times for each exercise to determine a moderate level (level 5 of OMNI-RES) of resistance for the elastic band [31]. Participants were then instructed to increase the resistance of the elastic band for each subsequent week. Participants in the control group performed none of the exercises.

2.7 Statistical analysis

The data were analyzed using IBM SPSS Statistical Version 20 (IBM Crop., USA). A normal distribution of data was determined by the Shapiro-Wilk normality test. Descriptive statistics were computed for each variable (mean and standard deviation). A paired sample $t$ -test was used to analyze the difference between pre- and post-tests within each group. An independent sample $t$ -test was used to compare the mean difference of pre- and post-tests between groups. The level of significance was set at $p$ -value $<$ 0.05.

3. Results

One of the participants in the exercise group did not complete the exercise program. Therefore, the exercise group was composed of 19 female participants, while the control group was composed of 20 participants. There was no significant difference in age, weight, height, and body mass index (BMI) between the groups (Table 1).

Figure 7.

The exercise for the serratus anterior muscle. Participants flexed both elbows 90 ${}^{\circ}$ and moved arms forward with protracted scapula.

Table 1

General descriptive characteristics of participants

Variables	Exercise group ( $n=$ 19)		Control group ( $n=$ 20)
	Mean	SD	Mean	SD	$p$ -value
Age (yr)	20.26	1.20	20.15	1.27	0.776
Weight (kg)	50.31	4.37	51.43	4.98	0.461
Height (cm)	156.68	4.45	158.30	6.43	0.370
BMI (kg/m ${}^{2}$ )	20.46	1.11	20.52	1.31	0.884

BMI: body mass index.

Table 2

Comparison of variables between pre and post-test of the scapular stabilization exercise and control group using paired sample $t$ -test

Parameter	Exercise group ( $n=$ 19) (mean $\pm$ SD)			Control group ( $n=$ 20) (mean $\pm$ SD)
	Pre	Post	$p$ -value	Pre	Post	$p$ -value
CA (degree)	49.63 $\pm$ 4.96	52.89 $\pm$ 4.85	$<$ 0.001 ${}^{\ast\ast}$	48.95 $\pm$ 4.27	48.85 $\pm$ 4.17	0.857
SHA
Right (degree)	39.58 $\pm$ 8.06	45.42 $\pm$ 6.52	$<$ 0.001 ${}^{\ast\ast}$	42.65 $\pm$ 7.70	41.90 $\pm$ 8.78	0.652
Left (degree)	42.05 $\pm$ 6.11	45.74 $\pm$ 5.31	0.001 ${}^{\ast}$	44.50 $\pm$ 6.14	40.00 $\pm$ 8.12	0.008 ${}^{\ast}$
Midthoracic curve (degree)	37.62 $\pm$ 8.88	32.97 $\pm$ 6.10	0.016 ${}^{\ast}$	41.58 $\pm$ 7.59	39.61 $\pm$ 7.61	0.100
Pectoralis minor length
Right (cm)	14.19 $\pm$ 0.83	15.01 $\pm$ 0.82	0.001 ${}^{\ast}$	14.39 $\pm$ 1.24	14.07 $\pm$ 1.02	0.082
Left (cm)	14.50 $\pm$ 0.87	15.44 $\pm$ 0.68	$<$ 0.001 ${}^{\ast\ast}$	14.91 $\pm$ 1.16	14.54 $\pm$ 1.12	0.033 ${}^{\ast}$
Rhomboid muscles
Right (kg)	5.06 $\pm$ 0.77	8.80 $\pm$ 1.24	$<$ 0.001 ${}^{\ast\ast}$	6.06 $\pm$ 0.89	5.18 $\pm$ 0.93	0.004 ${}^{\ast}$
Left (kg)	5.07 $\pm$ 0.86	8.11 $\pm$ 1.17	$<$ 0.001 ${}^{\ast\ast}$	5.67 $\pm$ 0.90	5.05 $\pm$ 1.15	0.028 ${}^{\ast}$
Middle trapezius muscles
Right (kg)	4.97 $\pm$ 1.48	7.81 $\pm$ 1.34	$<$ 0.001 ${}^{\ast\ast}$	4.76 $\pm$ 0.97	4.66 $\pm$ 1.02	0.505
Left (kg)	4.72 $\pm$ 1.71	9.05 $\pm$ 1.79	$<$ 0.001 ${}^{\ast\ast}$	4.62 $\pm$ 1.42	4.46 $\pm$ 1.23	0.245
Lower trapezius muscles
Right (kg)	5.25 $\pm$ 1.40	8.95 $\pm$ 1.42	$<$ 0.001 ${}^{\ast\ast}$	5.60 $\pm$ 1.44	4.78 $\pm$ 1.06	$<$ 0.001 ${}^{\ast\ast}$
Left (kg)	5.23 $\pm$ 1.64	8.83 $\pm$ 1.80	$<$ 0.001 ${}^{\ast\ast}$	5.92 $\pm$ 1.36	4.94 $\pm$ 0.69	0.001 ${}^{\ast}$
Serratus anterior muscles
Right (kg)	9.38 $\pm$ 1.19	12.77 $\pm$ 1.43	$<$ 0.001 ${}^{\ast\ast}$	10.06 $\pm$ 1.34	9.36 $\pm$ 1.16	$<$ 0.001 ${}^{\ast\ast}$
Left (kg)	8.69 $\pm$ 1.39	12.03 $\pm$ 1.40	$<$ 0.001 ${}^{\ast\ast}$	9.73 $\pm$ 1.17	9.20 $\pm$ 1.34	0.006 ${}^{\ast}$

CA: cervical angle, SHA: shoulder angle, ${}^{*}$ significance level ( $p<$ 0.05), ${}^{**}$ significance level ( $p<$ 0.001).

Table 3

Comparison of the mean difference between pre and post-test of the scapular stabilization exercise and control group

Parameter	Mean difference		$p$ -value
	Exercise group ( $n=$ 19)	Control group ( $n=$ 20)
CA (degree)	3.26 $\pm$ 2.13	$-$ 0.10 $\pm$ 2.45	$<$ 0.001 ${}^{\ast\ast}$
SHA
Right (degree)	5.84 $\pm$ 4.35	$-$ 0.70 $\pm$ 6.83	0.001 ${}^{\ast}$
Left (degree)	3.68 $\pm$ 4.28	$-$ 4.50 $\pm$ 6.84	$<$ 0.001 ${}^{\ast\ast}$
Midthoracic curve (degree)	$-$ 4.65 $\pm$ 7.64	$-$ 1.97 $\pm$ 5.10	0.209
Pectoralis minor length
Right (cm)	0.81 $\pm$ 0.91	$-$ 0.33 $\pm$ 0.80	$<$ 0.001 ${}^{\ast\ast}$
Left (cm)	0.87 $\pm$ 0.61	$-$ 0.38 $\pm$ 0.74	$<$ 0.001 ${}^{\ast\ast}$
Rhomboid muscle strength
Right (kg)	3.74 $\pm$ 1.02	$-$ 0.88 $\pm$ 1.22	$<$ 0.001 ${}^{\ast\ast}$
Left (kg)	3.04 $\pm$ 1.19	$-$ 0.62 $\pm$ 1.17	$<$ 0.001 ${}^{\ast\ast}$
Middle trapezius muscle strength
Right (kg)	2.84 $\pm$ 0.96	$-$ 0.10 $\pm$ 0.67	$<$ 0.001 ${}^{\ast\ast}$
Left (kg)	4.33 $\pm$ 1.74	$-$ 0.17 $\pm$ 0.63	$<$ 0.001 ${}^{\ast\ast}$
Lower trapezius muscle strength
Right (kg)	3.71 $\pm$ 1.47	$-$ 0.82 $\pm$ 0.80	$<$ 0.001 ${}^{\ast\ast}$
Left (kg)	3.60 $\pm$ 1.73	$-$ 0.97 $\pm$ 1.10	$<$ 0.001 ${}^{\ast\ast}$
Serratus anterior muscle strength
Right (kg)	3.38 $\pm$ 1.03	$-$ 0.70 $\pm$ 0.74	$<$ 0.001 ${}^{\ast\ast}$
Left (kg)	3.34 $\pm$ 1.28	$-$ 0.52 $\pm$ 0.75	$<$ 0.001 ${}^{\ast\ast}$

CA: cervical angle, SHA: shoulder angle, ${}^{*}$ significance level ( $p<$ 0.05), ${}^{**}$ significance level ( $p<$ 0.001).

The results of pre- and post-tests revealed significant differences in the increase ( $p<$ 0.05) of all variables (CA; SHA; pectoralis minor length; rhomboid, middle and lower trapezius, and serratus anterior muscle strength) within the exercise group. Within the control group, there was a significant difference in the decrease of the left SHA, left pectoralis minor length, and strength of both sides of the rhomboid, lower trapezius, and serratus anterior muscles ( $p<$ 0.05) (Table 2). The results of the mean difference of pre- and post-test between the exercise and control group revealed a significant difference ( $p<$ 0.05) in all variables except the degree of the mid-thoracic curve (Table 3).

4. Discussion

The purpose of the current study was to examine the effects of scapular stabilization exercises on posture and scapular muscle imbalance by measuring CA, SHA, degree of mid-thoracic curve, length of the pectoralis minor muscle, and strength of the rhomboid, middle and lower trapezius and serratus anterior muscles. Our results indicated that four weeks of three scapular stabilization exercises significantly increased cervical and shoulder angles, pectoralis minor muscle length and scapular muscle strength. The results suggested that this exercise protocol can correct forward head and shoulder posture and improve scapular muscle imbalance.

According to our study, scapular stabilization exercises could improve cervical and shoulder angles. The scapular stabilizer muscles, including the rhomboid, middle and lower trapezius, and serratus anterior muscles, are the primary muscles that control the movement of the scapula [32]. Furthermore, muscles that attach to the cervical spine and nuchal ligament, such as the trapezius and rhomboids muscles, are important muscles associated with controlling the extension of the head and neck. Therefore, scapular stabilization exercises can adjust and correct head, neck and shoulder postures, which subsequently lead to increased cervical and shoulder angles [16, 19].

In addition, the exercise program in our study can improve pectoralis minor length. The primary muscles that control scapular movement include the rhomboid, trapezius, serratus anterior, levator scapulae, and pectoralis minor muscles [32]. Strengthening of scapular stabilizer muscles can improve the scapular position and forward shoulder posture, adjusting and increasing the flexibility of the pectoralis minor muscle, which results from reciprocal inhibition. When an antagonist muscle is in voluntary maximal contraction, it makes the agonist muscle relax because of the neural mechanism of reciprocal inhibition. At the spinal level, the neural signal from Ia afferent fibers of the antagonist muscle enters the spinal cord and interacts with inhibitory interneurons synapsed to alpha motor neurons of the agonist muscle which result in the inhibition of agonist muscle activity and causes its relaxation. This mechanism creates stretching force for the agonist muscle and elongates agonist muscle fibers. In the case of the increased muscle length of the pectoralis minor (agonist muscle) in our results, the contraction of the posterior muscle (scapular muscle) may inhibit the neural activity of the anterior (pectoral) muscle, leading to the release of the tension in the pectoral muscle [24]. Furthermore, participants in our study were asymptomatic on both sides of their shoulder joint with full range of motion. Our study may indicate that in the case of minimal or no structural change in non-contractile structures, scapular stabilizer exercises can increase the flexibility of the pectoralis minor muscle.

To the best of our knowledge, no previous studies have investigated the effects of scapular stabilization exercises on the mid-thoracic curve. Our study showed no significant difference in the degree of the mid-thoracic curve between groups. There were several reasons for this. First, the majority of participants did not exhibit thoracic hyperkyphosis (thoracic angle $>$ 40 ${}^{\circ}$ ) [33]. The effects of scapular stabilization may not change the mid-thoracic curve. Second, we included sedentary young adults who spend most of their time sitting and using the computer. Prolonged sitting leads to spinal stiffness [34, 35], which may not cause changes in mid-thoracic kyphosis. Finally, scapular stabilization exercises may be inadequate to correct thoracic kyphosis. The weakness of the extensor muscle of the spine allows anterior pulling of gravitational force on the thoracic spine, leading to an increasing thoracic curve. Therefore, strengthening the extensor muscle of the spine should be seriously taken into consideration, particularly to re-correct the posture of mid-thoracic kyphosis [36].

The exercise program of our study, including exercises for the lower trapezius and serratus anterior muscles, was recommended from a previous study that reported on highly activating the lower trapezius and serratus anterior muscles [25]. Additionally, exercises for the middle trapezius and rhomboids muscles by performing external rotation with the elbow flexed at 90 ${}^{\circ}$ in a side-lying position were recommended [25]. Although our study selected a sitting position because participants could perform the exercise more comfortably than in a side-lying position, our results showed a significant increase in the strength of the middle trapezius and rhomboid muscles.

There were many variations of the exercise protocol from previous studies. Lynch and colleagues [16] studied the effects of eight weeks of scapular stabilization exercises on the alteration of scapular muscle strength, cervical angle and shoulder displacement. The scapular stabilization exercise program of this study included the isometric strengthening of scapular stabilizer muscles (Y to W, L to Y, and scapular protraction exercise) combined with pectoral muscle stretching at 3 sets of 10 repetitions three times per week. The study found a significant decrease in forward head and shoulder posture, but there was no significant difference in scapular muscle strength when compared to the control group. The decrease in forward head and shoulder posture without increasing scapular muscle strength may be due to the effects of the stretching exercise on the pectoral muscle. In comparison, the exercise program in our study revealed more explicit effects of increasing scapular muscle strength and pectoralis minor length in a shorter exercise training duration of four weeks. This fact might be because our study applied progressive resisted exercise using an elastic band, which leads to enhance activation of the scapular muscle.

In addition, other previous studies investigated the effects of scapular stabilization exercises on neck and shoulder posture. Thakur and colleagues [17] applied the resistance of an elastic band with the proprioceptive neuromuscular facilitation (D2 PNF pattern) exercise, holding 30 seconds for 4 repetitions; shoulder external rotation and low row exercise for 12 repetitions at 3 times a week for two weeks. The results of this study showed a significant decrease in forward head posture. However, this study did not investigate the scapular muscle strength. From a study on the physiological adaptation of resistance exercises, 2–3 weeks would increase the strength resulting from neural adaptation [37]. Therefore, strengthening exercises for two weeks may not be effective for a significant increase in muscle strength.

Hajihosseini and colleagues [20] studied the effects of shoulder retraction, external rotation, and flexion on forward shoulder posture by using an elastic band at 3 sets of 10 repetitions per week for six weeks. This study showed a significant decrease in forward shoulder posture. However, the strengthening of the scapular muscles was not investigated. Although six weeks of scapular stabilization exercises can increase scapular muscle strength, our study showed a significant increase in muscle strength at four weeks, which was shorter than the previous study.

The advantages of our study compared to previous studies included the following aspects. First, we investigated all variables of posture and muscle imbalance that could be affected by scapular stabilization exercises. Our study suggested that only scapular stabilization exercises can improve neck and shoulder posture, scapular muscle strength, and pectoralis minor muscle length. Second, we selected the optimal scapular stabilization exercise protocol according to the literature review [25] and the full term of the ACSM’s guidelines for exercise testing and prescription [26]. A moderate level of resistance for the elastic band was recommended, which was appropriate for strengthening exercises [31]. Finally, the individual exercises were instructed and monitored under strict supervision.

Several limitations were noted in our study. First, the participants were disclosed regarding their groups. Second, the various activities of the participants during the training program could not be regulated in the exercise and control groups. Third, information on activities and work characteristics, such as type of computer use, sitting posture and specific type of work, must be determined. In addition, we recruited only female participants; male participants should be included to generalize the results in future studies. Lastly, this RCT was not registered in the trial registry, however, it was approved by the ethical committee for human research of Walailak University prior to the commencement of this study. In addition, this study was conducted according to the CONSORT checklist for validation and standardization of the overall framework as shown in the supplementary data.

5. Conclusion

The current study suggested that a 4-week scapular stabilization exercise program was effective in improving forward head and shoulder posture, increasing the flexibility of the pectoralis minor muscle and strengthening scapular muscles, which include the rhomboids, middle and lower trapezius and serratus anterior muscles, particularly in young women.

Footnotes

Acknowledgments

We would like to express our special thanks to Assistant Professor Jiraphat Nawarat for providing excellent guidance and encouragement to this project. Our sincere thanks also go to Miss Thitika Khongsankhum, Miss Kanmanee Dama-u, Miss Yasmin Waeyusoh, and Miss Rowaida Hajisamoh for providing their assistance and making excellent efforts to accomplish this project. This study was supported by the Internal Research Funding for Health Science at the Institute of Research for Health Science, Walailak University, Thailand.

This study was supported by the Internal Research funding for health science (WU-IRG-62-014) at the Institute of Research for Health Science, Walailak University, Thailand.

Conflict of interest

The authors have no conflict of interest to report.

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