**1. Introduction**

Overuse or repetitive motion injuries are very common in sports [1]. This type of injury can be a major source of disability and pain and can leave players unable to engage in physical activity or competition. There is little evidence on the real impact and severity in sport, due to the methodological difficulties involved in recording them [1]. Volleyball is one of the sports affected by overuse injuries [2–4]. The biomechanics of the different movements involved in volleyball, in particular the spike and serve, together with the anatomy of volleyball players coincide with the most prevalent overuse injuries, the shoulder region (19.0 ± 11.2%) and the spine (16.8 ± 9.7%) [3,4]. These structures are subjected not only to repetition of movement but also to high torsional values and amplitude of movement in short periods of time [3].

The assessment of posture and biomechanics is a useful clinical tool in shoulder pain. Shoulder antepulsion, also known as "rounded shoulder", is characterized by a position in which the scapula rotates downwards and remains tilted forward, increasing cervical lordosis and upper thoracic kyphosis [5]. It has been previously published that postural or biomechanical alterations such as forward head position, shoulder antepulsion, altered scapular kinematics, and imbalance of muscle activity are associated with shoulder pain [6]. This could lead to the development of pain depending on the tolerance and adaptive capacity of the central nervous system [7]. In addition, there is evidence that a high percentage of patients with non-specific arm pain have their shoulders in antepulsion and head forward (78% and 71%, respectively) [6,8].

Loss of activity of the lower trapezius and serratus anterior, a marked thoracic kyphosis and the anatomy of the scapula itself can cause the shoulder antepulsion. In addition, tension of the pectoralis minor [5], which, together with the downward displacement of the coracoid process, may affect the gliding of the brachial plexus cords. Complete scapular protraction (due to its junctions with soft tissues and surrounding structures) may reduce the space between the clavicle and the first rib, restricting nerve gliding, and as a result of a combination of movements of structures of the shoulder girdle itself, anterior displacement of the humeral head may occur [9]. This can lead to some postural alterations such as an antepulsion of the shoulder can alter the mechanosensitivity of different tissues, thus decreasing their tolerance to mechanical stress even if it does not provoke a nociceptive response [10]. A recent study adds data on this association by concluding that individuals with shoulder impingement syndrome had a greater thoracic kyphosis and less extension movement than age- and gender-matched healthy controls [11]. For all these reasons, more studies are needed in the sports population to observe whether these types of associations or relationships exist and what clinical implications they may have.

Currently there are no studies that correlate the forward shoulder position with photometry with the shortening of the pectoralis minor in any type of population. Nor has it been studied whether these postural alterations are related to pain and mechanical hyperalgesia in volleyball players.

For the above reasons, the objectives of this study are as follows:


### **2. Materials and Methods**

This research was a cross-sectional study conducted according to the Strengthening the Reporting of Observational studies in Epidemiology (STROBE) statement [12], and following the declaration of Helsinki. The study protocol received approval from the Research Ethics Committee of University CEU Cardenal Herrera from Valencia (CEI16/0112). All the participants agreed to participate and signed an informed consent form. The study was conducted at the Physiotherapy and Pain Research Center in Alcalá de Henares (Spain).

#### *2.1. Participants*

Recruitment was carried out through an invitation to participate in the study was sent to the different volleyball sports clubs in the local area. Eligible participants were women and men from the age of 18 onwards and being a regular volleyball player (3 or more hours per week). Exclusion criteria included participants who has previous neck and shoulder injuries or surgery and were unable to read or speak in Spanish.

#### *2.2. Procedure*

Shoulder position was assessed in volleyball players with and without shoulder pain by determining the position of the humeral head and the length of the pectoralis minor muscle [6,13]. In addition, the mechanosensitivity of neuromuscular structures related to the upper quadrant of the evaluated subjects was assessed [14].

The assessments were performed by three physiotherapists independently and none of them knew the subject's condition in relation to shoulder pain, this was done to ensure the simple blind. Study participants were randomly assigned to each assessment by choosing a ballot with a number on it. Subjects who selected number 1 were first assessed for shoulder position, subjects who selected number 2 were assessed for mechanosensitivity of the musculature, and subjects who selected number 3 were assessed for mechanosensitivity in the nerve trunks.

#### *2.3. Outcome Measures*

#### 2.3.1. Forward Shoulder Angle (Forward Shoulder Position)

Posture was assessed using a postural analysis software [14,15]. Markers were placed on the acromion and the spinous process of C7. Participants were placed in a standing position, sideways 40 cm from the wall, and instructed to maintain a natural resting position. A reflex camera (Nikon Model D5300 SLR, Tokyo, Japan) was placed on a tripod one meter high and three meters from the wall. One photograph was taken from the right side and one photograph from the left side.

The forward shoulder angle (FSA) was determined by calculating the angle formed by a vertical line passing posterior to the marker at C7 and a line connecting C7 and the acromion marker. Those participants who showed values equal to or greater than 52◦ were considered a forward shoulder position (FSP) [6]. This procedure has shown good reliability with an very high intraclass correlation coefficient (ICC) (0.89) [6]. The photographs were taken by a physiotherapist with more than 10 years of experience in the management of musculoskeletal pain and was blinded to the values obtained from the other assessments.

#### 2.3.2. Pectoralis Minor Length Measurements

Pectoralis minor (PM) length is expressed as the pectoralis minor index (PMI) which is calculated as PM length (cm)/subject height × 100. This normalization index is used to allow for the variety of soft tissue and body structure between subjects [16]. To measure the length of the PM muscle the reference points were the inferior medial angles of the coracoid process, and lateral to the sternocostal junction of the fourth rib on its underside. These landmarks have shown an ICC of 0.96 [16]. A digital caliper (Mitutoyo/200 mm, Kawasaki, Japan) was used to measure the distance between these two points. Measurement using a caliper has shown an ICC of 0.83 to 0.87 [17].

Participants were placed supine with both hands on the abdomen, with the shoulders slightly abducted, and in a relaxed elbow flexion position. The elbows were flexed to eliminate the passive influence of the biceps brachii muscle [18]. PMI assessment was performed by a second evaluator blinded to the other assessments values and with more than 10 years of experience in the management of musculoskeletal problems.

#### 2.3.3. Tissue Mechanosensitivity (Muscle and Nerve Trunks)

The degree of tissue mechanosensitivity was assessed by determining the pressure pain threshold (PPT) with algometry, i.e., quantitative assessment of the sensory perception of the mechanical stimulus [19]. The PPT was measured with a manual algometer (Wagner Force Dial, Model FDK20) which has a head of 1 cm<sup>2</sup> and determines the pressure in kg/cm<sup>2</sup> [19]. The pressure was increased by 1 kg per second until the subject indicated changes in pressure sensation. The assessor stopped applying pressure when the participant expressed pain. Three measurements were made at each location with a 30-s rest period in between. The mean value of the three measurements was used for statistical analysis. A third evaluator blinded to the other assessments measured the PPT of the following muscles: serratus anterior, lower trapezius, infraspinatus, teres minor, upper trapezius, levator scapulae, pectoralis major. The muscle was palpated to locate the most mechanosensitive point and perform the PPT measurement. Measurement of the PPT with an algometer has been shown to have good reliability with an ICC of 0.87 to 0.89 [19].

A fourth evaluator performed bilateral PPT measurements of the nerve trunks of the upper extremity: median nerve, radial nerve, and ulnar nerve. The nerves were evaluated at the locations described by Sterling et al. [19], which have shown good reliability with an ICC of 0.92 to 0.97. The median nerve identified in the ulnar fossa inside the tendon of the biceps muscle. The radial nerve was located in the lateral intermuscular septum between the medial and lateral head of the triceps brachii muscle, and the ulnar nerve was located in the ulnar canal of the elbow. Both evaluators had more than 10 years of experience in the management of musculoskeletal system alterations.

#### 2.3.4. Pain Intensity

Shoulder pain was assessed using a visual analog scale (VAS). The subject with pain indicated what their pain was on a 10-cm line where 0 represented no pain and 10 the worst pain imaginable. This tool has been shown to be reliable with an ICC of 0.94 [20]. The VAS measure was expressed in cm. Participants with more than 0 cm in the VAS were placed in the shoulder pain group (SPG), the rest of participants were placed in the control group (CG).

#### *2.4. Sample Size*

Sample size and power calculations were performed with an appropriate software (G\*Power 3.1) [21]. This study was based on a model of correlations, and the FSA was the primary outcome, with an effect size of 0.75. Given an alpha level of 0.05 and a power of 0.80, two groups were generated with a total sample size of 50. The groups included shoulder pain and without pain (control) with a minimum of 25 participants per group.

#### *2.5. Data Analysis*

Statistical analysis was performed with the Statistical Package for the Social Sciences, version 28 (IBM Corporation, Armonk, NY, USA). The normality of the study variables was tested using the Shapiro–Wilk test. A normal distribution of the variables was not obtained in the Shapiro–Wilk test (*p* < 0.05). Qualitative variables are presented as an absolute value and the percentage of the relative frequency [*n* (%)]. Continuous variables are represented as median (1st and 3rd quartiles). All statistical tests were interpreted at a significance level of 5% (*p* < 0.05). To test the differences between groups for FSA, PMI, muscle PPT, and nerve PPT, the Mann–Whitney U test was performed to verify which ones entailed statistically significant differences. Finally, the correlations between the study variables were analyzed

for each group with the Spearman's Rho test considering the results as 0.01 to 0.19 very low correlation, 0.2 to 0.39 low correlation, 0.4 to 0.69 moderate correlation, 0.7 to 0.89 high correlation, 0.9 to 0.99 very high correlation, and 1 large or perfect correlation.

#### **3. Results**

#### *3.1. Participants and Descriptive Data*

A total of 56 volleyball players met the inclusion criteria and agreed to enter the study, leaving a sample of 28 in the SPG group and 28 in the CG. The median age of the sample was 22.5 (19 and 24), with most of them being female *n* = 33 (58.9%). No statistically significant differences were found in the descriptive characteristics measured in both groups (*p* > 0.05). The descriptive data of the participants are shown in Table 1.

**Table 1.** Characteristics of the groups. Values are median (first and third quartiles) and *n* (%).


VAS, visual analogue scale; FSP, forward shoulder position.

#### *3.2. Comparison between Groups*

The Mann–Whitney U test revealed no statistical differences between groups for FSA (*p* = 0.33) and for PMI (*p* = 0.29), see Table 2. On the other hand, significant statistical differences between groups for muscle PPT in Lower Trapezius (*p* = 0.019), Infraspinatus (*p* < 0.01), Teres Minor (*p* < 0.01), Upper Trapezius (*p* = 0.019), Pectoralis Major (*p* = 0.02), and for radial nerve PPT (*p* = 0.04), see Table 2.



\* *p* < 0.05, \*\* *p* < 0.01. FSA, forward shoulder angle; PMI, pectoralis minor index; PPT, pain pressure threshold.

#### *3.3. Correlations*

The Spearman's Rho test revealed in the SPG a negative moderate correlations between FSA and Infraspinatus-PPT (Rho = −0.43; *p* = 0.02); FSA and Levator Scapulae-PPT (Rho = −0.55; *p* < 0.01); FSA and Pectoralis Major-PPT (Rho = −0.41; *p* = 0.02); PMI and

Cubital Nerve-PPT (Rho = −0.44; *p* = 0.01); VAS and Upper Trapezius-PPT (Rho = −0.41; *p* = 0.02); VAS and Median Nerve-PPT (Rho = −0.51; *p* < 0.01). No significant correlations were found between posture measurements (FSA and PMI) and VAS, see Table 3.

**Table 3.** Spearman's Rho correlations in Shoulder pain Group.


\* *p* < 0.05; FSA, forward shoulder angle; PMI, pectoralis minor index; VAS, visual analogue scale.

No significant correlations were found according to the Spearman's Rho test in the CG, see Table 4.

**Table 4.** Spearman's Rho correlations in Control Group.


FSA, forward shoulder angle; PMI, pectoralis minor index; VAS, visual analogue scale.
