1. Introduction

For having favorable physical condition, coordination of different parts of the body is required. One of the areas that play a key role in maintaining good physical condition is the scapula. The role of the scapula has been significantly considered in recent years, as it is very important in the production of smooth and coordinated movements of the shoulder girdle. Kibler [2] stated that the role of the scapula in throwing and serving is to create the right movements and positions for facilitating shoulder function. In other words, proper shoulder movements are critical for natural shoulder mechanics and are effective during throwing balls. The position of the scapula and its orientation are clearly different in throwing and non- throwing athletes, which indicates that adaptations in these athletes depend on their field of activity [4]. The inability of the scapula to perform these roles results in loss of physiological and biomechanical efficiency, and, therefore, causes shoulder dysfunction. This can lead to poor performance and increased shoulder injuries [5]. 

One of the main roles of the scapula is that it acts as a base for connecting muscles, and its displacement can affect the function of the muscles around the shoulder girdle, especially the muscles that support the scapula. In addition, the position of the scapula is directly related to the production of muscle forces as well as the range of motion of the shoulder girdle [5]. Therefore, it seems that displacement of the scapula may affect the function and production of the muscle force in the shoulder girdle, especially the scapular stabilizer muscles. In volleyball athletes, repetitive movements such as abduction and external rotation are accompanied by extension and internal rotation, and hitting the ball at maximal abduction increases the traction force. Muscle imbalances and rotation muscles weakness are risk factors for the shoulder injury in overhead athletes [6]. Regarding the main cause of changes in the distance of the scapula from the spine, some researchers have stated that the position of the scapula is an inherited condition that some people are born with [7]. However, others believe that if the muscles that support the scapula do not have the necessary endurance, strength, and flexibility, there will be many changes in this area, given the important role they play in the position of the scapulae relative to each other.

The literature review showed that very little research has been done on the strength and range of motion of the muscles around the shoulder, as well as the ratio of strength between the muscles around the glenohumeral joint in elite athletes, and most of them have examined the external/internal rotation ratio in shoulder. On the other hand, some studies have mainly compared the physical strength of female and male athletes or athletic and non-athletic women and have generally concluded that male athletes have more physical strength than female athletes, or female athletes are in a better position than non-athletic women in terms of physical strength or other physical fitness factors. Therefore, due to the fact that the balance of strength between the agonist and antagonist muscles of the glenohumeral joint can cause damage to the shoulder joint and impingement syndrome, it is necessary to conduct research in this area. To our knowledge, no study has been conducted in this area, and none of them have examined the effect of the position of the scapula on the strength of the shoulder girdle muscles in elite athletes. In this regard, this study aimed to assess and compare the strength and Range of Motion (ROM) of external and internal rotation of shoulder muscles in male volleyball players

2. Methods

The study population consists of all male volleyball players in Mahabad County with at least 3 years of regular exercise experience and having practice at least 5 sessions per week. Of them, 15 with uneven shoulders and 15 healthy peers were randomly selected as the statistical sample of this study and entered the study after obtaining written consent from them. A Manual Muscle Testing (MMT) dynamometer (CI=95%) was used to measure the isometric strength of the shoulder muscles. Strength of internal and external rotators was measured in a neutral position (zero degrees of abduction). The ROM of the internal and external rotation of the glenohumeral joint in one of the shoulders was evaluated twice a day with a standard goniometer (Universal, LTD Co., Japan) with an accuracy of 3 degrees. To measure uneven shoulders, a scoliometer (Mizuho OSI, Japan) with acceptable validity and reliability.

The collected data were analyzed in SPSS V. 20 using descriptive (mean, standard deviation) and inferential statistics. Independent t-test was used to compare the strength and ROM of even and uneven shoulders, and paired t-test and ANOVA were used to compare two study groups. The normality of data distribution was performed by Kolmogorov-Smirnov test due to the sample size >30. The significance level was set as P<0.05.

3. Results

The results shown in Table 1 showed that there was a significant difference in internal (P=0.03) and external rotation strength (P=0.02) of volleyball players with and without uneven shoulders.

A significant difference was also reported in their internal (P=0.02) and external rotation ROM (P=0.03).

4. Conclusion

The high muscle strength of volleyball players with uneven shoulders has been identified as a risk factor in many studies. In volleyball, it can also be considered a long-term cause of injury. Volleyball, like other sports, involves repetitive movement patterns in which hands have activity overhead, and in the long term, it can lead to the asymmetry of shoulder on the dominant side which can cause muscle imbalance and limitation in the ROM of rotator muscles in the shoulder joint of these athletes. It requires attention to preventive approaches by creating corrective solutions. It is recommended that volleyball coaches and players, given the specific biomechanics of repetitive volleyball movements and to prevent imbalances in muscle strength and ROM, emphasize equally in strengthening and stretching the agonist and antagonist muscles of the shoulder girdle, and when exercising, pay attention to the muscles that are undergoing strengthening or flexibility exercises.

Ethical Considerations

Compliance with ethical guidelines

This article is taken from Research plan in Physical Education and Sport Sciences Lab. All subjects voluntarily participated in the study and signed the written consent form. IR. IAUETB.98001 Article Code of Ethics

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions

All authors contributed in preparing this article.

Conflicts of interest

The authors declared no conflict of interest.

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