Dynamic testing of volleyball players ’ body posture using a formetric 3D device

: Volleyball activities involve several factors such as asymmetrical movements, muscle imbalance, and continual overloading that damage body posture. This study aimed to determine postural changes of volleyball players in spike overhand front set, overhand pass ball, and mat ball motions, and if there is any postural variation difference between short training and long training volleyball players. Methods: Fifteen long training and short training male volleyball players aged 21 – 23 years were recruited in this study. The upper-thoracic angle, thoracic-lumbar segment angle, lower lumbar-sacral segment, pelvic inclination, and inclination of the acromion were measured using DIERS formetric 3D device in a dynamic way. During the overhand pass motion, long training participants showed a significant larger peak upper-thoracic and peak thoracic-lumbar angles than short training participants. During the mat ball motion, the long training volleyball players demonstrated a significant higher peak upper-thoracic angle, but a smaller peak inclination of the acromion than short trainers. During the spike overhand front set motion, long trainers showed a significant smaller peak upper-thoracic angle than short trainers. Athletes should consider developing an appropriate dynamic trunk stabilization to reduce posture imbalance during volleyball activities.


Introduction
Clinical research evidence shows that a good posture can be developed with the appropriate sport; however, asymmetrical movement, muscle imbalance, and intensive overloading over a long period leads to some undesirable postures such as asymmetric and abnormal postures. Undesirable postures is a prevailing problem among various types of athletes, and it is characterized by asymmetrical muscle force, deformed spine, asymmetric scapula in all planes, etc., eventually leading to cosmetic deformities.
Volleyball is one of the most popular sports in the world, but there is an estimation of a higher prevalence of deformation in the spine and shoulder as participation increases. Serve and attack constitute a series of asymmetric movements, which impose adverse effects on the body posture, especially in developing adolescents [1]. Mostly, a high-level player needs to perform hard and one-sided repeated exercises for a long period in unbalanced positions, which are associated with dynamic loading [2]. The potential influences of physical activity on the musculoskeletal system is one of the significant factors in maintaining a body posture. Previous reports show that back pain accounts for a large proportion of injuries in volleyball (80%), a contrast to other sports [3]. Spinal twisting, anterior-posterior bending, and asymmetrical motions are the main elements that contribute to the increase in low back injuries [4,5]. Low back pain leads to dysfunctioning of the trunk muscles, which has a significant negative effect on coactivation during dynamic trunk stabilization [6]. Duan et al. [7] reported that physical activity which led to curvature of the trunk produced a 10-fold increase in vertebral compression compared to erect standing. The line of gravity in longissimus and iliocostalis muscles can be altered by spinal flexion; consequently, it decreases their resistance to shear force but increases their resistance to compression force [8]. The maximal compressive force which the vertebral body can bear is related to bone density and is between 2 and 12 kN [9]. Spike presents the highest loading in the vertebrae, and peak loading is observed in phases of take-off and landing [3].
Thoracic kyphosis is another prominent body feature in volleyball athletes. Lichota et al. [2] noted that 75% of volleyball players in their study were kyphotic. Conversely, lordosis was the least frequent when compared with handball and taekwondo participants. However, authors failed to provide a specific explanation on the high proportion of kyphosis among these volleyball players. A typical postural pattern was detected in fifty female volleyball players in a study by Varěková et al. [1], and it showed a higher acromion, scapula, and iliac crest on the left lateral position leading to an asymmetric posture.
Numerous scientists have developed great interest in the relationship between playing volleyball and body posture [10][11][12]. However, most studies conducted static measurements in participants, which can only describe the superficial features. The intrinsic reason for body shape asymmetry due to volleyball activities should be detected through dynamic testing. Therefore, this study aimed to determine the postural changes of volleyball players in spike overhand front set, overhand jump back set, and mat ball motions. The second aim of this study was to determine whether the postural variations differs between short training and long training volleyball players. We hypothesized that participants who experienced the long training period will show a greater imbalance in the spine and shoulder, as compared to the short training players.

Methodology
In this study, 30 college-aged male volleyball athletes who were long or short trainers were recruited from the schools' professional volleyball team. Long training participants were experienced participants who had volleyball training for at least 3 years, and the short training participants had volleyball training for 1 year. All participants had no musculoskeletal injury and other diseases that could influence sports performance. All participants signed the consent form, and this study was approved by the Research Academy of Grand Health before data captured.

Physical evaluation
During body posture evaluation, the spine and shoulder symmetry were estimated between the long training and short training volleyball players. The upper-thoracic segment angle, thoracic-lumbar segment angle, lower lumbar-sacral segment angle ( Fig. 1), pelvic inclination in the frontal plane, and inclination of the acromion were measured.

Instrumentation
Data on body posture were collected during the motions with the DIERS formetric 3D device, which is associated with a dynamic capture system. It is a non-invasive electronic measuring system which is implemented when two images are merged into one information. The projector projects a stripe onto the surface of the object, then the camera matrix allocates a single pixel to corresponding three-dimensional coordinates (Fig. 2). By comparing the original stripe projection with the one caught by the camera matrix, it is possible to recalculate the shape of the object in 3D.

Experimental procedure
Participants were instructed to stand in front of the device and perform volleyball specific motions including the spike overhand front set, overhand pass ball, and mat ball motions. All motions were performed with a ball that was cast by an experimenter to simulate the real sport movements. Participants were required to wear tight-fitting clothes to perform all movements, in order to reduce the clothe interferences during data collection. For data precision, each motion was performed three times. The device can capture the movement process of the subjects' specified movement, and automatically analyse the data in a specific moment when it is needed. The data was used only if the participant performs the motions accurately.

Data analysis
Independent sample t-test was used to estimate discrepancies of the spine and shoulder between long training and short training volleyball players in the following motions: a spike overhand front set, an overhand jump back set, and a mat ball. Statistical analysis was conducted using SPSS 20.0 (SPSS Inc., Chicago, IL, USA), and the level of significance was set at p < 0.05.

Results
Results from body posture testing (Table 1) in mat ball motion show that no significant differences were found in the lumbar-sacral angle, pelvic inclination, and inclination of the acromion between long and short training participants. However, the long training participants had significantly larger peak upper-thoracic angles than the short trainers (long trainers, 42.1 ± 5.2; short trainer, 35.2 ± 4.4; P = 0.001). Compared with the short trainers, long trainers had significantly greater peak thoracic-lumbar angle (long trainers, 40.0 ± 3.5; short trainers, 32 ± 5.6; P < 0.001). Table 2 summarizes the results from the overhand pass ball motion. A comparative analysis of the anterior-posterior curvatures of spine showed a significant difference for some evaluation parameters between the long and short trainers. The long training volleyball players demonstrated a significantly higher peak angle at the upper-thoracic (long trainers, 45.4 ± 4.8; short trainers, 38.2 ± 3.7; P = 0.001) but a lower peak inclination of the acromion when compared with the short trainers (long trainers, 7.2 ± 2.7; short trainers, 12.8 ± 2.5; P = 0.001). Nevertheless, there was no significant difference found at the thoracic-lumbar angle, lumbarsacral angle, and pelvic inclination between the two groups. Table 3 demonstrates the results of the overhand pass ball motion between the two groups. Results show that trunk movement was more thoracic, and significant differences were found between the    Lumbar-sacral angle, pelvic inclination, and inclination of the acromion did not show any significant difference between the long and short training participants.

Discussion
This research was conducted in a dynamic way, which is different from previous methods where testing was conducted under static conditions. The advanced nature of the DIERS formetric 3D device provides us with an efficient and reliable way to measure spinal and shoulder changes during motions. It is thus necessary to understand the effect of volleyball training on spinal morphology based on real dynamic motions. Results of the current study demonstrated that the long volleyball trainers have high occurrence of kyphosis than short training players in three different motions. A larger inclination of the acromion was found in the short trainers, although no difference was found at the lumbar-sacral angle and pelvic-inclination. A larger kyphotic angle was found in long training volleyball players. This result is supported by previous studies that presented the shape of anterior-posterior curvatures of the spine, which indicated thoracic kyphosis curvature in volleyball players [2,13]. This might have resulted from an intensive-oriented physical activity over the long practice period [14] since volleyball players need repeated maximal effort jumps, frequent sudden change of direction sprints, and repeated overhand movement to spike or block [15]. Based on previous studies, spinal variations are related to spinal twisting, flexing, and unbalanced movements, and spikers/attackers seem to have more characterized increase in kyphotic angle [16]. In terms of athletic demands, a volleyball player needs high coordination between the lower and upper extremities and requires a stronger trunk to perform intensive activities. Indeed, hypermobility of the upper extremity introduces additional requirements in volleyball activities. Trunk muscle coactivation and recruitment pattern is more important than the muscle strength. It also requires that muscle function contributes to dynamic stability [17], since dynamic trunk stability is an important factor in controlling intervertebral and unstable movements [18]. Furthermore, dynamic stability also has a positive effect on the control of movement and loading in the distal segment by coordinating trunk muscle recruitment. Coordination of muscle recruitment is as a result of expected or unexpected movement changes such as, whether the correct posture in static stability or the expected path of dynamic motion can be maintained [19]. Effective dynamic trunk stability is essential in preventing back injuries and improving athletic performance [20,21]. Researchers have indicated that frequent repetitive movements may lead to tissue damage due to the long period of intensive strength, hyperflexibility, fatigue, and abnormal neuromuscular control [22,23]. In addition, trunk muscle coactivation can reduce the risk of spine injury by providing firmness and stability for the trunk [24].
Previous biomechanical studies of volleyball movements have estimated a larger proximo-distal peak force during bump set, dig, and spike motions. Crisco and Panjabi [25,26] noted that, without muscular contribution, the spinal buckling will bear 90 N of compressive loading. The compressive force on the spine during movements may be one of the important factors in back injuries. Although only the anterior-posterior direction of the spine was tested in this study, it may not provide sufficient evidence for the development of back pain in volleyball activities. Smith et al. [18] reported that in back pain, neuromuscular fatigue, inflammation, and pain, are associated with lower extremity segment dysfunction and atrophy in the muscles. Lumbar spine flexion causes muscle tension, disc degeneration, and herniation [27]. The intervertebral space is narrowed anteriorly, and the nucleus pulposus protrudes backward. However, when the trunk is overextended, the vertebral space is narrowed and the nucleus pulposus is pushed forward, increasing lumbar spondylolysis, which is the most common reason for back pain in adolescents [28].
A previous study showed that 80% of right-handed elite female volleyball players had their left acromion, scapula, and ilium higher than the right side, showing a 'typical' postural pattern [29]. In this study, the short training players presented with a high inclination of the acromion than the long trainers. This may have led to the trunk muscle instability that occurred in the short training group, leading to a larger imbalance of the shoulder from the left to right, especially when the upper arm was lifted above the shoulder level. Kugler et al. [30] detected a negative dominant shoulder and lateralized scapula in 30 volleyball attackers; however, no asymmetry was noted on both sides of the shoulder in recreational athletes. Furthermore, the previous author observed the presence of scapular asymmetry in elite male volleyball players by using the scapula lateral slide test and also indicated that the dominant arm showed a significant limitation in shoulder internal rotation [31]. Because few dynamic studies which assess shoulder and spinal changes in volleyball activities have been conducted, explanation of these changes are in reference to results from static posture measurements.
Although several studies demonstrate negative effects of volleyball activities on posture, it is essential to train effectively and intensely to reach a high level in competitions; avoidance of negative influences on the body posture during these activities is inevitable. Thus, an appropriate dynamic trunk stabilization during volleyball training, which helps in the development of an effective trunk muscle coactivation should be ensured [18]. Bauman [32] suggested that athletes should associate effective volleyball performance with dynamic trunk stability to enhance neuromotor control during training programs [32]. In basic volleyball training programs, elementary agility, balance, and coordination activities such as specific exercises for the core as well as muscle training for the abdominal and lower back regions should be incorporated, except in the fundamental running, jumping, and throwing. At the same time, muscle endurance, core strength-power, and kinaesthetic awareness also need to be developed with body weight resistance [33]. The player should participate in different activities to build good sensorimotor abilities, rather than focusing on a single sports program [34][35][36].
This current study has several limitations. Participants with greater height differences were included without stratification because the heights of the high-level volleyball team was naturally better than amateurs, and it is difficult to find the same height found in a professional team in amateur players, although we screened for tall players. In addition, we observed the trunk segment in only the sagittal plane; the coronal and transverse plane were not included. The asymmetric rotational and bending strain in the upper extremity are important factors that lead to deformation of the body posture. In future studies, the dynamic spine morphometry should be widely employed in body posture evaluation. A comprehensive study of spinal movements can provide more visual data to help understand how the spine changes during movements. It will provide a reference for coaches to guide training and prevent spinal injury.

Conclusion
In summary, after comparing dynamic testing for spine curvature and the shoulder between long training volleyball players and short training players based on volleyball movements, which include the overland jump back set, deep bump set, and spike, long training volleyball players had a larger thoracic kyphosis angle than short training players. This may have resulted from an intensive-oriented physical activity over the longer practice period. However, there was a smaller shoulder inclination in the long trainers compared with the short trainers. This may be due to trunk muscle instability that occurred in the short training group, leading to a larger imbalance of the shoulder, from the left to the right. No difference was found in the lumbar lordosis angle between the two groups. This study suggests that athletes should develop an appropriate dynamic trunk stabilization to reduce posture imbalance during volleyball activities.