Ⅰ. Introduction

In modern society, due to incorrect posture, 80% of adults experience musculoskeletal disorders. In particular, many symptoms are observed in the shoulder area, such as forward head posture and rounded shoulder posture (Lee HK, 2022; Ahn SJ, Cho EH, and Kim M, 2019).

Rounded shoulders are characterized by anatomical features such as scapular protraction, downward rotation, and anterior tilt, along with increased cervical flexion and thoracic kyphosis (Choi JU, Jeong YS, and Kwong O, 2020). Maintaining such a rounded shoulder posture for an extended period can lead to issues with the jaw joint, neck and back vertebral joints, reduced range of motion in the ribs, muscle imbalance, and pain. Specifically, reduced rib movement and decreased spacing between the ribs during breathing can result in lower changes in chest cavity volume, thereby affecting pulmonary function (Yoon YJ, 2022).

To improve pulmonary function in patients with rounded shoulders, various studies have been conducted on intervention methods such as Kinesio taping, Pilates, and equestrian sports to strengthen the respiratory muscles and deep muscles (Park JH et al., 2023;Jung ES, 2020). Equestrian sports not only strengthen the respiratory and deep muscles but also provide the benefits of aerobic exercise and special effects on posture correction (Kang SR et al., 2013;Kim MJ et al., 2015;Hun LJ, 2013).

Balance is a specific state of the postural system that maintains the vertical orientation of the body through the balance of forces and moments acting on the body (Bober and Zawadzki, 2006). The human body, as a biomechanical system composed of multiple segments, requires appropriate muscle tension to maintain the balance of each segment. The interaction between the moments of forces originating from gravity and those generated by muscle tension keeps the entire body in a state of motion to maintain balance. To maintain balance, the flexibility and physiological curvatures of the spinal column are required, which allow the line of gravity to fall within the base of the support entire structure (Zagrobelny and Woźniewski 1999).

In equestrian activities, the center of gravity (COG) is located at a different position, shifted forward to approximately 10 cm anterior to the 6th thoracic vertebra due to the position of the hands holding the reins (Swift et al., 2004). Factors such as the rider's posture, movements of the upper and lower body, and the instability of the support surface can exacerbate the multi-planar and oscillatory movements of the pelvis, which are destabilizing factors (Mrozkowiak M. and Ambroży D., 2014). Both slow and fast speeds can also act as destabilizing factors for posture. Under dynamic conditions, postural control relies on the regulation of head position in space, and the movements of the trunk and head become important elements in maintaining balance (Mrozkowiak M. and Ambroży D., 2014).

There are studies related to equestrian sports focusing on deep muscles, postural alignment, and abdominal pressure (Kim DH and Lim JM, 2020), but these studies have typically targeted healthy individuals, and there is a lack of research related to pulmonary function. Therefore, this study aimed to investigate the effects of equestrian exercise speed on the Pectoralis Minor Index (PMI), abdominal pressure, and pulmonary function in participants with rounded shoulders.

Ⅱ. Methods

1. Subjects

The A University located in Gyeonggi-do province posted a recruitment notice for participants in the current study, and 27 individuals volunteered. The 27 participants were 20-year-old adults currently enrolled at A University. To confirm whether the participants met the selection criteria of having rounded shoulder posture, the researchers referred to a previous study and conducted an assessment. Participants were evaluated as having rounded shoulder posture if the height from the base to the acromion in the supine position was 2.5 cm or more (Seo TH, Kim MS, and Jung YW, 2019). Considering the gender ratio and weight, the researchers used stratified random assignment to randomly allocate the participants into three groups: low-speed, mediumspeed, and high-speed, with 9 individuals in each group.

The exclusion criteria were as follows: First, individuals who had previously engaged in equestrianrelated exercises were excluded, as their ability to control the movements of their upper limbs, lower limbs, trunk, and head in order to maintain balance may have differed from that of novice participants (Mrozkowiak M. and Ambroży D., 2014). Additionally, individuals with cardiopulmonary diseases, limb pain, or mental disorders who would be unable to perform the study normally were also excluded.

The participants of this study were informed in detail about the research in advance, and only those who consented were included. Furthermore, the study was conducted at University A, with a health clinic available in case of any injuries.

2. Measurements

The sample size calculation for the subjects was performed using the G*power program (G*power ver. 3.1.9.2, University of Kiel, Germany). Since there were no prior studies with the same research design as this experiment, a moderate effect size was used to determine the appropriate sample size (Faul F et al., 2007). The sample size calculation was performed by selecting F test's ANOVA: repeated measures, within-between interaction, and inputting an effect size of 0.81, a significance level of 0.05, a power of 0.8, 3 groups, and 2 measurements into the G*power program. As a result, 9 participants per group were calculated, totaling 27 participants.

After screening, the 27 selected participants were divided into three groups according to the intervention method: a low-speed training group with 9 participants, a medium-speed training group with 9 participants, and a high-speed training group with 9 participants. Each training group used an equestrian apparatus (Panasonic EU-6441, Matsushita Electric Works, Ltd., Osaka, Japan) and conducted exercises for 7 minutes on each course (flat ground, uphill, downhill), with a 7-minute rest period between courses, totaling 21 minutes of exercise. To eliminate the influence of vision, sleep masks were worn. The evaluation items included measurements of body composition, PMI (Pectoralis Minor Index), abdominal pressure, and pulmonary function (Figure 1).

1) General Characteristics

The general characteristics of the participants were measured using an automatic height meter (BSM330, South Korea) and a body fat analyzer (InBody720, South Korea) to determine height, weight, and BMI. Additionally, In this study, the shoulder height measurement method used by Seo TH et al. (2019) was also employed to assess rounded shoulders (Seo TH, Kim MS, and Jung YW, 2019). A rod was used to measure the height from the base surface to the acromion (Table 1) (Figure 2).

2) PMI (Pectoralis Minor Index; PMI) Changes

Following the method presented in previous studies, participants sat comfortably on a chair with their hands resting naturally at their sides. The examiner placed stickers on the junction of the 4th rib and the sternum as well as on the coracoid process, then measured the distance between them using a tape measure. The PMI was then calculated and standardized using the formula (ICC = 0.96) (Borstad J, 2008): PMI = (Length of the Pectoralis Minor / Height of the Participant) x 100 (Figure 3).

3) Abdominal Pressure

The pressure of the stabilizer was increased to 40 mmHg, and then a pressure cuff was placed under the second lumbar vertebra of the subject. Starting in a supine position, on the start signal, subjects were instructed to pull their navel towards the floor, applying pressure to the cuff solely with abdominal strength. To prevent compensatory actions, subjects were cautioned to relax the strength in their heels and avoid posterior pelvic tilt. Afterward, the examiner measured and recorded the pressure in the stabilizer 10 seconds later (Figure 4).

4) Pulmonary Function

Pulmonary function was assessed using a Portable Spirometer (AVAD9, IEMBIO Inc., Korea) for the measurement of FVC, FEV1 and PEF. After blocking the nose with a nose clip, the participant, sitting, bites the mouthpiece and, following the examiner's "start" command, begins breathing normally at least three times. Then, irrespective of speed, the participant is instructed to inhale as much air as possible. On the examiner's "exhale" command, the participant exhales as quickly as possible and continues for at least 6 seconds or until it becomes difficult to exhale further (Figure 5).

3. Data analysis

This experiment utilized SPSS version 22.0. To compare pre- and post-experiment within groups, a paired t-test was conducted. For comparisons between groups, analysis of covariance (ANCOVA) was performed, with the exogenous variable (shoulder height) set as the covariate followed by Bonferroni post-hoc test.

Ⅲ. Results

1. Within Groups Pre- and Post-Exercise comparisons

Significant differences were observed only in the PMI value in the low-speed group. In the mediumspeed group, significant differences were observed in all variables except FVC after exercise. In the highspeed group, significant differences were observed in PMI and abdominal pressure (Table 2).

1) PMI

For PMI, there was an increase in all exercise groups: from 7.95±0.77 to 8.44±1.09 in the lowspeed group, from 8.36±0.77 to 8.99±0.89 in the medium-speed group, and from 8.36±0.77 to 8.89±0.76 in the high-speed group, with these increases being statistically significant (p<0.05).

2) Abdominal Pressure

For abdominal pressure, significant differences were observed in the medium and high-speed exercise groups, with values increasing from 78.00±35.74 to 96.66±53.22 and from 60.88±18.89 to 80.66±24.65, respectively (p<0.05). In the low-speed exercise group, although there was an increase in values, the difference was not statistically significant (p>0.05).

3) Pulmonary Function

Pulmonary function can be divided into FVC, FEV1, and PEF. Firstly, FVC did not show a significant difference in the low-speed, medium-speed, and high-speed exercise groups (p>0.05). For FEV1 and PEF, significant differences were observed in the medium-speed exercise group, with values increasing from 1.59±0.77 to 2.00±0.89 and from 2.60±1.82 to 3.13±2.01 respectively (p<0.05).

2. Between Groups Comparisons

The results of the between-group comparison showed that both PMI and abdominal pressure did not exhibit significant differences among the low-speed, medium-speed, and high-speed exercise groups (p>0.05). This indicates that the PMI and abdominal pressure indicators changed similarly across all speed exercise groups (Table 3). When comparing the pulmonary function measurement values of FVC, FEV1, and PEF, FVC did not show significant differences in any of the exercise groups (p>0.05). However, FEV1 and PEF showed significant improvements in the medium-speed exercise group (p<0.05). This suggests that medium-speed exercise may have a positive effect on certain pulmonary function measurements (Table 3).

1) PMI

In the comparison between groups, PMI did not show significant differences among the low-speed, medium-speed, and high-speed exercise groups (p>0.05).

2) Abdominal Pressure

In the comparison between groups, abdominal pressure did not show significant differences among the low-speed, medium-speed, and high-speed exercise groups (p>0.05).

3) Pulmonary Function

In the comparison of pulmonary function between groups, which includes FVC, FEV1, and PEF: FVC did not show significant differences among the low-speed, medium-speed, and high-speed exercise groups (p>0.05). FEV1 showed significant improvements in the medium-speed group (p<0.05). PEF also showed significant improvements in the medium-speed group (p<0.05).

Ⅳ. Discussion

This study classified 27 adults in their 20s with rounded shoulders into three groups based on speed: low speed, medium speed, and high speed. After conducting Equestrian Equipment training, we measured PMI, intra-abdominal pressure, and pulmonary function, and compared the differences within and between groups. As a result of the study, the changes in PMI within the groups were statistically significant (p<0.05). In the low-speed training group, PMI increased from 13.35±1.75cm to 14.28±2.20cm; in the medium-speed training group, it increased from 14.11±1.91cm to 15.06±1.95cm; and in the high-speed training group, it increased from 14.25±1.21cm to 15.16±1.49cm. Horseback riding exercises stimulate the posture reflex muscles of the rider through the three-dimensional movements of the horse (left-right, front-back, up-down), enhancing spinal tension (Janura M et al., 2009;Ha DO, 2011). Horseback riding improves body alignment and reduces unnecessary movements while expanding the range of motion required for riding activities (Lee SL, 2008).

Additionally, research by Hun LJ (2013) on the effects of mechanical horse riding speed on spinal alignment in young adults found that rounded shoulder posture and medial rotation of the scapula lead to an increase in kyphosis, which in turn naturally increases the tendency to protrude the neck forward. After performing horseback riding exercises, an improvement in forward head posture was observed. Therefore, it is believed that controlling the forward head posture caused by rounded shoulders led to a decrease in kyphosis and an increase in PMI.

In terms of abdominal pressure, the low-speed training group showed an increase from 68.66±34.69 mmHg to 80.00±41.82 mmHg, the medium-speed training group from 78.00±35.74 mmHg to 96.66±53.22 mmHg, and the high-speed training group from 60.88±18.89 mmHg to 80.66±24.65 mmHg (p<0.05). These results suggest that as the horse's gait speed increases, the rider's pelvic movement speed also increases. Consequently, the activity of the superficial trunk muscles, such as the erector spinae and rectus abdominis, increases to control spinal movement (Van der Hulst M et al., 2010), leading to an increase in abdominal pressure. This finding is consistent with the study by Lee GW et al. (2014), which indicated that higher abdominal muscle activity results in increased abdominal pressure.

Regarding pulmonary function, in the mediumspeed training group, the FEV1 and PEF increased from 1.59±0.77 and 2.60±1.82 to 2.00±0.89 and 3.13±2.01, respectively (p<0.05). The change in the PMI among groups showed no significant difference, with the low-speed training group at 0.48±0.56, the medium-speed training group at 0.62±0.39, and the high-speed training group at 0.53±0.51 (p>0.05). The change in abdominal pressure was also not significantly different, with the low-speed training group at 11.33±20.00, the medium-speed training group at 18.66±22.24, and the high-speed training group at 19.77±22.57 (p>0.05). Pulmonary function showed significant differences among groups in two variables: FEV1 and PEF (p<0.05). There was no significant difference between the low-speed and high-speed groups, but in the medium-speed group, the FEV1 increased by 0.41±0.48 and PEF by 0.52±0.51, showing significant differences.

Abdominal pressure was effective in both the medium-speed and high-speed training groups, while pulmonary function showed significant improvement in the medium-speed training group. Bae W et al. (2020) concluded that abdominal muscle exercises can secure space for breathing through posture control along with an increase in respiratory function. Additionally, the study by Park SJ et al. (2017) indicated that abdominal strengthening exercises increase the activation of abdominal muscles during exhalation, especially enhancing the ability to expel residual air from the lungs, suggesting that the increase in abdominal pressure after horseback riding exercises is related to the improvement in pulmonary function. On the other hand, Lee HG et al. (2022) found that there was no statistically significant difference in vital capacity with the improvement of rounded shoulder posture, indicating that the rounded shoulder posture does not affect pulmonary function.

Based on previous studies, it appeared that an increase in abdominal pressure was associated with an improvement in pulmonary function, and The increase in pectoralis minor length was shown regardless of speed. Therefore, medium-speed training was considered to be helpful in simultaneously improving pulmonary function and correcting rounded shoulder posture. Through this study, it was determined that horseback riding simulator exercises increased rounded shoulder posture and abdominal pressure, and that the speed of exercise needs to be considered. This can also be applied and utilized in the field of sports physical therapy.

However, this study faced limitations in generalizing its results due to reasons such as the number of participants and the intervention period. Therefore, future research should be conducted based on these results, with a sufficient intervention period to enable the generalization of the findings.

Ⅴ. Conclusion

This study suggests that horseback riding simulator exercises can significantly improve the posture and pulmonary function of individuals with rounded shoulders. The results indicated improvements in PMI across all speeds (low, medium, high) of horseback riding exercises, an increase in abdominal pressure during medium and high-speed exercises, and enhanced pulmonary functions such as FEV1 and PEF specifically with medium-speed exercises compared to low or high speeds. These findings imply the importance of speed regulation in horseback riding exercises, highlighting their clinical relevance for application in sports training scenarios.