Smartphones have become a part of daily life. They evolved from being a tool for making phone calls, text messages, and enjoying the internet to a tool for building a new environment [1]. Despite this, the continuous use of smartphones in a sitting position can cause changes in posture, such as the cervical flexion angle and cervical pain [2], and cause inappropriate postures, such as slouched posture or forward head posture (FHP) [3]. Moreover, using a video terminal such as a smartphone for a long time ultimately causes thoracic malalignment due to the rounded shoulder posture or FHP, which leads to a decrease in the function of the respiratory system [4]. A loss of lung function can occur if it continues [5].

Modern people mainly work or live while sitting on a chair. The sitting position places three times more stress on the lower back than the standing position and seven times more than the lying position [6-8]. Most people tend to sit in a bent posture for a long time, which is often accompanied by an altered posture of the spine and pelvis [9]. Although many people maintain a sitting position for a long time throughout the day, they can often find themselves sitting in a cross-legged posture, with one leg placed over the knee of the other leg. Most people adopt the crossed leg posture because it is comfortable or habitual, and it compensates for the height difference between both pelvises [10,11]. This results in changes in muscle tension and range of motion (ROM) [12].

Human posture is determined by muscle coordination, proprioception, balance, joint position, and joint function. Movement is reduced when one posture is maintained for a long time, which reduces proprioceptive information input to the central nervous system through proprioceptors distributed within joints and muscles. In addition, it can cause muscle imbalance and affect muscle fatigue [13-15]. Muscle movements caused by muscle coordination begin with electrical activity; the resulting signals are recorded using electromyography (EMG) [16]. Recording muscle electrical activity is used widely in studies of muscle coordination, muscle contraction characteristics, motor unit recruitment, and muscle firing [17]. In addition, the respiratory function is related to gas metabolism, but it can also cause other physical problems, such as poor posture and body balance [18].

Previous studies have reported that smartphone use affects various muscles, and their use in a sitting position affects the cervical erector muscles and upper trapezius muscles [19]. A previous study reported changes in neck and torso posture when standing and sitting while using a smartphone. They found that the neck and torso angles in the sitting posture changed more significantly than in the standing posture [20]. Pelvic deformation and pelvic tilt caused by asymmetrical postures, such as crossedlegged posture, can cause permanent spinal deformity and chronic low back pain [21]. The habitual use of the crossed-leg posture for a long time can cause kyphosis or scoliosis, a musculoskeletal disease that occurs in the spine [22]. The thoracic malalignment causes contraction and expansion of the lungs during breathing and a decrease in overall pulmonary function [23]. A study measuring the respiratory function after using a smartphone in a sitting position for one hour reported that the forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) decreased compared to the control group that spent free time [24].

Despite the many studies on inappropriate posture with smartphone use, most studies on the crossed-legged posture focused on the trunk muscles, and the smartphone use posture focused on the upper body muscles. Few studies have examined the effects of leg muscle activity and respiratory function on the changes in other body parts depending on the sitting posture or the type of sitting posture when using a smartphone. Therefore, this study analyzed the muscle activity of the trunk and leg muscles and respiratory function in young adults according to the type of crossed-legged posture when using a smartphone. This study proposed the following hypotheses. First, the muscle activity in a crossed-leg sitting posture or tailored crossed-leg sitting posture would be significantly higher than in an upright sitting posture. Second, the muscle activity increases significantly before and after each of the three sitting postures. Third, the respiratory function in the crossed-leg sitting posture or tailored crossed-leg sitting posture would be significantly lower than in the upright sitting posture. Fourth, the respiratory function would differ significantly between before and after each of the three sitting postures.

1. Participants

This study involved 30 young adults attending the Department of Physical Therapy at N University in Cheonan. A sample size calculation for a MANOVA, repeated measurements, and the within-between interaction were tested (effect size d = .60, significance level α = .05, power 1-β = .80) using G*Power 3.1.9.7 software (G*power 3.1.9.7 software (Heinrich Heine University Düsseldorf, Germany).

Based on the declaration of Helsinki [25], this study was performed on those who voluntarily signed an informed consent form after fully understanding the purpose and methods of this study. The exclusion criteria were those who had a musculoskeletal disease that affected the experiments, those who had a history of pain or surgery in the joints of both lower extremities, and those who had diseases in the respiratory system [26].

2. Apparatus

A body composition analyzer (InBody720, Biospace, Seoul, Republic of Korea) was used to determine the physical characteristics of the participants. An EMG system (Free EMG 1000, BTS Bioengineering, Milan, Italy) was used to measure muscle activity, and spirometry (Cardio7, BIONET, Seoul, Republic of Korea) was used to measure the pulmonary capacity.

3. Study Procedure

Thirty subjects adopted three sitting postures during smartphone use after measuring their physical characteristics. The experiment was performed in a quiet place. The experimenter and subject were present at the time of the measurement. They performed the crossed-legged posture in the first week, the tailored crossed-legged posture in the second week, and the upright sitting posture in the third week. They measured the muscle activity and respiratory function before and after performing each type of sitting posture. A one-week rest interval was allowed between the three postures [27]. They measured the muscle activity and respiratory function before and after performing each type of sitting posture.

1) Measuring methods

(1) Muscle activity measurement

A Free EMG 1000 was used to measure the muscle activity. The device was a wireless surface EMG, and the data were processed using the EMG Analyzer software on a computer. The EMG signals were recorded at a sampling rate of 1 kHz, bandpass filtered between 20 and 500 Hz [28]. The subject's muscle activity was measured. The subject was instructed to use the smartphone continuously for 15 minutes and concentrate on the smartphone while maintaining each type of sitting posture. The EMG signals were collected before and after the three types of sitting posture for each week. Before the EMG measurement, the subjects wore shorts [29]. The hair in the area was removed to minimize resistance to the EMG signals generated from the skin [30]. Foreign substances were wiped off with an alcohol swab. Medical electrodes (2223H, HUREV Co. Ltd., Republic of Korea) were attached to the muscle area [31]. The electrode attachment sites included the ES [32,33], the external oblique (EO) muscle, internal oblique (IO) muscle [31, 34], rectus femoris muscle [35], vastus lateralis muscle [36], and tensor fascia latae muscle [37-38] (Appendix Ⅰ).

The measured EMG values were normalized using the maximum voluntary isometric contraction (MVIC) value, and the MVIC value was measured using a manual muscle test [39]. When measuring, the value was repeated three times for five seconds in the position for each muscle, and the five seconds EMG data were processed as the root mean square (RMS). After processing the RMS with the middle three seconds excluding one second each at the first and last, the average EMG signal in the middle three seconds was obtained and defined as 100%MVIC [40].

The muscle activity for each type of sitting posture was measured while in the sitting position before and after the sitting postures. For the muscle activity measurements, the participants adopted a sitting position on a chair with a backrest with the hip, knee, and ankle joints flexed to 90° without crossing the legs [41]. When measuring the muscle activity, the measurement was conducted for five seconds to obtain the RMS value of the middle three seconds, excluding the first one second and the last one second. The average value was calculated after repeating the measurement three times. After normalizing using each RMS value and MVIC value, the %MVIC value was obtained [42].

(2) Respiratory function measurement

Spirometry was used to measure the respiratory function: FVC, FEV1, FEV1/FVC, and MVV. The subjects were instructed to breathe only through the mouth using a nose clip while sitting comfortably. The test was performed three times according to the American Thoracic Society guidelines; the highest measurement value was used as the result. The measurement method was explained to the subject in detail and demonstrated using the same examiner. The subjects placed the mouthpiece of the spirometer at the mouth [43]. When the examiner informed the subject of the start of the measurement, they took three or more breaths as usual and then inhaled air as quickly and thoroughly as possible. Subsequently, the subjects exhaled as much as they could and then inhaled the air completely again to complete one measurement. Through this, the maximum-effort expiratory spirogram (MES) was measured to calculate the FVC, FEV1, and the FEV1 to FVC ratio (FEV1/FVC) was measured to confirm the degree of pulmonary obstruction [44-45]. For each measurement, an approximately one-minute rest was allowed between the three measurements. If the subject complained of dizziness after the measurement, the measurement was re-measured when the subject was stable [46]. The maximal voluntary ventilation (MVV) was measured as breathing as deeply and quickly as possible for 12 seconds after at least three tidal breaths [47-49].

2) Sitting posture types

The subject maintained sitting postures of three types without leaning on a chair and watching an interesting video while holding the smartphone in their hand without resting their arm on the knee without discomfort [1]. The chair used in this study was 39 cm in height. They used a smartphone with their dominant hand [50]. The subjects were instructed to stop the experiment at any time if pain or unpleasant sensations occurred during the experiment. Before the experiment, the dominant side of the subjects was defined as the leg used to kick a ball. During the first and second weeks of the cross-legged sitting posture, the subjects were asked to maintain the posture by crossing their dominant leg [51].

The crossed-leg posture was performed in the first week. The crossed-leg posture was performed while maintaining a fully crossed-legged sitting position, with the thigh of one leg flexed at the knee joint at 90°, the thigh of the other leg raised, and the thighs touching each other [10] (Fig. 1). The tailored crossed-leg sitting posture was performed in the second week. The tailored crossed-leg sitting posture was performed by placing the ankle on the opposite knee, maintaining the posture of placing the ankle on the knee of one leg with the knee joint flexed at 90°. The upright sitting posture was performed in the third week. The upright sitting posture was achieved by placing both feet on the ground at shoulder width, the torso straight, and the knee joints of both legs flexed approximately 90° [52] (Fig. 2). After measuring each posture during each week, the subjects performed a stretching program provided by the examiner to restore the body function by maintaining the sitting posture [53-55] (Fig. 3).

Fig. 1. Crossed-leg sitting posture.

Fig. 2. Tailored crossed-leg sitting posture.

Fig. 3. Upright sitting posture.

4. Statistical analysis

The data were analyzed using SPSS for Windows version 29.0 (Statistical Package for the Social Sciences, IBM Corp., Armonk, NY, USA). A Kolmogorov–Smirnov test was used to verify normality. Multivariate analysis of variance (MANOVA) was used to compare the muscle activity and respiratory function between the groups according to the posture type and between the times before and after posture. A Scheffe test was used for multiple comparisons when a significant difference was found in MANOVA. The statistical significance level was set at α = .05.

1. General characteristics of subjects

The subjects were 30 young adults (15 females and 15 males). The average age, height, weight, and BMI of the subjects were 19.77 ± 1.70 years, 168.47 ± 8.06 cm, 64.32 ± 12.89 kg, and 22.53 ± 3.30 kg/m², respectively. The dominant sides of the hand and leg were two and 28 on the left and right sides, respectively (Table 1).

Table 1

General characteristics of the subjects (n = 30)

Variables Age (years)	Height (cm)	Weight (kg)	BMI (kg/m²)	Gender (F/M)	Dominant of (Left/Right)
Mean ± SD 19.77 ± 1.70	168.47 ± 8.06	64.32 ± 12.89	22.53 ± 3.30	15/15	2/28

2. Muscle Activity

The interaction effect showed no significant differences. A comparison of the changes in the three groups revealed a significant difference in the external oblique muscle. Multiple comparison analyses of the EO revealed the crossed-leg sitting posture to be significantly higher than the tailored crossed-leg and the upright sitting postures (p<.05). A comparison of the changes before and after the posture showed that the muscle activity of the ES in the cross-legged sitting posture was significantly increased after the posture (p<.05) (Table 2).

Table 2

Change in muscle activity according to the sitting posture types (%MVIC)

Muscle	Group	Pre-test	Post-test		p
EO	Crossed legb,c	6.50 ± 2.25	7.95 ± 3.66	Group	.000*
	Tailor crossed lega	4.96 ± 1.68	5.20 ± 2.97	Time	.167
	Uprighta	4.78 ± 2.75	4.89 ± 3.63	Group*Time	.385
IO	Crossed leg	7.08 ± 5.35	7.40 ± 4.99	Group	.337
	Tailor crossed leg	6.18 ± 4.34	8.03 ± 6.11	Time	.752
	Upright	9.39 ± 9.79	8.22 ± 9.45	Group*Time	.500
ES	Crossed leg	6.61 ± 3.13	6.91 ± 3.43	Group	.719
	Tailor crossed leg	5.14 ± 2.55	7.37 ± 3.70†	Time	.036*
	Upright	6.22 ± 3.86	6.94 ± 3.78	Group*Time	.271
RF	Crossed leg	1.57 ± 1.02	1.88 ± 1.54	Group	.127
	Tailor crossed leg	1.66 ± 1.13	1.31 ± 0.64	Time	.842
	Upright	3.34 ± 8.57	2.96 ± 7.79	Group*Time	.906
VL	Crossed leg	2.73 ± 2.07	3.33 ± 2.32	Group	.387
	Tailor-crossed leg	3.36 ± 3.25	2.59 ± 2.25	Time	.621
	Upright	6.86 ± 25.29	4.38 ± 14.08	Group*Time	.781
TFL	Crossed leg	4.4 ± 3.08	5.19 ± 3.95	Group	.930
	Tailor crossed leg	3.97 ± 3.1	6.03 ± 8.99	Time	.168
	Upright	4.3 ± 6.04	6.39 ± 15.12	Group*Time	.879

^*p<.05: Significant difference between the groups, ^†p<.05: Significant difference within the groups

^a: Crossed-leg sitting, ^b: Tailor-crossed leg sitting, ^c: Upright sitting

FVC: Forced Vital Capacity, FEV1: Forced Expiratory volume in one second, MVV: Maximal voluntary ventilation

3. Respiratory Function

The interaction effect showed no significant differences. A comparison of the changes in the three groups revealed a significant difference in the MVV. Multiple comparison analyses of the MVV showed that the crossed-leg sitting posture resulted in a significantly higher MVV than the tailor crossed-leg and upright sitting postures (p<.05). A comparison of the changes before and after the posture showed no significant difference (Table 3).

Table 3

Change in respiratory function according to the sitting posture types

Variables	Group	Pre-test	Post-test		p
FVC (L)	Crossed leg	3.91 ± 1.28	3.67 ± 1.00	Group	.700
	Tailor crossed leg	3.76 ± 1.13	4.22 ± 1.69	Time	.714
	Upright	3.92 ± 1.23	3.91 ± 1.19	Group*Time	.315
FEV1 (L)	Crossed leg	2.86 ± 1.02	2.89 ± 1.03	Group	.348
	Tailor crossed leg	3.01 ± 1.05	4.95 ± 10.23	Time	.316
	Upright	3.20 ± 0.98	3.16 ± 0.97	Group*Time	.356
FEV1/FVC (%)	Crossed leg	74.44 ± 17.57	77.88 ± 14.08	Group	.069
	Tailor crossed leg	79.06 ± 14.11	75.56 ± 19.68	Time	.965
	Upright	82.54 ± 12.92	82.30 ± 14.44	Group*Time	.448
MVV (L/min)	Crossed legb,c	67.06 ± 33.18	69.71 ± 32.13	Group	.000*
	Tailor crossed lega	85.79 ± 38.61	88.88 ± 38.47	Time	.646
	Uprighta	95.17 ± 41.17	97.33 ± 45.62	Group*Time	.998

^*p<.05: Significant difference between the groups, ^†p<.05: Significant difference within the groups

^a: Crossed-leg sitting, ^b: Tailor-crossed leg sitting, ^c: Upright sitting

EO: External oblique, IO: Internal oblique, ES: Erector spinae, RF: Rectus femoris, VL: Vastus lateralis, TFL: Tensor fascia latae

Once domesticated lifestyle habits become easy and comfortable, they are difficult to change with an individual's insufficient knowledge or attitude, whether good or bad. These incorrect lifestyle postures mean that certain muscles become weak or stretched, and other muscles become tense to compensate. The imbalance between muscles can eventually cause deformation of the musculoskeletal system and permanent damage [56]. This study compared the changes in muscle activity and respiratory function of the trunk and leg muscles according to the type of crossed-leg posture in young adults when using a smartphone. The muscle activity and respiratory function were significantly changed between the three sitting posture types.

A comparison of the change in muscle activity according to the type of posture revealed higher muscle activity of the EO in the crossed-leg sitting posture than in the tailored crossed-leg sitting and upright sitting posture. A previous study reported that various crossed-leg sitting postures (crossed-leg posture, tailored leg posture, and crossed-ankle posture) affect muscle activity in low back pain. The crossed-leg sitting postures showed the same muscle activation pattern as a posture with the trunk slightly rotated. Moreover, performing a crossed-leg position in one direction for a long time or a habitual crossed-leg posture can cause an imbalance in the trunk [57]. A study on the relationship between the muscle activity of the rectus abdominis, EO, and IO using needle EMG showed that the EO muscle acts on the opposite side, while the rectus abdominis and IO act on the same side during trunk rotation [58]. A crossed-leg posture can cause pelvic rotation and problems in the lower back [59]. The lack of the hip flexion range was compensated for by lumbar flexion when crossing the legs, which can increase the lumbar rotation moment [60]. A study on the lower extremity muscle activity according to the three different crossed-leg sitting postures showed that the change in muscle activity of the muscles around the hip joint according to each crossed-leg posture was related to the alignment of the pelvis and knee joint of each posture [41]. In this study, the muscle activity of the EO in the crossed-leg sitting posture was significantly higher than in the other two sitting postures. This was attributed to the crossed-leg posture causing trunk asymmetry and a spinal kyphotic curve at the same time as the trunk rotation posture [41,58-60], which also brought instability to the vertebral joint, with the EO compensating for it [60].

A comparison of the change in muscle activity according to the time revealed the muscle activity of the ES to be significantly increased in the tailored crossed-sitting posture. Previous studies reported that an increase in trunk extension due to the posterior tilt of the pelvis during tailored crossed-leg sitting can act as a factor that changes the muscle length and muscle fatigue, requiring excessive effort and energy from the spinal extensor muscles to maintain the correct sitting posture [61,62]. Previous studies reported that when using a smartphone in a sitting position, the lower back posterior tilt and slouching posture for a long time caused fatigue in the trunk muscles [63,64]. A comparison of the changes in muscle activity of the neck and trunk extensors according to the slouching posture and the upright sitting posture during the writing task, the neck and lumbar extensors were significantly higher in the slouching posture than the upright sitting posture. These results are because a slouched posture causes more joint movement and muscle activity in the cervical, lumbar, and erector spinae muscles [65]. In this study, the muscle activity of the ES was significantly increased in the tailor-crossed leg sitting posture after the posture, but no significant difference was found between EO and IO. This was attributed to the smartphone use posture and the tailor crossed-leg sitting increasing the posterior tilt of the pelvis through hip abduction and external rotation. The lumbar erector spinae muscles compensated for this by making a forward tilt of the pelvis to provide stability to the posture, and the vertebral extensors acted as an agonist work to maintain this posture [61-65].

A comparison of the change in respiratory function according to the group showed that the MVV was significantly lower in the crossed-leg sitting posture than in the tailored crossed-leg and upright sitting postures. A previous study measured the lung capacity by temporarily increasing the curvature angle of the spine in a sitting position in healthy adult men in their 20s. They reported that diaphragm movement was significantly decreased in the four postures. Furthermore, the diaphragm movement decreased significantly as the kyphotic angle was increased, which could adversely affect patients with decreased lung capacity [66]. Stimulation that strengthens the abdominal muscles improves the respiratory function, but weakness or fatigue of the abdominal muscles can reduce the respiratory function [67]. A previous study on the pelvis according to the crossed-leg sitting posture reported that the right pelvis was significantly higher only in the general crossed-leg posture than in the upright sitting posture [52]. Sitting with the crossed leg increases spinal curvature in the thoracic and lumbar spine and posterior tilting in the pelvis compared to a normal sitting posture [68]. The type of body posture affects the respiratory function, muscle tension, and muscle fatigue; these cause malalignment and function limitations [69]. A negative correlation was observed between increased thoracic curvature of the kyphotic vertebrae and the respiratory function [70]. The MVV represents the mechanical factor of breathing and provides respiratory muscles, lung compliance, and airway resistance [47,49,71]. Therefore, the MVV was significantly lower in the crossed-leg posture than in the tailored crossed-leg and upright sitting postures because the crossed-leg posture activated the EO and caused more thoracic kyphotic curvature and pelvic posterior tilting than the other two postures in this study. Hence, the tailored crossed-leg and upright sitting postures influenced the decrease in diaphragm movement [66-70]. In addition, although the EO in the crossed-leg sitting posture was more activated than in the other two postures, smartphone use caused malalignment of the overall spine for compensation, including the cervical and thoracic vertebrae. In addition, smartphone use had a more negative impact on the other causes of reduced respiratory function.

A comparison of the change in muscle activity according to the time showed no significant difference in the three posture types. A previous study on the effects of the cross-legged sitting posture in healthy adults showed that the thoracic wall mobility at rest and the maximum inspiration were significantly lower in a meditation sitting posture than in a correct sitting posture, but the crossed-leg posture for a short time did not affect the lung function. In addition, it was reported that the decreased spinal angle, decreased mobility of the thoracic wall, and imbalanced activation of the EO and IO due to the cross-legged sitting posture would not have had a clinical impact on the lung function of healthy people [72]. A previous study on crossed-leg sitting in healthy adults reported no significant difference in respiratory function between the crossed-leg and upright sitting postures. Moreover, the FVC and FEV1 in a crossed-leg sitting posture were significantly lower than in the normal sitting posture, but no significant difference was observed in the upright sitting posture. This was attributed to the insufficient study conducted to confirm the effect of crossed-leg sitting posture on physiological factors. Therefore, the exact mechanism that reduces respiratory function is unclear, but the decrease in elasticity and increase in stiffness of the spinal structures due to the kyphotic sitting posture helps decrease the respiratory function by limiting the inefficient movement of the trunk and the movement of the diaphragm [10]. A decrease in lung capacity has many causes, such as an abnormal spinal posture and the weakness and imbalance of respiratory muscles caused by this posture [73]. This lifestyle pattern reduces the cardiorespiratory endurance and reduces agility and muscular endurance, which are physical strengths based on the respiratory function, reducing the power to sustain certain physical activities [74]. Therefore, the reason why there was no significant difference in respiratory function before and after the posture in this study was attributed to the sitting posture types causing a posterior tilt of the pelvis [72-74]. On the other hand, these sitting postures with smartphone use did not have sufficient influence to significantly change the factors of respiratory function, such as decreased elasticity of the spinal structures and decreased mobility of the thoracic wall in the healthy young adults who were the subjects of this study.

This study had several limitations. First, the subjects were healthy young adults in their 20s, so it was limited in its applicability to various age groups. Second, this study did not include long-term effects. Third, the muscle activities of the abdominal muscle and lower back muscle on the one side where the legs are crossed were analyzed. Fourth, because the muscle activity was not measured during sitting, it was difficult to find changes in three sitting postures during smartphone use. Fifth, the upper extremity position was not immobilized while using the smartphone. Therefore, more studies will be needed to compare the effects in various age groups. In addition, a study is needed on the muscle activity of the muscles on the uncrossed side or both lower extremity muscles, the muscle activity of the muscles of the upper back, such as the neck and thoracic vertebrae above the lumbar vertebrae, and the muscle activity below the knee. In addition, more studies will be needed on how muscle activity changes in various sitting postures while using a smartphone.

This study analyzed the muscle activity and respiratory function in young adults according to three sitting postures while using a smartphone: crossed-leg sitting posture, tailored crossed-leg sitting posture, and upright sitting posture. The muscle activity of EO was significantly higher in the crossed-leg sitting postures than in the other two sitting posture types. The muscle activity of ES in tailor crossed-sitting posture was increased significantly after the sitting posture. The MVV in the crossed-leg sitting posture was significantly lower than in the other two sitting posture types. No significant difference was observed between the groups. Based on these results, a crossed-leg sitting posture can induce the muscle activity of EO in charge of contralateral rotation by increasing trunk and pelvic rotation. A tailored sitting posture can allow the muscle activity of ES to compensate for pelvic posterior tilting. In addition, a crossed-leg sitting posture can cause a decrease in respiratory function caused by changes in the position of the spine. Both crossed-leg sitting posture types can have adverse effects on the trunk muscle activity and respiratory function. Therefore, maintaining an upright sitting posture during smartphone use is important for proper spinal alignment and preventing decreased respiratory function.