Effects of smartphone use in various floor sitting positions on spinal and pelvic alignment and gluteal pressure in healthy adults

Abstract

[Purpose] This study investigated the effects of prolonged sitting and smartphone use on the craniocervical angle (CCA), trunk flexion angle (TFA), pelvic obliquity, and gluteal pressure asymmetry in healthy adults. [Participants and Methods] Thirty healthy participants used smartphones for 30 min in three sitting positions: cross-legged, side sitting, and long sitting. Cervical and lumbar angles, pelvic obliquity, and gluteal pressure asymmetry were measured at the start of upright sitting, 30 s, 10 min, 20 min, and 30 min. Visual 3D software was used for data collection and analysis. [Results] CCA and TFA progressively decreased in all three positions. Significant CCA reductions were observed at 30 min in both cross-legged and side sitting positions, and as early as 30 s in long sitting. TFA decreased significantly at all measured times in side sitting, and at most intervals in cross-legged sitting, but not in long sitting. Pelvic obliquity increased significantly in both cross-legged and side sitting, while remaining unchanged in long sitting. Gluteal pressure asymmetry significantly increased at 30 s in cross-legged sitting and at all intervals in side sitting, with no significant changes in long sitting. [Conclusion] If prolonged floor sitting is unavoidable, adopting a symmetrical long sitting posture is preferable.

INTRODUCTION

Floor sitting postures—such as cross-legged, seiza, and side-sitting—are common in Asian and Middle Eastern cultures¹⁾ and are also practiced in Western cultures during activities such as yoga, meditation, and certain social settings. In modern society, smartphones have rapidly become essential tools for activities such as music listening, internet browsing, video streaming, and photography²⁾. Smartphone use involves the head, neck, shoulders, wrists, hands, and trunk²⁾. Eitivipart et al. noted that neck flexion angles were greater when individuals used smartphones while sitting as opposed to standing³⁾. Poor sitting habits, such as looking at screens, slouching, crossing legs while seated, leaning forward even slightly, and extending to reach a mouse or a telephone, contribute to movement-related disorders, impair lumbar spine control, and lead to low back pain (LBP)^4,5,6).

Cross-legged sitting reduces muscle fatigue by decreasing activity in the external and internal obliques^{7, 8)}. Additionally, it contributes to joint stability by compressing the sacroiliac joints⁷⁾. However, sitting cross-legged in a slumped position places more stress on the intervertebral discs and spine, increasing disc pressure and exacerbating chronic LBP⁹⁾. Lee et al.¹⁰⁾ reported that during continuous cross-legged sitting, the craniocervical angle (CCA) increased, and the trunk flexion angle (TFA) decreased. Studies have examined smartphone use-related spinal and pelvic changes^{2, 11,12,13,14)}, and the effects of sitting cross-legged on the CCA, TFA, pelvic obliquity, and gluteal pressures^{10, 15,16,17)}, but systematic comparisons of the effects of various floor sitting postures are absent.

We hypothesized that prolonged smartphone use in various floor sitting positions would result in significant changes in spinal and pelvic alignment, with side sitting showing the greatest gluteal pressure asymmetry.

Therefore, this study investigated changes in CCA, TFA, pelvic obliquity, and gluteal pressure asymmetry in healthy adults while sitting and using a smartphone in various floor sitting positions for 30 min.

PARTICIPANTS AND METHODS

A total of 30 healthy adults (15 males, 15 females) participated in this study and were assigned to three groups: cross-legged (n=10; 7 males, 3 females), side sitting (n=10; 2 males, 8 females), and long sitting (n=10; 6 males, 4 females). The mean age was 24.8 ± 2.05 years, with no significant differences among groups (p=0.645). Average height and weight were 168.9 ± 9.04 cm and 64.5 ± 14.67 kg, respectively, with no significant group differences (height: p=0.531; weight: p=0.072). The inclusion criteria included no pain during 30 min of sitting and smartphone use for >1 hour daily for at least 1 year. The exclusion criteria were a body mass index >30 kg/m², musculoskeletal pain, recent back or pelvic treatments, and prior surgery. Standardized smartphone activities were performed during the 30-min study, with data collected by blinded observers. Ethical approval was received from the Dankook University IRB (approval DKU-2023-12-010-004), and informed consent was obtained. Power analysis (effect size: 0.25; power: 0.8; α=0.05) based on a pilot study confirmed the need for 30 participants.

CCA, TFA, and pelvic obliquity were assessed using a motion analysis system with six infrared cameras (Qualisys AB, Göteborg, Sweden) at a sampling frequency of 100 Hz. Reflective markers were placed by the same experienced researcher to ensure consistency as follows: CCA (tragus-OC and tragus-C7 lines), TFA (acromion-L1 and L1-greater trochanter lines), and pelvic obliquity (ASIS and PSIS). A pilot study confirmed acceptable marker displacement (<5 mm). Kinematic data were processed in Visual 3D (C-Motion Inc., Germantown, MD, USA), filtered with a Butterworth low-pass filter (6 Hz), and analyzed using Excel and GraphPad Prism 10.0. Positive values indicated increases in CCA, TFA, and pelvic tilt.

Gluteal pressure data were collected using a force plate (Advanced Mechanical Technology, Inc., Watertown, MA, USA) at 100 Hz. Participants sat centered on the force plates, divided into left and right regions, and the pressure was calculated. Gluteal pressure asymmetry was defined as the difference between the higher- and lower-pressure sides.

Participants performed a standardized activity (the most frequent daily task from the questionnaire) while seated in the long sitting, cross-legged, or side sitting position for 30 min, holding a smartphone with both hands in their line of sight. Reflective markers were placed on key anatomical landmarks. Measurements were taken at upright sitting, 30 s, 10 min, 20 min, and 30 min and analyzed with Visual 3D software.

Statistical analysis was performed using SPSS Statistics 29.0. The Shapiro–Wilk test assessed normality. MANOVA was conducted for continuous variables, while the Fisher–Freeman–Halton test evaluated categorical variables. Two-way mixed ANOVA examined the effects of time, group, and their interaction on CCA, TFA, pelvic obliquity, and gluteal pressure asymmetry. Significant interactions were analyzed with two-way ANOVA and Bonferroni corrections for pairwise comparisons. Statistical significance was set at p<0.05.

RESULTS

No significant differences among the three groups were found in baseline characteristics (p>0.05). CCA showed a significant main effect of time (p<0.001), with decreases in cross-legged sitting from 175.3° to 164.9° and side sitting from 180.7° to 171.1° over 30 min. The long sitting group showed a decrease only at 30 s (184.6° to 178.5°), with no significant group or interaction effects (Table 1). TFA results exhibited a significant main effect of time (p<0.001), with decreases in cross-legged sitting from 103.2° to 88.9° and in side sitting from 99.9° to 90.2°, while long sitting showed a minimal change from 94.6° to 90.0°. A significant group-by-time interaction (p=0.006) indicated greater TFA reduction in cross-legged sitting at 30 min compared to long sitting (Table 1). Pelvic obliquity showed significant effects of time (p<0.001), group (p<0.001), and group-by-time interaction (p<0.001). Angles increased in cross-legged sitting from 0.4° to 3.6° and side sitting from 0.4° to 4.5°, with no notable changes in long sitting (from 0.5° to 1.0°). Pelvic obliquity was highest in side sitting, with cross-legged and side sitting showing greater values at 30 min than upright and long sitting (p<0.05, Table 1). Gluteal pressure asymmetry increased significantly over time (p<0.001), with values rising from 31.9 to 72.6 in cross-legged sitting and from 44.0 to 180.5 in side sitting, while long sitting showed minimal changes (35.3 to 46.9). Cross-legged sitting showed a significant increase at 30 min compared to upright sitting (p<0.05), and side sitting exhibited significant increases at all time points (p<0.05). Significant group (p<0.001) and interaction (p<0.001) effects were observed, with side sitting exhibiting higher gluteal pressure asymmetry than cross-legged and long sitting at all time points (p<0.05, Table 2).

Table 1. Comparison of craniocervical angle, trunk flexion angle, and pelvic obliquity during smartphone use in three floor sitting positions (n=10 for each sitting position).

Variables	CS (CCA)	SS (CCA)	LS (CCA)	CS (TFA)	SS (TFA)	LS (TFA)	CS (PO)	SS (PO)	LS (PO)
Upright	175.3 ± 2.5	180.7 ± 2.5	184.6 ± 2.5	103.2 ± 2.4	99.9 ± 2.4	94.6 ± 2.4	0.4 ± 0.1	0.4 ± 0.1	0.5 ± 0.1
30 s	171.6 ± 2.9	177.0 ± 2.9	178.5 ± 2.9	101.5 ± 2.7	96.3 ± 2.7	92.1 ± 2.7	2.1 ± 0.3	4.5 ± 0.3	0.8 ± 0.3
10 min	166.8 ± 3.7	175.1 ± 3.7	176.6 ± 3.7	92.1 ± 2.9	92.3 ± 2.9	91.1 ± 2.9	2.5 ± 0.2	4.6 ± 0.2	0.8 ± 0.2
20 min	169.6 ± 4.2	174.1 ± 4.2	178.8 ± 4.2	91.0 ± 3.2	91.0 ± 3.2	89.4 ± 3.2	3.3 ± 0.3	4.8 ± 0.3	0.9 ± 0.3
30 min	164.9 ± 3.7	171.1 ± 3.5	176.8 ± 3.7	88.9 ± 2.7	90.2 ± 2.7	90.0 ± 2.7	3.6 ± 0.3	4.5 ± 0.3	1.0 ± 0.3
Upright vs. 30 s	3.7 ± 1.7	3.6 ± 1.7	6.1 ± 1.7*	1.6 ± 0.9	3.6 ± 0.9*	2.5 ± 0.9	1.6 ± 0.9	3.6 ± 0.9*	2.5 ± 0.9
Upright vs. 10 min	8.5 ± 3.0	5.5 ± 3.06	8.0 ± 3.06	11.0 ± 2.0*	7.6 ± 2.0*	3.5 ± 2.0	11.0 ± 2.0*	7.6 ± 2.0*	3.5 ± 2.0
Upright vs. 20 min	5.7 ± 3.0	6.6 ± 3.01	5.8 ± 3.01	12.2 ± 2.2*	8.8 ± 2.2*	5.1 ± 2.2	12.2 ± 2.2*	8.8 ± 2.2*	5.1 ± 2.2
Upright vs. 30 min	10.4 ± 2.6*	9.5 ± 2.6*	7.7 ± 2.6	14.3 ± 1.8*	9.7 ± 1.8*	4.5 ± 1.8	14.3 ± 1.8*	9.7 ± 1.8*	4.5 ± 1.8

*p<0.05.

CS: cross-legged sitting; SS: side sitting; LS: long sitting; CCA: craniocervical angle; TFA: trunk flexion angle; PO: pelvic obliquity.

Table 2. Comparison of gluteal pressure asymmetry during smartphone use in three floor sitting positions.

Variables	CL sitting (n=10)	Side sitting (n=10)	Long sitting (n=10)
Upright	31.9 ± 6.5	44.0 ± 6.5	35.3 ± 6.5
30 s	45.4 ± 12.1*	168.7 ± 12.1*	40.7 ± 12.1
10 min	53.5 ± 12.9*	178.4 ± 12.9*	46.3 ± 12.9
20 min	61.4 ± 13.3*	182.4 ± 13.3*	46.9 ± 13.3
30 min	72.6 ± 14.5*	180.5 ± 14.5*	41.6 ± 14.5
Upright vs. 30 s	13.5 ± 10.5	124.7 ± 10.5*	−5.3 ± 10.5
Upright vs. 10 min	21.6 ± 10.79	134.4 ± 10.79*	10.9 ± 10.79
Upright vs. 20 min	29.5 ± 12.36	138.4 ± 12.36*	11.6 ± 12.36
Upright vs. 30 min	40.6 ± 12.48*	136.4 ± 12.48*	−6.2 ± 12.48
CL sitting vs. Side sittinga		*
CL sitting vs. Long sittinga
Side sitting vs. Long sittinga		*

Unit: Pascals; CL sitting: cross-legged sitting, ^a: difference between the two postures in the post hoc test, Mean ± SD, *p<0.05.

DISCUSSION

This study investigated the effects of prolonged smartphone use in various floor sitting positions on spinal and pelvic alignment, finding significant changes, with side sitting showing the greatest gluteal pressure asymmetry, thereby supporting the hypothesis. Our results showed a significant decrease in the CCA, particularly in cross-legged and side-sitting positions, indicating increased cervical flexion and forward head posture. This posture increases stress on the cervical spine, as forward head flexion dramatically raises the load on spinal structures. Loss of the natural cervical curve can lead to early wear, degeneration, and the potential need for surgery¹⁸⁾. Prolonged smartphone use with forward head posture has been linked to “text neck syndrome”, causing neck and upper body pain due to strain on the cervical spine and muscular imbalances¹⁹⁾. Although our study did not assess pain, these postural deviations may increase the risk of neck discomfort over time. CCA, a key indicator of head and neck posture, progressively decreased in floor sitting postures, particularly in cross-legged and side sitting at 30 min, suggesting an increased risk of forward head posture. Prolonged smartphone use is associated with neck and shoulder pain due to increased neck flexion, head tilt, and muscle activity²⁰⁾. Unlike previous studies reporting increased CCA during cross-legged seated visual display terminal work¹⁰⁾, our results showed reductions, possibly due to differences in the viewing angle. Forward head posture, linked to increased muscle tension and cervical spine compression, aligns with the greater thoracic and neck flexion observed during slumped sitting^21,22,23). In this study, no significant gender differences were found in the measured variables, likely due to the standardized instructions provided to ensure consistent posture across participants. Gender differences in posture, with men often adopting more relaxed postures, may contribute to variations in neck flexion during prolonged sitting²⁴⁾. The absence of significant CCA reduction in long sitting could be attributed to posterior pelvic tilt and lumbar flexion, which reduce hamstring tension^{23, 25)}. Trunk flexion is linked to back pain risk through decreased multifidus activity, thoracic erector spinae relaxation, and increased intradiscal pressure²⁶⁾. In this study, trunk flexion decreased from upright to floor sitting, with significant reductions in cross-legged and side sitting. Unsupported postures, which require greater trunk muscle activity, explain the TFA decrease. Prolonged sitting is associated with LBP due to reduced lumbar lordosis and increased flexion^{27, 28)}, and unsupported floor sitting is not recommended for smartphone use due to an increased risk of neck pain and LBP.

Pelvic obliquity and gluteal pressure increased in cross-legged and side sitting. In cross-legged sitting, leg flexion and adduction lead to greater tilt. Side sitting may compromise spinal stability due to muscle activity imbalances, increasing LBP risk^{29, 30)}. Thus, a symmetrical long sitting posture is recommended for extended floor sitting.

In daily life, individuals unconsciously shift posture during prolonged smartphone use to relieve discomfort or meet task demands³¹⁾. Such use leads to gradual postural deviations, including increased trunk flexion, cervical bending, and asymmetrical weight distribution³⁾. Our findings mirror these patterns, with progressive CCA and TFA decreases—particularly in cross-legged and side sitting—potentially promoting forward head posture and musculoskeletal strain.

Limitations of this study include the effect of unsupported seating on tilt angles and stability. Future research should incorporate additional sitting positions and compare different supports.

In conclusion, significant decreases in CCA and TFA were found in all sitting positions, with the smallest changes in pelvic obliquity and gluteal pressure asymmetry in long sitting. It is important to address the risks of poor sitting postures. If prolonged floor sitting is necessary, a more symmetrical long sitting position is recommended.

Funding

This research received no external funding.

Conflict of interest

The author declares no conflicts of interest.