Background: Widespread smartphone utilization often leads to poor postural adaptations that alter musculoskeletal alignment, thereby adversely affecting muscle activity, respiratory function, and pain thresholds. Objectives: This study investigated the effects of desk-to-trunk distance on cervical muscle activity, respiratory function, and pressure pain threshold (PPT) during smartphone use in healthy young adults. Design: Randomized crossover trial. Methods: Twenty young adults who use smartphones for an average of four hours or more per day performed a 20-minute smartphone task under three distance conditions: 0, 15, and 30 cm. Muscle activity of the splenius capitis (SC) and upper trapezius (UT), respiratory function and PPT were assessed pre- and post-intervention. Results: Right UT muscle activity was significantly higher at 30 cm compared to 0 cm (P=.01). Significant decreases were observed in forced vital capacity (P=0.04) and slow vital capacity (P=.03) at 15 cm, and in maximum voluntary ventilation at 30 cm (P=.02) between the pre- and post-tests. The reduction in bilateral SC PPT was significantly more pronounced at 30 cm than at 0 cm (P=.01). Notably, significant PPT declines were observed across all conditions (right UT [P=.03], left SC [P=.01] at 0 cm; right UT [P=.03] at 15 cm; left SC, right SC, left UT, and right UT [P<.01] at 30 cm). Conclusion: Increased desk-to-trunk distance exacerbates forward head posture, reducing respiratory efficiency and increasing pain sensitivity, making minimal proximity (0 cm) an effective ergonomic strategy.

INTRODUCTION
SUBJECTS AND METHODS
    Subjects
    Outcome Measures
    Interventions
    Experimental Procedures
    Data and Statistical Analysis
RESULTS
    Muscle activity
    Respiratory function
    PPT
DISCUSSION
CONCLUSION
ACKNOWLEGEMENTS
FUNDING
CONFLICTS OF INTEREST
AUTHOR CONTRIBUTIONS
REFERENCES

• Kyungtae Yoo(Department of Physical Therapy, Graduate school of Namseoul University, Cheonan, Republic of Korea) Corresponding author