Gender-specific biomechanical effects of smartphone use with backpack load during standing and walking.

Smartphone use and backpack carrying are common among young adults, yet their combined effects on neck and shoulder biomechanics remain unclear. This study systematically examined the three-way interaction between gender, smartphone use, and backpack load under static and dynamic conditions. Thirty university students (15 males, 15 females) completed a repeated-measures protocol involving three backpack loads (0%, 5%, and 10% of body weight [BW]) and two postures (standing, walking), with and without smartphone use. Head-neck posture and muscle activity of the cervical erector spinae (CES) and upper trapezius (UTZ) were measured using motion capture and surface electromyography. Significant smartphone × load interactions were found for posture and muscle activation, especially during walking. Male participants showed greater head angles (19.3° vs. 14.6°) and upper trunk angles (28.8° vs. 26.4°) than female participants, whereas female participants consistently exhibited higher CES activity (7.9 vs. 6.8% of maximum voluntary contraction [MVC], p < 0.001). Gender differences in UTZ activation emerged only during smartphone use, with female participants showing disproportionately higher values when carrying 10% BW (15.6 vs. 13.8% MVC, p < 0.001). Critically, during walking with smartphone use and a 10% BW load, female participants' UTZ activity reached 16.8% MVC-exceeding the established 15% MVC fatigue threshold-while male participants approached this limit (14.5% MVC). UTZ activation increased progressively with backpack load in both genders, particularly during walking. These results provide the first empirical evidence that the simultaneous use of smartphones and backpacks can push neck-shoulder muscle activity to near-fatigue levels, especially in female participants. To reduce musculoskeletal strain, backpack loads should be kept below 10% BW during smartphone use, particularly when walking. These findings support ergonomic guidelines for minimizing technology-related musculoskeletal risks in young adults.