Walking Speed Modulates Neck–Shoulder Strain During Smartphone Use with Backpack Load

<b>Background/Objectives:</b> The concurrent use of smartphones and backpacks presents notable ergonomic challenges for students and young adults. However, the influence of walking speed on this dual-task biomechanical strain remains unclear. This study investigated how walking speed, backpack load, and gender affect postural alignment and neck-shoulder muscle activity during smartphone use. <b>Methods:</b> Thirty healthy university students (15 males and 15 females) were assessed using a motion capture system and surface electromyography to quantify postural alignment and muscle activity. Each participant completed twelve randomized conditions comprising three backpack loads (0%, 5%, and 10% of body weight) combined with four locomotor states (standing and walking at slow, normal, and fast speeds). Outcome measures included neck flexion, upper-trunk angle, lumbosacral angle, and normalized surface electromyography of the cervical erector spinae (CES) and upper trapezius (UTZ). A three-way ANOVA was used to evaluate main and interaction effects. <b>Results:</b> Increasing backpack load significantly increased neck flexion and upper-trunk angle while reducing the lumbosacral angle (all <i>p</i> < 0.001). Muscle activity rose proportionally with load, with UTZ activation nearly doubling from 10.7% to 21.1% maximum voluntary contraction (MVC) at 10% body weight. Faster walking increased lumbar flexion and elevated CES and UTZ activation (<i>p</i> < 0.05), while neck and upper-trunk postures remained stable across speeds. Females maintained more upright postures but exhibited higher muscle activity than males (<i>p</i> < 0.01). UTZ activation frequently exceeded the 15% MVC fatigue threshold during walking with backpacks. <b>Conclusions:</b> Walking speed critically modulates musculoskeletal strain during concurrent smartphone use and load carriage. The combined effects of backpack load and smartphone use can elevate neck-shoulder muscle demands, with UTZ activity occasionally approaching fatigue thresholds under moderate load and faster walking. Based on the current findings, backpack loads above 5% of body weight may increase the risk of neck-shoulder strain. Additionally, reducing smartphone use during fast walking may help prevent neck-shoulder fatigue and related musculoskeletal discomfort.