Smartphone-Related Alteration in Lumbo-Pelvic Rhythm During Functional Activities Among Young Adults: A Cross-Sectional Biomechanical Study

Abstract Background: Prolonged smartphone usage induces habitual forward head posture and thoracic flexion, potentially disrupting the integrated spinal kinetic chain. While static postural deviations are well-documented, smartphone-related alterations in dynamic spinal movement patterns— specifically lumbo-pelvic rhythm during functional activities—remain inadequately explored. Understanding these biomechanical adaptations is critical, as abnormal lumbo-pelvic coordination represents a risk factor for chronic low back pain and movement dysfunction. Objective: To investigate whether chronic smartphone usage alters lumbo-pelvic rhythm during forward bending and functional lifting tasks in young adults, and to examine the relationship between smartphone exposure duration and spinal movement coordination patterns. Methods: This cross-sectional comparative study enrolled 52 young adults (aged 18-30 years) stratified into two groups: heavy smartphone users (>4 hours/day, n=26) and light users (<1 hour/ day, n=26). Lumbo-pelvic rhythm was assessed using dual inclinometry and three-dimensional motion capture during standardized forward bending tests, functional lifting tasks, and functional reach tests. Primary outcomes included lumbo-pelvic rhythm ratio (lumbar flexion contribution/ pelvic rotation contribution), trunk flexion timing, and pelvic rotation angles. Secondary measures included thoracic kyphosis angle, craniovertebral angle, and core muscle endurance. A subgroup (n=18 heavy users) completed a 6-week physiotherapy intervention comprising movement re patterning exercises, core neuromuscular control training, and thoracic extension mobility program. Statistical analyses employed independent t-tests, paired t-tests, and Pearson correlations. Results: Heavy smartphone users demonstrated significantly reduced lumbar contribution to forward bending compared to light users (36.2±7.4% vs 51.3±6.8%, p<0.001), with compensatory increased pelvic rotation (28.4±5.7° vs 20.1±4.3°, p<0.001). Trunk flexion timing was significantly delayed in smartphone users (0.87±0.21 seconds vs 0.62±0.15 seconds, p<0.001), indicating altered motor control sequencing. Strong negative correlations existed between daily smartphone usage and lumbar contribution (r=-0.746, p<0.001). Heavy users exhibited greater thoracic kyphosis (47.8±6.2° vs 38.4±5.1°, p<0.001) and reduced craniovertebral angles (45.2±4.8° vs 54.7±5.3°, p<0.001). Following the 6-week intervention, significant improvements occurred in lumbo-pelvic rhythm ratio (36.2±7.4% to 46.8±6.9%, p<0.001), pelvic rotation (28.4±5.7° to 22.7±4.8°, p=0.003), and core endurance (21.4±6.3 to 34.7±7.8 seconds, p<0.001). Conclusions: Chronic smartphone usage induces maladaptive alterations in lumbo-pelvic rhythm characterized by reduced lumbar mobility contribution, excessive compensatory pelvic motion, and delayed neuromuscular timing during functional activities. These findings establish a mechanistic link between habitual smartphone posture and whole-spine movement dysfunction, extending beyond previously documented static postural changes to dynamic biomechanical impairments. Movement re-patterning, core neuromuscular training, and thoracic mobility interventions effectively restore normal lumbo-pelvic coordination. Preventive strategies addressing smartphone ergonomics and movement quality may mitigate risk of chronic low back disorders in smartphone dependent populations.