Acute Changes in Acromial Distance, Pulmonary Function, and Peripheral Microvascular Perfusion Following One-Handed Versus Two-Handed Smartphone Use in University Students: A Secondary Analysis of a Randomized Parallel-Group Trial

Background and Objectives: The acute effects of one-handed versus two-handed smartphone use on shoulder position, pulmonary function, and local cutaneous microvascular perfusion remain unclear. This secondary analysis of a randomized parallel-group trial aimed to explore differences in forced vital capacity (FVC), other pulmonary function parameters, bilateral supine acromial distance, and local cutaneous microvascular perfusion between the two smartphone-handling conditions. Methods: In this randomized parallel-group pretest–posttest trial, 63 university students were allocated to either one-handed (n = 33) or two-handed (n = 30) smartphone use for 30 min. Outcomes were assessed before and after the task. Baseline-adjusted between-group comparisons were conducted using post hoc exploratory analyses of covariance. Results: FVC decreased in the two-handed group (mean change: −0.13 ± 0.06 L), whereas no significant change was observed in the one-handed group. Baseline-adjusted post-task FVC was higher in the one-handed group (adjusted mean difference, 0.16 L; 95% CI, 0.06–0.26 L; p = 0.002). The FEV1/FVC ratio also differed between groups after baseline adjustment (p = 0.007). Vital capacity decreased significantly in both groups, while inspiratory reserve volume increased and expiratory reserve volume decreased significantly within the one-handed group. Supine acromial distance increased by 0.5–0.6 cm in both groups, without significant baseline-adjusted between-group differences. Right little finger perfusion decreased significantly within the one-handed group (p = 0.032), whereas no significant within-group changes were observed at the other measurement sites. No significant baseline-adjusted between-group differences in local cutaneous microvascular perfusion were detected. Conclusions: Thirty minutes of one-handed or two-handed smartphone use was followed by small, acute changes in selected pulmonary function parameters and increased supine acromial distance in healthy university students. An exploratory baseline-adjusted analysis identified differences in post-task FVC and FEV1/FVC between the two handling conditions. Although a within-group reduction in right little finger perfusion was observed in the one-handed group, no significant between-group difference was detected. As a post hoc secondary analysis with multiple unadjusted comparisons, the findings are exploratory and subject to potential Type I error and selective emphasis on significant results. The clinical significance and reproducibility of these changes remain uncertain.