Content Interest, Neck Posture, and Discomfort During Smartphone Use

This is a single-center, two-condition, two-period randomized crossover experiment at Karabuk University, Faculty of Health Sciences, Department of Physiotherapy and Rehabilitation. One hundred participants will be allocated 1:1 to sequence AB or BA. A is preferred/high-interest video content; B is neutral/lower-interest content. Sequence allocation will be computer-generated by a researcher not involved in measurement and concealed in numbered sealed envelopes. Sessions will be separated by 48-72 hours and scheduled at similar times of day where possible.

A separate preparatory pilot involving up to 12 volunteers will test workflow and technical procedures. Pilot observations will not enter the main hypothesis analyses or the main enrollment target of 100. Substantial protocol changes after the pilot will require an ethics amendment before the main study.

Videos will be selected from a standardized pool matched as closely as possible for duration, resolution, volume, subtitles, and presentation. After short previews, interest/enjoyment ratings will identify preferred/high-interest and lower-interest content that does not evoke negative emotion. Violent, frightening, sexual, or strongly negative material will not be used. Both conditions involve video viewing only.

Participants will sit comfortably in an adjustable chair with back support and use the same study smartphone and adjustable stand. Screen position, distance, brightness, sound, room lighting, seating, and camera setup will be reproduced across sessions. Natural head and trunk movements will be allowed during each 30-minute session.

Neck and bilateral shoulder discomfort will be recorded at baseline and minutes 10, 20, and 30 using the 0-10 Borg CR-10 discomfort scale. The primary endpoint is neck discomfort at minute 30. Sagittal video-photogrammetry will assess neck and trunk angles at minutes 0, 5, 10, 15, 20, 25, and 30. Markers will be placed at the tragus, C7, T12, and greater trochanter. Neck flexion is defined by the tragus-C7 line relative to vertical; trunk angle is formed by the C7-T12 and T12-greater trochanter lines. Kinovea will be used for coded-image analysis by a trained assessor who will be unaware of the content condition where possible. Participants cannot be blinded to their own preferences.

Digital algometry will measure pressure pain threshold bilaterally at the upper trapezius and at the dominant tibialis anterior before and immediately after viewing. A 1 cm2 probe will apply pressure at approximately 30 kPa/s, stopping at first pain. Three measurements per site will be averaged, with 30-60 seconds between measurements. Measurement order will be maintained across sessions.

The Turkish Extended Nordic Musculoskeletal Questionnaire will describe baseline symptoms. End-of-session assessments will include the Turkish Flow Short Scale, interest/enjoyment subscale of the Turkish Intrinsic Motivation Inventory, and Self-Assessment Manikin valence and arousal ratings. Baseline sleep, recent intense physical activity, caffeine, analgesic use, and discomfort will be recorded to assess session comparability and deviations.

Mixed-effects models will include condition, time, condition-by-time, period, and sequence, with participant as a random effect. The primary analysis compares minute-30 neck discomfort within participants. Additional models will include neck and trunk angles to explore the relationship independent of measured postural exposure. Pressure pain threshold will be examined with condition-by-time (pre/post) interaction. Effect sizes and 95% confidence intervals will be reported with two-sided alpha 0.05.

Participants may stop at any time. Marked pain, dizziness, numbness/tingling, visual discomfort, or unwillingness to continue will end the task immediately. All data and images will be coded. Identifiable images will not be published or presented without separate explicit written permission.