Purposes

Mobile devices with various screen sizes are now popular.  Screen size may affect how persons choose to interact when typing with such devices.  Self-reported duration and frequency of text messaging has been associated with musculoskeletal disorder (MSD) symptoms, particularly in the shoulder, neck, and thumb (1, 2).  It is unknown which aspects of mobile device usage may increase MSD risk.  However, non-neutral neck and wrist postures are associated with MSDs in office workers engaging in keyboarding (3-8).  The study aim was to determine if mobile device screen size affects neck and/or wrist posture during self-determined texting positions.  Also, fingers used in texting were tabulated by screen size, as this may affect MSD risk.

Methods

Three touch screen mobile devices of different screen sizes – 3.5” (Apple iPod Touch MC544LL), 7” (Samsung Galaxy Tab GT-P1010) and 9.5” (Apple iPad 2) -- were randomly presented to asymptomatic college students (n = 20).  Each device was made ready for simulated texting by activating the “Notes” or a similar application.  Devices were placed screen down on a table 1 m away from the participant.  Participants retrieved the device, resumed sitting in a chair, and spent as much time as desired to determine how they wanted to text “hi how are you” repeatedly for 10 seconds.  Wrist and neck postures were measured through electrogoniometry and sagittal plane motion analysis, respectively.  Fingers used to text and whether the device was placed on the lap were noted.  Excel (Microsoft Corp., Redmond, WA) and SAS V9.3 (SAS Institute Inc., Cary, NC) were used in data analysis.  Outcome variable trends by screen size were determined using SAS PROC GLM for repeated measures, with p < 0.05 denoting statistical significance.

Results

Ninety percent of subjects using the 3.5” screen chose to text with both thumbs (table).  As screen size increased, more used the right index finger as more varied texting styles were chosen.  Additionally, more placed the entire device or its bottom edge on their lap.  Wrist extension and ulnar deviation, and cervical flexion increased with increasing screen size (test for trend, all p < 0.01, table).

Conclusions

Greater prevalence of non-neutral neck and wrist postures were found during texting with increasing touch screen size.  These factors may increase MSD risk.  However, decreased thumb usage occurs with greater screen size.  Such larger screens may be protective against thumb symptoms.  Further studies are warranted.